The oldest Archaeopteryx (Theropoda: Avialiae): a new specimen from the Kimmeridgian/Tithonian boundary of Schamhaupten, Bavaria

Axial skeleton

The neck was obviously disrupted by the left forelimb, so that the eighth cervical vertebra (assuming a cervical count of nine vertebrae, as in other specimens of Archaeopteryx; Wellnhofer, 2008, 2009) is disarticulated from the more anterior elements (Fig. 5). Anterior to this vertebra, only one cervical is recognisable on the other side of the left humerus, close to the skull. However, this vertebra (presumably a posterior mid-cervical) is very poorly preserved, and most of the lateral wall of the element is missing, so that the interior of the vertebra is exposed (Fig. 6B). The vertebral centrum is elongate (more than 7 mm long at an anterior centrum height of c. 2.4 mm) and seems to be at least slightly convex anteriorly, although the articular surface is too poorly preserved to determine this with certainty. The prezygapophysis is marked as a slender process that overhangs the vertebral centrum anteriorly and is flexed ventrally in its anterior part. Both the vertebral centrum and neural arch are strongly pneumatised as found in the Berlin specimen (Britt et al., 1998); in the case of the neural arch small cavities extend even into the stalk of the prezygapophysis (Fig. 6B).

The last two cervical vertebrae are preserved in articulation with the dorsal vertebral column (Fig. 17).

The length decreases rapidly from the eighth (8.1 mm) to the ninth cervical (6.1 mm). The more anterior of these two cervicals has an elongate centrum that is approximately 2.8 times as long as its anterior height. The centrum is only slightly constricted in lateral view, and the articular ends are not offset from one another. The anterior end of the centrum is strongly convex, indicating an opisthocoelous condition for at least the posterior cervical vertebrae of this specimen (Fig. 17). Dames (1884) assumed an amphicoelous (‘biconcave’) condition for the cervical vertebrae of the Berlin specimen of Archaeopteryx, although he noted that this condition cannot be established with certainty. Wellnhofer (2008, 2009) considered the vertebrae to be platycoelous in this specimen. However, as the cervical vertebrae of this specimen are preserved in articulation, the condition is difficult to assess, and this is also true for the Eichstätt specimen, which also preserves a complete cervical vertebral column (Wellnhofer, 1974, 2008). In the 11th specimen, the neck is also preserved in articulation (Foth, Tischlinger & Rauhut, 2014), but at the strongest flexure of the neck, the anterior end of the seventh cervical is slightly disarticulated from the sixth element, and also seems to be at least slightly convex. In only slightly more dervied avialan taxa, such as Confuciusornis (Chiappe et al., 1999) and Sapeornis (Zhou & Zhang, 2003b), the cervical vertebrae are already hetercoelous; the condition is currently unknown in Jeholornis (Zhou & Zhang, 2003a; O’Connor et al., 2012; Lefèvre et al., 2014).

In the anterior end of the centrum of the eighth cervical, the lateral wall of the bone is missing, so the presence of a pneumatic foramen can only be assumed by the presence of a cavity in this area. However, the centrum is clearly strongly pneumatized, with the preserved structures of the broken anterior end indicating a camellate structure, as present in Aerosteon, carcharodontosaurids, ornithomimosaurs, tyrannosaurids, oviraptorosaurs, and modern birds (Britt, 1993; Sereno et al., 2008; Evers et al., 2015). The neural arch of this vertebra is poorly preserved, but the postzygapophysis seems to overhang the centrum posteriorly, while the prezygapophysis seems to be placed entirely above the anterior end of the centrum. The probably last cervical vertebra is poorly preserved, and little can be said about its morphology (Fig. 17). The anterior end of the centrum seems to be flat, but this might be an artifact of preservation. The neural arch is high, being approximately as high as the centrum.

In contrast to most other theropods, Archaeopteryx (including the new specimen) possesses 14 dorsal vertebrae. The first dorsal vertebra is slightly shorter (5.3 mm) than the last cervical vertebra. As in other specimens of Archaeopteryx (Britt et al., 1998; Christiansen & Bonde, 2000), it has a well-developed pneumatic foramen on the anteroventral side of the centrum (Fig. 17). This foramen seems to be placed below the parapophysis, which also seems to be the case in the 11th specimen. A ventral keel or hypapophysis is absent. As in the last cervical vertebra, the neural arch is anteroposteriorly short and high.

The remaining dorsal vertebrae are poorly preserved and/or overlapped by the heads of the dorsal ribs, and little can be said about their morphology (Fig. 18). The posteriormost dorsal and the sacral vertebrae are largely missing due to a break going through the slab here, so that details of the transition between the two vertebral regions cannot be studied. The centra and parts of the neural arches of the 7th to 10th dorsal vertebrae are exposed (Fig. 19). The centra are amphi- to platycoelous and elongate, the eighth dorsal being c. 6.2 mm long and 4 mm high posteriorly. The centra are evenly constricted ventrally in lateral view and have a flattened ventral surface. No pneumatic foramina are present in these centra, but a large, shallow, anteroposteriorly elongate depression is present on the dorsal part of the lateral side. A part of the lateral lamination of the neural arch is preserved in the eighth dorsal (Fig. 19). The anteroposteriorly extensive transverse process is supported ventrally by a short, but stout and vertically oriented posterior centrodiapophyseal lamina (pcdl), which is placed above the posterior third of the centrum. Anteriorly, a well-developed, though more slender paradiapophyseal lamina (ppdl) extends posterodorsally at an angle of approximately 45° in relation to the long axis of the centrum and meets the base of the transverse process well in front of the pcdl, so that there is a large centrodiapophyseal fossa in between these laminae that is roofed dorsally by the transverse process. A well-developed prezygodiapophyseal lamina (prdl) is present anterior to the point where the ppdl meets the transverse process. Anterior to the ppdl, but originating ventrally at approximately the same point as the latter, there is a thin, but well-developed prezygoparapophyseal lamina (prpl), which extends anterodorsally at an angle of slightly more than 90° towards the ppdl and reaches the anterior end of the prezygapophysis anteriorly (Fig. 19). The prezygapophysis slightly overhangs the vertebral centrum anteriorly. The neural spines of the mid-dorsal vertebrae are anteroposteriorly long (70–75% of centrum length in D9), rectangular in outline, and slightly lower dorsoventrally than long anteroposteriorly (Figs. 18 and 19). They are placed over the posterior part of the centrum, so that their posterior margin is approximately flush with the posterior end of the latter. The postzygapophysis extends posterior to the base of the neural spine and thus overhangs the centrum for its entire length. Its dorsal margin is connected to the neural spine by a short, curved spinopostzygapophyseal lamina (spol).

Because the sacral region is highly damaged the number of sacral vertebrae cannot be estimated. Only a few fragments of the neural arches of the sacral vertebrae are discernible. The neural spines of the sacral vertebrae were obviously similar to those of the dorsal vertebrae and are well separated (Fig. 18).

Due to the damage in the sacral region, the proximalmost caudal vertebrae are very poorly preserved and mainly indicated by impressions. Thus, the transition between the sacral and caudal vertebrae is difficult to establish, but we take a rather abrupt change in length of the vertebrae to indicate this transition. Based on this assumption, there are 22 caudal vertebrae preserved (Fig. 20A), as in the Eichstätt (Wellnhofer, 1974, 2008, 2009) and 11th specimen. However, it cannot be ruled out that a very small 23rd vertebra might originally have been present posterior to the last preserved element. When compared, Eosinopteryx possesses 20 caudals (Godefroit et al., 2013a), Anchiornis has approximately 31–32 vertebrae (Pei et al., 2017), while the caudal series of Jeholornis varies between 22 and 27 vertebrae (Zhou & Zhang, 2002, 2003a; O’Connor et al., 2012).

The proximal caudals have rather short centra, as in Jeholornis (O’Connor et al., 2012), Anchiornis (Xu et al., 2009a), and several other paravians, but nothing can be said about their morphology. The length of the caudal vertebrae increases rapidly from 6.2 mm in the sixth to 9.7 mm in the ninth vertebra. The longest caudal vertebrae are the 11th and 12th elements, which measure 10.1 mm each and in which the vertebral centrum is more than five times as long as high, being thus more elongate than in Jeholornis (O’Connor et al., 2012). From here, caudal vertebral length decreases gradually to 8.7 mm in the 18th caudal, and then rapidly to 2.4 mm in the 22nd vertebra (Fig. 20C). Vertebral centra are generally spool-shaped and only slightly constricted in the middle in lateral view (Fig. 20). A shallow longitudinal depression is present on the lateral side in the middle and distal caudal vertebrae (at least to caudal 17) and become more pronounced towards the articular ends (Fig. 20B). These depressions might be slightly exaggerated by compression, as a damaged mid-caudal vertebra shows that the elements were hollow. However, similar lateral depressions have also been noted in Anchiornis (Hu et al., 2009).

The anterior mid-caudals (e.g. CA6) have short and stout prezygapophyses that point anterodorsally at an angle of slightly less than 45° towards the long axis of the centrum and do not overhang the centrum anteriorly. In contrast, the postzygapophyses are slender, point more posteriorly than dorsally and overhang the centrum posteriorly. From the eighth caudal onwards, the prezygapophyses become more slender, point more anteriorly than dorsally, and overhang the centrum anteriorly. In the distal caudal vertebrae, the prezygapophyses are developed as thin, anteriorly pointing processes that overlap the posterior end of the preceeding centrum. However, in contrast to many tetanuran theropods, in which this overlap often accounts for one-third or more of the length of the centrum, it reaches maximally 24% in this specimen of Archaeopteryx. The postzygapophyses form a broad, posteriorly pointing and posteriorly forked platform in the middle and distal caudals that overhangs the following centrum approximately as much as the prezygapophysis. From the seventh vertebra onwards, the pre- and postzygapophyses are connected by a pronounced longitudinal ridge on the laterodorsal side of the neural arch (Fig. 20). A small, apparently triangular transverse process is present at about mid-length of the centrum in the sixth caudal vertebra. This transverse process is transversely short and placed just below the mid-height of the centrum. Transverse processes are also still present in the seventh and eighth caudal, where it is only a small triangular flange of bone. These processes seem to be completely absent from the ninth caudal onwards.

The neural spine is first visible in the eighth caudal vertebra, where it is developed as a low, dorsally straight dorsal ridge that extends over almost the entire length of the vertebra, but is maximally as high as the dorsal edges of the zygapophyses. The last vertebra with a spine is the 10th caudal; posterior to this element, the dorsal surface of the neural arch is flat.

Several dorsal ribs are at least partially preserved. As in all dinosaurs, the rib head is two-headed (Fig. 18). Where the rib heads are preserved, in the mid-dorsal section, they have a long and slender capitulum that is continuous with the proximal curvature of the rib shaft, and a much shorter, dorsally pointing tuberculum (Fig. 19). As in other specimens of Archaeopteryx (Wellnhofer, 1974), the first dorsal rib is short and spike-like. The anterior dorsal (thoracic) ribs have long shafts that are almost straight proximally, but curve strongly medially in their distal half (Fig. 18). This morphology is also found in the Eichstätt (Wellnhofer, 1974), Solnhofen (Wellnhofer, 1992) and 11th specimen, whereas the ribs are only gently and more gradually curved in the Berlin and Munich specimens (Wellnhofer, 1993, 2008, 2009). As all specimens are flattened into two dimensions, it is unclear in how far these differences might be due to compression and differences in the orientation of the ribs during burial and which of these morphologies represents the original condition. However that may be, the more posterior ribs of the 12th specimen are more gradually curved. Many of the rib shafts seem to have a narrow and shallow longitudinal furrow anterolaterally (Fig. 18). This furrow is also present in the 11th specimen, Microraptor (Pei et al., 2014) and Anchiornis (Pei et al., 2017). Uncinate processes, as they are present in Microraptor (Hwang et al., 2002), some other dromaeosaurids (Norell & Makovicky, 1999) and oviraptorids (Clark, Norell & Chiappe, 1999), are absent in this and all other specimens of Archaeopteryx. Several broken sections of rib shafts indicate that these bones were hollow (Fig. 21). As in several other specimens of Archaeopteryx (Wellnhofer, 2008, 2009), the distal end of several of the anterior dorsal ribs is notably thickened (Figs. 18 and 21), indicating the attachment of cartilagenous sternal ribs (see Foth, 2014).

The anterior caudal chevrons are poorly preserved, but the element between the fourth and fifth caudal vertebra is dorsoventrally short and expanded distally and thus corresponds to the morphology figured by Wellnhofer (2008). Posterior to this element, a recognisable chevron is first present between the 9th and 10th caudal vertebrae (Fig. 20B). This chevron is skid-like, with a longer anterior than posterior process. The anterior process is more slender than the posterior process and tapers to a point, whereas the posterior process is rounded posteroventrally. The contact to the chevron facets of the vertebrae is developed as a small, triangular flange. The anteroposterior length of this chevron is approximately 5.5 mm (3.8 mm anterior and 1.7 mm posterior process), whereas its dorsoventral height is only 1 mm. More posteriorly placed chevrons are generally similar in morphology, but more symmetrical (Fig. 20B). The last chevron seems to be present between the 21st and 22nd caudal (Fig. 20C). In contrast to Jeholornis (O’Connor et al., 2012) and dromaeosaurids (Ostrom, 1969; Norell & Makovicky, 1999), the adjacent chevrons do not overlap.

Gastral ribs are present in the thoracic region and all the way to the pubis in the abdominal region (Figs. 4, 5 and 18), as in the Eichstätt and Munich specimens of Archaeopteryx (Wellnhofer, 2008, 2009). The anterior elements are more robust than posterior elements and the medial elements are more robust than lateral gastralia (Fig. 21). Whereas the latter remain of subequal width throughout their length, the medial gastralia taper laterally. In contrast to the situation in Velociraptor (Norell & Makovicky, 1997), Sinornithoides (Russell & Dong, 1993) and Linheraptor (Xu et al., 2010), but similar to the condition in Microraptor (Pei et al., 2014), the lateral elements seem to be subequal in length to the medial elements.

Appendicular skeleton

As noted above, the forelimbs are disarticulated from the rest of the body, but kept their natural articulation between the forelimb elements, especially in the left limb. Unfortunately, however, a break ran through the right shoulder girdle and forelimb when the specimen was discovered, and parts of the left forelimb were lost due to erosion or when the slab was split originally. Furthermore, all limb bones are strongly compressed and cracked, and their preservation is thus rather poor.

As in most other specimens of Archaeopteryx, the shoulder girdle comprises the scapula, coracoid and furcula (Fig. 22), but an ossified sternum is not present, as it is the case in most non-pennaraptoran theropods, Anchiornis, Mei, Jianianhualong, and Sapeornis (Zheng et al., 2014; Foth, 2014; Xu et al., 2017). The scapulocoracoid is described in reference to its original position, with the scapula being oriented more or less parallel to the vertebral column, and the coracoid at an angle of 100–110° towards it (as in the Thermopolis (Mayr et al., 2007) and 11th specimen). The scapula is long and slender (Figs. 22 and 23), with the ratio of scapular length (43 mm) to minimal shaft width (2.8 mm) being more than 15 in the left scapula. The shaft is of subequal width through most of its length, although a small distal expansion to maximally 4.4 mm is present in the right scapula (Fig. 22; the distal end of the left scapula is crushed into the vertebral column and therefore poorly preserved). The shaft is gently curved in its proximal two-thirds, but less so than reconstructed by Wellnhofer (2008: fig. 6.10). The shaft is transversely broader ventrally than dorsally, and a shallow longitudinal depression is present on the lateral side in its proximal two thirds, although this might be exaggerated by compression, especially proximally (Fig. 23). Whereas the ventral margin of the shaft is rounded transversely over most of its length, it becomes sharp edged towards the margin of the glenoid, and a ventrally and slightly medially facing flat surface is present medial to it.

Proximally, the scapula expands ventrally towards the glenoid, with a maximal expansion of 2.5 mm below the scapular shaft (Fig. 23). In contrast, there is only a very slight dorsal expansion towards the acromion process, which is more anteriorly than dorsally directed, as in other paravians (Forster et al., 1998; Norell & Makovicky, 1999; Xu, Wang & Wu, 1999; Xu et al., 2002, 2011; Gianechini & Apesteguia, 2011; Brusatte et al., 2013; Lefèvre et al., 2014). Thus, the suture between the scapula and coracoid is an oblique line extending from the anterior margin of the glenoid anterodorsally, but its anterior extend is uncertain due to breakage in this area. Most of the glenoid articular surface is therefore placed on the scapula, where the rim of the glenoid forms a semioval arch between the ventral expansion at the base of the shaft and the coracoid suture (Fig. 23). The articular surface of the scapular glenoid faces more laterally than ventrally; given the position of the scapula high on the ribcage parallel to the dorsal vertebrae, this orientation in life was certainly even more or entirely lateral, as in Unenlagia (Novas & Puerta, 1997), Rahonavis (Forster et al., 1998) and Sinornithosaurus (Xu, Wang & Wu, 1999). The dorsal rim of the glenoid lacks any ledge and is even very slightly depressed in its central part. Only towards the suture with the coracoid, which is developed as a lateral ridge in this area, the rim of the glenoid expands somewhat laterally. The supraglenoid fossa on the lateral side of the proximal scapula is large and elongate triangular in outline (Fig. 23), as in other paravians, but its margins are not clearly defined, which might be due to compression. Dorsally, the low acromion process is largely overlapped by the very robust epicleideal process of the furcula, the contact with which extends to almost the level of the posterior margin of the glenoid (Fig. 23). Although the furcula is preserved in close association with the scapula, it is slightly displaced from its facet on the latter. The facet is placed on the laterodorsal edge of the anterior part of the acromion process and extends slightly more ventrally anteriorly. This indicates that the furcula bridged the gap between the left and right should girdles, as in other pennaraptorans (Nesbitt et al., 2009), but was not as strongly ventrally directed as reconstructed by Elzanowski (2002) and Wellnhofer (2008).

Only the proximalmost portion of the left coracoid is preserved in articulation with the scapula. The rest of the bone is broken and was flattened below the rest of the scapulocoracoid. Thus, only the medial side of the main body of the coracoid is visible, and its anterior and posterior margins are overlapped by the scapula and the humerus, respectively (Fig. 23). The portion of the coracoid preserved in articulation with the scapula indicates that this element was angled at approximately 100–110° towards the latter. Thus, the angle between these two elements is smaller than in Velociraptor (Norell & Makovicky, 1999), but larger than reconstructed by Elzanowski (2002) and Wellnhofer (2008). The coracoid was a rather large, plate-like element, as in other specimens of Archaeopteryx (Wellnhofer & Tischlinger, 2004; Mayr et al., 2007), troodontids (Russell & Dong, 1993), dromaeosaurids (Norell & Makovicky, 1999), and Sapeornis (Zhou & Zhang, 2003b; Provini, Zhou & Zhang, 2009), with an estimated dorsoventral length of 9.5–10 mm. In contrast, Confuciusornis and Jeholornis possess an elongated coracoid, as present in modern birds (Chiappe et al., 1999; Zhou & Zhang, 2003a). Proximally, the coracoid forms only a small part of the anterior margin of the glenoid fossa. The coracoid glenoid facet is triangular in outline and faces mainly posteriorly and only slightly laterally.

Only the left ramus of the furcula is preserved (Figs. 22 and 24), although the preserved portion includes the ventral bend of the element and the medialmost part of the right ramus (Fig. 24), in contrast to the indication in the reconstructed element, which implies that exactly half of the bone is present. As in other specimens of Archaeopteryx, it was a rather robust, U-shaped element. The bone is slightly convex anteriorly; this curvature has certainly been reduced by compression, and might originally have been similar to that seen in other pennaraptorans (Nesbitt et al., 2009). A very small, rounded flange is present at the flexure point ventrally (Fig. 24) and might indicate an incipient hypocleidium (though see Nesbitt et al., 2009, who questioned the homology of such tubercles, which are common in theropods, with the hypocleidium of birds). The epicleideal process of the furcula is slightly flattened and triangular in outline, tapering to a rounded point mediodorsally. The largely collapsed ramus of the furcula indicates that this element might have been hollow (Fig. 24), as in Aerosteon (Sereno et al., 2008) and Buitreraptor (Makovicky, Apesteguia & Agnolin, 2005).

Both humeri are crushed and rather poorly preserved. As far as can be established, their morphology seems to correspond closely to that found in other specimens of Archaeopteryx (see Wellnhofer, 2008, 2009). The right humerus is exposed in posterior view, but most of the proximal end is missing. The deltopectoral crest is well developed and its distal end raises rather abruptly out of the shaft (Fig. 22). Assuming a similar length of the left and right humerus, the deltopectoral crest reached approximately for 30% of the length of the humerus down the shaft, as in Balaur (Brusatte et al., 2013), while it measures only 25% in Anchiornis (Pei et al., 2017). A slightly raised, elongate oval tubercle is present on the lateral side of the shaft at the distal end of the deltopectoral crest (Fig. 22). The distal end is transversely expanded, apparently more so medially than laterally. Its posterior side is flat to very slightly concave, but a large posterior depression adjacent to the articular end, as it is found in many non-avialan theropods and also in the ninth specimen of Archaeopteryx (Ottmann & Steil specimen, so called ‘chicken wing’), is absent. The medial side of the distal end, which is exposed in the left humerus, is flattened. Based on the more complete left humerus, which is exposed in medial view, the shaft seems to be less flexed posteriorly in its proximal portion than in other specimens of Archaeopteryx.

As with the humeri, the radia and ulnae are poorly preserved, and little detail can be gained from them. As with most of the other appendicular elements, the shafts of the bones are collapsed, indicating that they were hollow. The more complete right ulna and radius are exposed in medial and/or anteromedial view (Fig. 22).

The ulna is slightly longer than the radius, and both elements are shorter than the humerus, as in other specimens of Archaeopteryx, non-avialan theropods and Confuciusornis (Chiappe et al., 1999), whereas the ulna is as long or longer than the humerus in Jeholornis (Lefèvre et al., 2014), Sapeornis (Zhou & Zhang, 2003; Provini, Zhou & Zhang, 2009) and more derived avilans. The ulna is slightly bowed posteriorly in its proximal half, while the distal half of the shaft is straight (Fig. 22). The proximal end is very slightly expanded anteroposteriorly and there is a pronounced angle of slightly less than 90° between the articular surface and the shaft, but an olecranon process is absent. A short, sharply defined ridge extends from the proximal end distally at about the mid-width of the bone. Given the compaction of the element, it seems plausible that this ridge marks the rather sharply defined boundary between the anterior and the medial side of the ulna, so that both sides are exposed here, pressed into a single plane. The distal end of the right ulna is exposed in medial view. There is no anteroposterior expansion and the distal articular surface is strongly convex, extending proximally in a small, triangular process medially, as in Balaur (Brusatte et al., 2013). The left ulna, the distal end of which is exposed in posterolateral view, shows furthermore that the articular end was strongly convex anteroposteriorly.

The radius is slightly more slender than the ulna and straight (Fig. 22). The proximal end is not preserved in the left element and hidden by the humerus in the right bone. The distal end shows little to no expansion, but is too poorly preserved to say anything about its morphology.

Both carpi and mani are poorly preserved, and only few elements are present. Of the carpus, only the poorly preserved semilunate carpal of the right manus is preserved (Fig. 22), but does not provide much additional information on the carpus of Archaeopteryx than that known from other specimens (Ostrom, 1976; Wellnhofer, 2008, 2009). A conspicuous proximodistally oriented ridge extends across the dorsal surface of the carpal and divides the smaller medial portion, that would have articulated with metacarpal I, from the wider lateral part that articulated with metacarpals II and III. The articular surface for metacarpal I is very slightly angled medially in respect to that for metacarpals II and III.

Only large parts of metacarpals II and III are preserved in the left hand (Fig. 25). As in the Berlin (Wellnhofer, 1985, 2008), Eichstätt (Wellnhofer, 1974), Munich (Wellnhofer, 1993), and Thermopolis (Mayr et al., 2007) specimens of Archaeopteryx, metacarpal III is subequal in length to metacarpal II, but only slightly more than half its width, straight, and closely appressed to the latter over its entire length. In contrast, the third metacarpal of the London (BSPG cast) and Solnhofen (Wellnhofer, 1988b) specimens are slightly more curved and separated from metacarpal II over most of their length by a notable gap.

Only parts of the phalanges are, largely poorly, preserved, and they do not show much detail (Figs. 22 and 25). As in other specimens of Archaeopteryx, the phalanges of the hand were slender and elongate, showing no signs of laterally or medially located longitudinal furrows. The unguals are strongly recurved, with strongly expanded proximal flexor tubercles (Figs. 25 and 26). Unlike the situation in many maniraptoran theropods (Rauhut & Werner, 1995), there is only a small, incipient dorsal ‘lip’ on the proximal articular end (Fig. 26). This is also different from the situation in Xiaotingia (Xu et al., 2011) and Anchiornis (Pei et al., 2017) and most other Pennaraptora, but similar to Balaur (Brusatte et al., 2013). The unguals preserve parts of their horny sheaths, which enlarge the bony core for approximately one third of the total length of the unguals and add to the pronounced curvature of these elements (Fig. 26).

Of the pelvis, only the shafts and distal ends of the pubis and ischium are preserved (Figs. 27 and 28). As in all non-avialan maniraptoran theropods, with the possible exception of Scansoriopterygidae (Zhang et al., 2008; Czerkas & Feduccia, 2014), the pubis is considerably longer than the ischium (Fig. 28). As in the Berlin, Eichstätt, Solnhofen, Thermopolis, and 11th specimen (Dames, 1884; Wellnhofer, 1974, 1992, 2008, 2009; Mayr et al., 2007; Foth, Tischlinger & Rauhut, 2014), the pubic shafts are parallel to the ischium and directed posteroventrally in respect to the long axis of the vertebral column, indicating that this might have been the natural orientation, in contrast to the more ventral orientation advocated by Wellnhofer (1985, 2008, 2009). This interpretation is supported by the preserved gastralia, which extend up to the margin of the pubis in the Eichstätt (Wellnhofer, 1974, 2008), 11th (Foth, Tischlinger & Rauhut, 2014) and the current specimen (Fig. 28), which should not be the case if the pubis was rotated posteriorly post-mortem. Curiously, however, in all of these specimens, as well as the Munich (Wellnhofer, 1993) and Haarlem specimens (Ostrom, 1970; Wellnhofer, 2008), the gastralia are placed at about the half length of the pubis, not at its distal end, as would be suspected (see also discussion). This is also the case in Jianianhualong (Xu et al., 2017), Anchiornis (Zheng et al., 2014; Pei et al., 2017), Sapeornis (Zhou & Zhang, 2003b) and various dromaeosaurids (Hone et al., 2010; Xu et al., 2010; Han et al., 2014; Lü & Brusatte, 2015).

The pubic shafts are slender and very slightly flexed posteroventrally. The proximal break of the right pubis shows that the bone was hollow (Fig. 28). Xu et al. (2011) stated that a lateral expansion of the pubic shafts was present in the Solnhofen specimen. Such an expansion is clearly absent in the current specimen. Moreover, examination of the Solnhofen specimen by two of us (OR, CF) indicates that the only exposed right pubic shaft is only slightly convex laterally, and this might furthermore be partially or even entirely caused by the compaction of the femur onto the lateral side of the proximal pubic shaft. Thus, a noted lateral expansion, as it is found in microraptorine dromaeosaurids (Hwang et al., 2002; Longrich & Currie, 2009; Pei et al., 2014), is clearly absent.

The pubic apron is narrow and extended for approximately half the length of the pubic shafts (Fig. 28), as in the London and Thermopolis specimens (de Beer, 1954; Mayr et al., 2007). The extent of the pubic apron is thus considerably less than in non-coelurosaurian theropods (Gilmore, 1920; Galton & Jensen, 1979; Madsen, 1976; Currie & Zhao, 1993a), slightly less than in non-maniraptoran coelurosaurs (Osmólska, Roniewicz & Barsbold, 1972; Brochu, 2003), and comparable to the situation in dromaeosaurids (Norell & Makovicky, 1999; Hwang et al., 2002). In more derived avialans, the pubic apron is further reduced; it extends over slightly less than half the length of the pubis in Jeholornis (O’Connor et al., 2012), about a third in Confuciusornis (Chiappe et al., 1999) and Sapeornis (Zhou & Zhang, 2003b), roughly the distal fourth in Sinornis (Sereno, Rao & Li, 2002), and the left and right pubes are separated over their entire length in Ornithurae (Clarke & Norell, 2002). As in most theropods, the apron is placed anteriorly on the medial side of the pubic shafts. Towards the proximal end of the median contact of the aprons of the left and right pubis, the medial edge of the flange becomes sharp edged and flexes very slightly posteriorly.

Whereas the left and right pubes have slightly disarticulated over most of the length of the pubic apron, exposing a longitudinal furrow in the edge of the apron of the left pubis proximally, they remain in contact distally, and no suture is visible at the distal boot. The distal boot only expands posteriorly and has a hook-like appearance, with an angled ventral margin and a slight proximal expansion posteriorly (Fig. 28). An anterior expansion of the pubic boot is also absent in some (e.g. Velociraptor: Norell & Makovicky, 1997; Unenlagia: Novas & Puerta, 1997; Microraptor: Pei et al., 2014; Zhenyuanlong: Lü & Brusatte, 2015), but not all (e.g. Deinonychus: Ostrom, 1976; Achillobator: Perle, Norell & Clark, 1999) dromaeosaurids, Anchiornis (Hu et al., 2009; Pei et al., 2017), and many basal avialans that retain a pubic boot (e.g. Sapeornis: Zhou & Zhang, 2003b; Sinornis: Sereno, Rao & Li, 2002). The lateral surface of the pubic boot is poorly preserved but seems to be flat.

The ischia are approximately half the length of the pubis and considerably more massive (Fig. 28), which might be slightly exaggerated by compression. Only the distal end of the right ischium seems to be preserved, whereas the more proximal portion visible obviously belongs to the underlying left ischium. Thus, the ischia were in close contact distally, but obviously not fused. A posteriorly rounded, triangular ‘intermediate process’ is present at about half length of the ischial shaft (Fig. 28), as in all other specimens of Archaeopteryx where the ischium is preserved (Elzanowski, 2002; Foth, Tischlinger & Rauhut, 2014), as well as in Sinovenator (Xu et al., 2002), Jianianhualong (Xu et al., 2017), Microraptor (Hwang et al., 2002), and Xiaotingia (Xu et al., 2011). It projects posteriorly and is proximodistally more elongated than anteroposteriorly expanded.

As in all specimens of Archaeopteryx, the distal end of the ischium is bifurcated, with the posterior ramus representing the ischial shaft, whereas the anterior flange corresponds to the obturator process (Fig. 28). Thus, the bifurcated appearance of these structures stems from a well-developed distal incision between the ischial shaft and the obturator process, similar to the morphology in many basal, non-coelurosaurian theropods (Rauhut, 2003). Within coelurosaurs, in which the obturator process usually disappears gradually into the ischial shaft distally, such an incision is only present in Anchiornis (IVPP V 14378; Xu et al., 2009a, 2011), Serikornis (Lefèvre et al., 2017) and, to a lesser degree, in Buitreraptor (Makovicky, Apesteguia & Agnolin, 2005; Agnolin & Novas, 2013). The distal end of the ischial shaft is slightly flexed posteriorly and tapers to a rounded tip. The obturator process is damaged, but it was obviously considerably larger than the distal tip of the ischial shaft; photographs taken prior to preparation of the specimen show several bone fragments anterior to the preserved part, indicating that the process was considerably expanded anteriorly, as in the Eichstätt, Munich and Thermopolis specimens (Wellnhofer, 1974, 1993, 2008, 2009; Mayr et al., 2007). de Beer (1954: figs. 6), Mayr et al. (2007: fig. 11C) and Agnolin & Novas (2013: fig. 3.5f) illustrated the London specimen with a much smaller, ventrally rather than anteriorly directed process. However, the ischium of the London specimen is not entirely exposed in lateral view, but somewhat inclined into the slab anteriorly, and the anterior end of the obturator process is covered by the pubis (BSPG cast of NHMUK 37001; see also Wellnhofer, 1985: fig. 2), so nothing can be said about its shape. Likewise, in the Berlin specimen, for which de Beer (1954: fig. 7) also illustrated a small, ventrally directed process, the ischium is poorly preserved, but UV photographs show that the obturator process is similarly anteriorly expanded as in other specimens (Tischlinger & Unwin, 2004; Wellnhofer, 2008: fig. 5.55).

The femora are poorly preserved and largely collapsed (Fig. 27). The proximal ends of both elements are missing. The shaft is slender, slightly curved posterodistally and seems to become more robust distally, being approximately 120–130% of the width of the proximal shaft just proximal to the distal expansion. A slightly raised ridge on the posteromedial edge of the proximal shaft of the left femur probably represents the fourth trochanter, which was thus strongly reduced, as in most coelurosaurian theropods. The collapsed shafts show that the femora were hollow and thin-walled.

Both tibiae are preserved, the right element being exposed in lateral and the left element in medial and, partially anteromedial views (Figs. 27 and 29). Both elements are preserved in articulation with the fibulae and the proximal tarsals. The tibiae are straight and slender elements, being longer than the femur. The proximal end is expanded, more so posteriorly than anteriorly, to a maximal anteroposterior width of 8.3 mm (measured on the left tibia). The posterior expansion forms a rounded posterior tubercle of c. 1.5 mm anteroposterior length that abruptly arises from the shaft directly distal to the proximal end, as also figured for the Eichstätt specimen by Wellnhofer (1974: fig. 11). The proximal end of the right tibia shows that this expansion is entirely formed by the medial part of the proximal tibia, whereas the posterior margin of the fibular condyle seems to be more or less flush with the posterior margin of the proximal tibial shaft and thus considerably offset anteriorly from the posterior end of the medial side. Two probably nutrient foramina (see Seymour et al., 2012) enter the tibia on the posterolateral side of the proximal end: a smaller, slightly more distally placed foramen at the level of the distal end of the posterior expansion that opens laterally from the base of this expansion, and a larger, more proximally placed foramen in the posterior end of the base of the fibular condyle, which opens laterodistally.

The cnemial crest is small, expanding c. 1.3 mm anteriorly from the fibular condyle, and only offset from the shaft by a small concavity in the anterior margin of the tibia distally (Fig. 30), as also figured by Wellnhofer (1974) for the Eichstätt specimen. In contrast to many basal theropods, its proximal end does not extend proximally beyond the proximal surface of the main body of the tibia. A weakly developed longitudinal lateral ridge is present close to the anterior end of the cnemial crest, as in most theropods, and helps to define a well-developed incisura tibialis.

A well-developed lateral longitudinal crest of the contact with the fibula is present on the anterior part of the lateral side of the shaft (Figs. 29 and 30), as in all theropods (Gauthier, 1986). The crest is straight and slender and 7.7 mm long. It becomes lower proximally, but is clearly offset from the proximal end, its proximal end being placed some 7 mm below the articular surface.

Distal to the cnemial crest, the shaft of the tibia gradually narrows distally, but it exact shape and outline cannot be established due to compression. The distal end is clearly considerably broader transversely than deep anteroposteriorly. However, only the lateral malleolus is slightly expanded from the shaft, whereas the medial malleolus is more or less continuous with the latter. As in most theropods (Currie & Zhao, 1993a; Brochu, 2003; Burnham, 2004), the medial side of the distal end is somewhat deeper anteroposteriorly than the lateral side. Whereas the medial side of the distal tibia is flattened, the lateral side is anteroposteriorly gently rounded. A short, but well-developed longitudinal ridge is present at the medial edge of the anterior side of the distal end of the tibia and braces the ascending process of the astragalus medially. The ridge expands medially distally and its medial edge curves onto the medial side of the bone. Such a ridge is also present in the Thermopolis (Mayr et al., 2007) and the 11th specimens (Foth, Tischlinger & Rauhut, 2014).

The fibula is extremely slender and closely attached to the tibia. Both fibulae are preserved, but only the distal half is visible of the left element. The proximal end is expanded to a maximal anteroposterior width of 5.6 mm, or approximately 67% of the proximal width of the tibia. As in most theropods, the bone is here more strongly posteriorly than anteriorly expanded. Parts of the central part of the proximal expansion are broken away, revealing that the bone was very thin in this area, considerably more so than at the base of the shaft or the proximal end, indicating that a large and extensive medial groove was present in the fibula, as it is the case in many coelurosaurian theropods (Rauhut, 2003).

The shaft rapidly narrows in its proximal part towards the iliofibularis tubercle. The latter is developed as a low ridge on the anterolateral side of the fibula that becomes wider and less conspicuous distally. Its position roughly coincides with the area of the fibula that contacts the lateral fibular crest of the tibia. In contrast to many maniraptorian theropods (Rauhut, 2003: fig. 48), the fibular shaft does not narrow abruptly distal to the tubercle, but gradually decreases in width from this area to approximately the mid-shaft. Thus, whereas the shaft is 1 mm wide at the level of the beginning of the iliofibularis tubercle, its minimal anteroposterior width is 0.6 mm. The shaft is flattened, so that its transverse width is even less. Distally, the fibula is closely appressed to the anterolateral side of the tibia, and its distal end is slightly and gradually expanded from the shaft to a maximal width of 0.9 mm. The distal end is rounded and sits in a concave facet of the calcaneum.

Astragalus and calcaneum are preserved in close contact with the tibia and fibula, but neither seems to be fused with the elements of the lower leg or with each other. The left astragalus is exposed in anteromedial and partially distal view (Fig. 31), whereas the right tarsus is visible in lateral and slightly posterior view (Fig. 32). As in the Thermopolis specimen (Mayr et al., 2007), the bodies of the astragalus and calcaneum are rather low and the medial condyle of the astragalus is bulbous and more pronounced than the lateral condyle (Fig. 31). Both condyles are separated by a deep concavity anteriorly and distally. A transverse groove across the condyles is absent; such a groove is present in many non-maniraptoran theropods (Rauhut, 2003), and probably represents a remnant of the suture between separate ossifications of the astragalar body and the ascending process (Rauhut & Pol, 2017). Although the condyles of the astragalus face anterodistally, as it is usual in tetanuran theropods (Sereno et al., 1994; Rauhut, 2003), the medial condyle is placed entirely distal to the tibia, as in basal theropods (Rauhut & Carrano, 2016). Laterally, the tibia extends slightly further distally so that the lateral condyle is at least partially placed anterodistal to the tibia. On the medial side of the astragalar body, a well-developed depression is present. This depression becomes deeper posteriorly and is bound posterodistally by a raised ridge.

The ascending process of the astragalus is largely present, but poorly preserved (Fig. 31). As in most coelurosaurian theropods (Rauhut, 2003), its base extends approximately over the entire width of the astragalar body, and it is considerably higher (6.9 mm) than the latter (2.4 mm) and thus slightly more than 10% of the length of the tibia. Anteriorly, it is separated from the astragalar condyles by a well-developed groove. Proximally, a portion of the ascending process is broken away, revealing a well-developed facet on the anterior side of the distal tibia adjacent to the medial ridge.

Mayr et al. (2007) noted that, in the Thermopolis specimen, the ascending process of the astragalus was slightly separated from the medial ridge on the anterior side of the distal tibia by a groove and the distal end of the tibia extended slightly medially to the body of the astragalus. They interpreted this as an original feature and compared it to the situation in ornithomimosaurs and Jeholornis (Mayr et al., 2007: 110). In the new specimen, however, the medial margin of the ascending process is closely appressed to the medial ridge and the medial side of the distal tibia is more or less flush with the medial side of the astragalar body, as in most theropods, indicating that the situation in the Thermopolis specimen might be due to a slight displacement of the tarsus.

Only the right calcaneum is visible in posterolateral view (Fig. 32), and most of its lateral surface has suffered from erosion. Mayr et al. (2007) noted that the calcaneum in the Thermopolis specimen was approximately as wide as the distal end of the fibula in anterior view. This also seems to be the case in the 12th specimen, but the calcaneum narrows posteriorly and is thus wedge-shaped in distal view. Posteriorly, the calcaneum does not reach the posterior edge of the tibia, but the astragalus expands below the latter to take part in the posterolateral edge of the tarsus, as in most tetanuran theropods. The facet for the tibia on the calcaneum is steeply inclined posteroproximally and is slightly concave proximodistally, so that much of the calcaneum is placed anterior to the tibia. On the anterior part of the proximal side of the calcaneum a smaller, anteroposteriorly concave facet for the fibula is present. It is placed more proximal than the distal end of the facet for the tibia and separated from the latter by a pointed proximal process of the calcaneum. Distally, the calcaneum is rounded.

The left tarsometatarsus is exposed in ventromedial view (Fig. 31), whereas the right is visible in lateral and slightly dorsal view (Fig. 32). The presence, identity and probable fusion of the distal tarsals with the metatarsals in Archaeopteryx have remained enigmatic. No separate distal tarsals were identified in the London specimen by de Beer (1954), who left the question open if this might be due to fusion of these elements with the metatarsals. de Beer (1954: 29) noted that the latter state was reported for the Berlin specimen, but, unfortunately, did not cite by whom; Dames (1884: 37) only discussed the possible fusion of the metatarsals with each other. Likewise, Heller (1959) noted the possible fusion of the proximal ends of the metatarsals of the Maxberg specimens, but did not comment on the distal tarsals. Wellnhofer (2008: fig. 5.72) figured the Maxberg specimen as having two unfused distal tarsals above metatarsal III and IV, based on the X-ray image of this specimen, but does not comment on this in the text. For the Eichstätt specimen, Wellnhofer (1974: 199) described a thin, plate-like bone proximal to metatarsal III, which he considered to be largely fused with the metatarsal, whereas a possible, small and rounded distal tarsal IV seems to have been separate from the metatarsal. However, in the right foot of the Eichstätt specimen, a clear suture between the distal tarsal and metatarsal III is visible (O. W. M. Rauhut and C. Foth, 2014, personal observation). For the Solnhofen specimen, Wellnhofer (1988b, 1992) identified two distal tarsals, but placed above metatarsals II and III and not fused to these elements. Tischlinger & Unwin (2004) identified an unfused lateral distal tarsal in the Berlin specimen. No distal tarsals are mentioned in the description of the Thermopolis specimen (Mayr et al., 2007). Finally, in his reconstruction of the skeleton of Archaeopteryx, Wellnhofer (2008: fig. 6.13) figures three separate distal tarsals, proximally overlying metatarsals II–IV.

The presence of three distal tarsals in Archaeopteryx would be extremely unusual, since only two such elements are generally present in non-avialan theropod dinosaurs (Ostrom, 1969; Osmólska, Roniewicz & Barsbold, 1972; Madsen, 1976; Norell & Makovicky, 1997). In these animals, the two distal tarsals are usually interpreted as distal tarsals III and IV, with the former mainly overlapping metatarsal III (and often partially or even largely metatarsal II; Ostrom, 1969) and the latter overlapping metatarsal IV. In avialans more derived than Archaeopteryx, the distal tarsals seem to be already completely fused with the metatarsals into a tarsometatarsus (Chiappe et al., 1999; Zhou & Zhang, 2002).

In the new specimen, there is a single large element capping the proximal ends of metatarsals III and IV, whereas the proximal end of metatarsal II is expanded slightly more proximally than the other metatarsals to be approximately level with the proximal surface of this distal tarsal (Fig. 31), as it is the case in Mei, for example (IVPP V 12733). A small indentation is present posteriorly in this single element above the contact between metatarsals III and IV. This is similar to the situation in a specimen of Velociraptor described by Norell & Makovicky (1997), which also has a single distal tarsal with an indentation above the contacts of the two metatarsals. Norell & Makovicky (1997) interpreted this tarsal in Velociraptor as the fused distal tarsals III and IV, given that the relationship of this element with the metatarsals is the same as that of these two tarsals in other theropod dinosaurs, and the indentation above the contact between the metatarsals indicates a line of fusion. We agree with this interpretation and suggest that this is the same in at least this specimen of Archaeopteryx.

The distal tarsal is thus a blocky element, with an almost straight posterior margin, with a small indentation above the contact between metatarsals III and IV (Fig. 31). The proximal surface is slightly convex both anteroposteriorly and transversely over metatarsal IV, but slightly depressed above metatarsal III. In the right pes, the lateral edge of the distal tarsal is broken, revealing two large cavities in the interior of the element. As for the question of fusion between the distal tarsal and the metatarsals, the element in the new specimen is closely appressed to the proximal ends of metatarsals III and IV, but a suture line is clearly visible both under normal and UV light, indicating that fusion, if present, is at best partial. Distal tarsals are fused to the metatarsals in various Pennaraptora (Chiappe et al., 1999; Norell & Makovicky, 1999; Hwang et al., 2002; Zhou & Zhang, 2003b; Yuan, 2008; Balanoff & Norell, 2012; Brusatte et al., 2013; Lefèvre et al., 2014), while they seem to be unfused in Sinornithosaurus (Xu & Wang, 2000), Deinonychus (Ostrom, 1969) and Anchiornis (Pei et al., 2017).

As in other specimens of Archaeopteryx, the metatarsus is long and slender, with metatarsals II to IV being closely appressed to each other over their entire length (Figs. 31 and 32). However, in contrast to the situation reported in the Maxberg and Solnhofen specimens (Heller, 1959; Wellnhofer, 1988b, 1992), the metatarsals are not fused proximally, but separated over their entire length. The left metatarsus is exposed in mainly posterior and slightly medial view (Fig. 31), whereas the right metatarsus is exposed in lateral view (Fig. 32). As in all theropods, metatarsal I does not reach the tarsus proximally, and metatarsal V is reduced to a small splint of bone.

Metatarsal I is a short element, slightly more than one fourth of the length of metatarsal II, to which it is attached in the distal third of the latter (Fig. 31), as in other Pennaraptora. The proximal third of the bone tapers to a point; however, in contrast to many tetanuran theropods, in which metatarsal I tapers almost from the distal articular trochlea proximally with little or no shaft in between, there is a distinct shaft in between the articular end and the proximal point in Archaeopteryx, which accounts for slightly more than 1/3 of the length of the bone, as in Sinornithosaurus (Xu & Wang, 2000). Another unusual feature of metatarsal I is that the shaft is flexed posteromedially at about its mid-length, so that its medial margin is concave and its lateral margin convex longitudinally (Fig. 31). This curvature of the shaft of metatarsal I also seems to be present in the Solnhofen, Thermopolis (Mayr et al., 2007) and 11th specimens, but cannot be established for most specimens of Archaeopteryx, due to incomplete preservation or the way the feet are exposed.

The distal articular end of metatarsal I is proximodistally short and seems to have been slightly ginglymoidal. A well-developed, but not sharply defined extensor groove is present anteriorly, although this might be exaggerated by compression. The distal condyles are too poorly preserved to say anything about their detailed morphology.

A contentious issue concerning the foot of Archaeopteryx is the possible opposability of the first digit. Whereas many authors considered the first digit to be at least somewhat opposable towards the other digits of the pes (Dames, 1884; de Beer, 1954; Wellnhofer, 1974, 1988b, 1992, 1993, 2008; Tarsitano & Hecht, 1980), Mayr, Pohl & Peters (2005) and Mayr et al. (2007) recently argued that metatarsal I is attached medially, not posteromedially to metatarsal II and lacks the torsion that constitutes most to opposability in modern birds (Middleton, 2001), indicating that the condition in Archaeopteryx was similar to other non-avialan theropods in that the first digit is parallel to the other pedal digits. Specifically, Mayr, Pohl & Peters (2005) and Mayr et al. (2007) argued that, in the Thermopolis specimen, metatarsal I was attached medially to metatarsal II, with its distal end even being placed slightly dorsal (anterior) to the distal ends of the other metatarsals, and that the first phalanx of digit I is exposed in dorsomedial view, similar to the other digits, which are exposed in dorsal view. The new specimen, however, seems to support the traditional view that the first digit was at least somewhat opposable. In the left foot, which is exposed in posterior view, metatarsal I is attached to the posteromedial side of metatarsal II (Fig. 31). Non-avialan theropods and also Archaeopteryx lack a distinct fossa for the attachment of metatarsal I on metatarsal II (the position of which is another indicator for opposability according to Middleton, 2001), so the exact position of the attachment should, however, be seen with caution, as post-mortem compression might have altered it. However, this argument is valid for both the posteromedial orientation seen in several specimens, including the new one described here, and the medial position in the Thermopolis specimen: if the digit was not fully opposed, but in a posteromedial position, compaction of the foot in the anteroposterior plane would move the metatarsal into a more medial position. This might be supported by the left foot of the Thermopolis specimen, which is exposed in slightly anteromedial view (WDC-CSG-100; Mayr et al., 2007). Whereas the observation by Mayr et al. (2007) that the distal end of metatarsal I is placed slightly dorsal (anterior) in relation to the other metatarsals is true for the right foot, the metatarsal is angled posterodistally in the left foot, displacing its distal end below the level of the other metatarsals.

However that may be, in the new specimen, the area medial to the attachment of metatarsal I as preserved is gently rounded and does not show a facet for the attachment of a metatarsal, indicating that, if displaced, the displacement is probably small. Furthermore, the curve in the shaft of metatarsal I would displace the distal end posteriorly even if the metatarsal was placed more medially on metatarsal II, and the same can be seen in the feet of the Eichstätt, Solnhofen, and 11th specimens. Furthermore, in the current specimen, the distal end of metatarsal I is visible in anterior view, whereas the other metatarsals are exposed in posteromedial view. Even accounting for slight rotation from a more medial position and compaction, the distal condyles would thus, at best, be placed at approximately 90° in relation to the distal condyles of the other metatarsals, facing laterally. Both of these observations, the posterior displacement and lateral orientation of the distal condyle, are confirmed by the right foot: although metatarsals III and IV are exposed in lateral and slightly anterior view, the distal end of metatarsal I is exposed posterior to the shaft of metatarsal IV, and its distal articular end faces laterally, with the supposedly posterior side (if aligned with the other metatarsals) facing even slightly anteriorly (Fig. 32). Thus, although metatarsal I of this specimen does not necessarily confirm a fully opposable digit I, it indicates that the orientation of this digit differed considerably from that of the other digits, and was not parallel to them (see also description of the phalanges below). A fully opposed first digit is present in all avilans more derived than Archaeopteryx, such as Jeholornis (Zhou & Zhang, 2002) and Sapeornis (Zhou & Zhang, 2003b).

Metatarsals II to IV are closely appressed, with only their distal articular ends slightly diverging (Fig. 31). In posterior view, metatarsal II is the broadest of the metatarsals, followed by metatarsal IV (Fig. 31). This is different to the situation in troodontids (Kurzanov & Osmólska, 1991; Currie & Peng, 1993; Xu et al., 2002; Xu & Wang, 2004; Zanno et al., 2011) and dromaeosaurids (Ostrom, 1969; Norell & Makovicky, 1999; Hwang et al., 2002), where metatarsal IV is significantly more robust than metatarsal II, but similar to the condition in Confuciusornis (Chiappe et al., 1999). Metatarsal III is slightly more slender than metatarsals II and IV at the proximal end, and almost completely pinched between the other two elements distally.

As noted above, metatarsal II extends more proximally than metatarsals III and IV to reach the same proximal level as the distal tarsal capping the other two metatarsals (Fig. 31). Its proximal end is slightly convex anteroposteriorly and shows a well-developed, smooth articular surface, bordered by a gently raised rim, distal from which the bone surface is slightly pitted. The proximal part of the shaft is flattened laterally for approximately two-thirds of its length, whereas the medial side is rounded anteroposteriorly. Just below mid-length, proximal to the attachment of metatarsal I, the posterolateral edge is drawn out into a marked ridge (Fig. 31). Distal to this ridge, the lateral side becomes more rounded. The distal end flexes slightly medially and seems to be slightly wider than deep, probably in contrast to the proximal part and mid-shaft. A slender, but well-developed flexor ridge is present on the posterior side of the distal end medially, but no collateral ligament groove seems to be present on the medial side of the metatarsal, although the bone is slightly damaged here. The distal end is well rounded anteroposteriorly, but does not seem to be ginglymoidal, in contrast to the situation in dromaeosaurids (Ostrom, 1969; Norell & Makovicky, 1999), although this cannot be said with absolute certainty due to articulation of the digit with the metatarsal.

Metatarsal III has a flattened posterior side proximally, but becomes triangular in cross-section towards the mid-shaft, tapering posteriorly (Fig. 31). The anterior side becomes flattened distally, and there seems to have been a shallow extensor groove on its anterior side at the distal end (Fig. 32). The distal articular end seems to be more posteriorly than anteriorly expanded and is well-rounded anteroposteriorly, being not or only very slightly ginglymoidal. A well-developed collateral ligament groove is present at least laterally and is placed slightly anterior to the mid-depth of the articular end. The distal end seems to be wider than deep, but not much can be said about the condition at mid-shaft. Just proximal to the ligament groove, a depression is present on the lateral side of the metatarsal, marking the attachment with the distal end of metatarsal IV.

The proximal end of metatarsal IV is flattened medially, where it contacts metatarsal III, but a posterior medial process overlapping the posterior side of the proximal end of metatarsal III, as it is present in many basal tetanurans (Madsen, 1976; Currie & Zhao, 1993a) is missing (Fig. 31). In posterior view, the shaft narrows gradually over its proximal three-fourths, until it reaches its narrowest point just where the shaft starts to deflect slightly laterally from metatarsal III. There seems to be a slight longitudinal depression on the posterior side of the shaft, although it is certainly exaggerated by compaction over most of the length of the bone. Thus the posterior side is rather flattened, being offset from both the medial and the lateral sides by marked edges. The distal end extends slightly further distally than the end of metatarsal II, but not as far as metatarsal III. It is slightly expanded both anteriorly and posteriorly and is transversely narrow anteriorly, whereas the posterior part is drawn out laterally (Fig. 32). A posterior tubercle is only present medially, and a collateral ligament groove is absent laterally. A shallow, not clearly defined extensor groove is present on the anterior side. Both the shaft and the distal end are deeper anteroposteriorly than broad transversely.

Metatarsal V is a slender rod of bone that is attached posterolaterally to metatarsal IV and is slightly less than one-fourth of the length of the latter (Figs. 31 and 32), similar to the condition in other basal avialans (Chiappe et al., 1999; Zhou & Zhang, 2002, 2003b). Its proximal end seems to be slightly deeper anteroposteriorly than wide transversely, but without being markedly flattened, as it is the case in many basal tetanurans (Rauhut, 2003). Distally, the shaft is slightly flexed medially and tapers (Fig. 31), lacking an articular end or phalanges, as in all neotheropods. The shaft is broken in both sides, revealing a thin-walled, hollow condition for the bone.

The digits of the feet are complete on both sides, but partially overlapping each other as exposed. The phalangeal formula is 2-3-4-5-0, as in the Berlin, Eichstätt, Munich, Thermopolis and 11th specimens (and most other theropods), but in contrast to the Solnhofen specimen, which only has four phalanges on the fourth digit in the only completely preserved right pes (Wellnhofer, 1988b, 1992; Elzanowski, 2001b). The situation in the Maxberg (Heller, 1959) and Haarlem specimens is uncertain, although Ostrom (1972) and Wellnhofer (2008, 2009) assumed five phalanges to be present in the latter. However, as most phalanges are preserved as impressions in this specimen, nothing can be said with certainty. For the London specimen, de Beer (1954) assumed four phalanges to be present, but as the fourth digit in the only preserved foot is mainly covered by the second digit, nothing can be said with certainty.

Digit III is the longest digit, followed by digit IV (88.5% of digit III) and digit II (74% of digit III). Digit I is the shortest, being slightly half the length of digit II.

Both digits of the first toe are preserved and visible on either side. Phalanx I-1 is slightly longer than the ungual of this digit. Its proximal end is expanded transversely, and the proximal articular facet seems to be twisted in relation to the distal articular end (Fig. 33), similar to the condition found in manual phalanx I-1 in theropods (Galton, 1971). This twist in the articular ends slightly displaces the distal end medially in relation to the proximal end and thus helps to further bring the ungual of digit I in opposition to the other digits. The distal end of phalanx I-1 is transversely narrow and ginglymoidal, with well-developed, dorsally displaced collateral ligament grooves. The ungual of the first digit has a large flexor tubercle proximally that accounts for approximately one-third of the height of the proximal end. The bony claw is moderately curved, so that the distal tip is placed ventral to the ventral border of the proximal end, being displaced for approximately half the height of the proximal end in relation to the latter (if the claw is held with the proximal articulation being vertical). A single, well-developed claw groove is present, and the part of the ungual ventral to the groove is wider than the portion dorsal to it. The horny sheath of the claw is preserved on both sides (Fig. 33). It extends the claw for approximately half the length of the bony claw and follows the curve of the latter, so that the distal end of the horny claw is placed slightly more than the proximal height of the bony claw ventral to the ventral margin of the flexor tubercle.

In the second digit, the two non-ungual phalanges are subequal in length, which is different from the situation in various dromaeosaurids (Ostrom, 1969; Forster et al., 1998; Xu & Wang, 2000, 2004) and troodontids (Currie & Peng, 1993; Zanno et al., 2011), in which the first phalanx is notably longer than the second. The first phalanx is elongate, but stout, and has a well-developed longitudinal ridge proximally on the medial margin of its ventral side; the lateral side of the phalanx is not visible in either foot. Distally, the phalanx has a strongly ginglymoidal articular end, which is extended proximally on the dorsal surface approximately as far as on the ventral surface, indicating a hyperextensible first toe, as in the Thermopolis (Mayr, Pohl & Peters, 2005; Mayr et al., 2007) and Eichstätt specimens (Rauhut & Erickson, 2010: fig. 5). The collateral ligament groove is centrally placed on the distal end of the medial side of the phalanx, and a broad, rounded, notable medial tubercle is present proximoventral to it. A well-developed extensor groove seems to be present on the dorsal side of the phalanx. The second phalanx is slightly more slender and has a flattened ventral surface, in contrast to many dromaeosaurids (Ostrom, 1969; Rauhut & Werner, 1995; Norell & Makovicky, 1997; Forster et al., 1998; Novas et al., 2009; Turner, Makovicky & Norell, 2012) and troodontids (Currie & Peng, 1993; Zanno et al., 2011), where PII-2 possesses a pronounced ventral heel proximally. The distal end is ginglymoidal, with widely and deeply separated, slender condyles, which extend considerably further proximally on the ventral than on the dorsal side. The collateral ligament groove is well-developed, seems to be deeper on the lateral than on the medial side and is displaced dorsally (Fig. 34). The ungual of the second digit is the largest ungual of the foot. Its morphology closely corresponds to that of the ungual of the first digit, but it is slightly more notably curved. In contrast to the situation in dromaeosaurids, which have an asymmetric arrangement of the claw grooves and a sharp ventral margin in the second pedal ungual (Kirkland, Burge & Gaston, 1993; Rauhut & Werner, 1995; Norell & Makovicky, 1997), the claw grooves are symmetrically or near-symmetrically arranged (based on a comparison of the left and right unguals, which are visible in medial and lateral view, respectively) and the ventral margin of the claw is broader than the dorsal margin and flattened.

The three non-ungual phalanges of the third digit become successively smaller, and phalanx III-1 is the largest of the foot. The phalanges are elongate, with strongly ginglymoidal distal ends. Whereas the collateral ligament grooves are centrally placed at the distal end of the lateral and medial side and are deeper on the lateral than on the medial side in the first two phalanges, the third phalanx has the grooves displaced dorsally (Fig. 34) and subequally developed on either side. Well-developed dorsal extensor grooves are present in phalanges III-1 and III-2, but apparently not in III-3. In at least the second and third phalanx of the digit, the ventral side is notably flattened proximally and offset from the lateral and medial sides by marked ventrolateral and ventromedial edges. The ungual of digit III is notably smaller than that of digit II and only slightly larger than ungual IV. It is also less strongly recurved and has a less bulbous and less notably expanded flexor tubercle than the ungual of the second digit (Fig. 34).

The four non-ungual phalanges of the fourth digit are less elongate than those of the second and third digit, as it is usual in theropods. The length successively decreases from phalanx IV-1 to IV-3, but then slightly increases again in phalanx IV-4. All phalanges are notably ginglymoidal and have well-developed collateral ligament grooves. The latter seem to be centrally placed in IV-1, but displaced dorsally in the other phalanges, most notably so in the penultimate phalanx (Fig. 34). Well-developed extensor grooves are present at least in phalanges IV-2 and IV-3 (Fig. 34), whereas only a shallow depression seems to be present in IV-1; nothing can be said in this respect about phalanx IV-4. The ungual of digit IV is the smallest pedal ungual, subequal in size to ungual I, and has the least conspicuous flexor tubercle.

Cranial osteology

Postorbital. Although Archaeopteryx is a rather well-known taxon, and numerous anatomical descriptions have been published, the detailed analysis of the new specimen resulted in several insights into its osteology. Most importantly, for the first time, the postorbital is well preserved in a specimen of Archaeopteryx (Figs. 6, 7 and 9–11). Although the presence of this bone and of a closed postorbital bar had been assumed for this taxon several times (Wellnhofer, 1974; Tischlinger, 2005; Rauhut, 2014) many recent reconstructions of the skull show Archaeopteryx without or with a reduced postorbital and an open postorbital bar (Ji & Ji, 2007; Wellnhofer, 2008, 2009; Xu et al., 2011; Wang & Hu, 2017), a condition that is important or even essential for the inference of an avian-style cranial kinesis in this taxon (see Bühler, 1985; Wellnhofer, 2008, 2009). The new specimen confirms the presence of a triradiate postorbital, with a long ventral process that reaches the postorbital process of the jugal, as reconstructed by Wellnhofer (1974) and Rauhut (2014). As discussed by Rauhut (2014), this indicates that an avian style cranial kinesis was absent in Archaeopteryx.

The postorbital of Archaeopteryx conforms well with the same bone in other paravians in that the anterior process is upturned, resulting in a Y-shaped postorbital (Figs. 6, 7 and 9), rather than the T-shaped element seen in basal tetanurans, including basal coelurosaurs, such as Zuolong (Choiniere et al., 2010) and tyrannosauroids (Currie, 2003; Xu et al., 2004). However, in contrast to most dromaeosaurids (see e.g. Ostrom, 1969; Barsbold & Osmólska, 1999; Norell et al., 2006; Xu et al., 2010; Turner, Makovicky & Norell, 2012), troodontids (Currie, 1985; Norell et al., 2009), Anchiornis (Hu et al., 2009; Pei et al., 2017), and Aurornis (Godefroit et al., 2013b), in which the postorbital is a rather plate-like element with a notably short ventral process, this bone is slender in Archaeopteryx, with the ventral process being the longest of the three processes. This resembles the condition in more basal taxa, such as oviraptorosaurs (Clark, Norell & Rowe, 2002; Balanoff & Norell, 2012), but also seems to be the case in at least some slightly more derived avialans, such as Pengornis (Zhou, Clarke & Zhang, 2008; O’Connor & Chiappe, 2011), and in the troodontid Gobivenator (Tsuihiji et al., 2014).

Prefrontal. A separate prefrontal ossification has so far usually been said to be absent in Archaeopteryx (Elzanowski, 2001a, 2002; Mayr et al., 2007; Wellnhofer, 2008, 2009). This bone has previously only been postulated to be present in the Eichstätt specimen (Wellnhofer, 1974), although the element identified as such was later correctly reinterpreted as part of the lacrimal (Elzanowski & Wellnhofer, 1996). However, the prefrontal is actually preserved in both the Eichstätt and the Thermopolis specimens (Fig. 37). In the former, it is an elongate, splint-like bone that has been slightly displaced from the medial side of the lacrimal and the orbital margin to slightly obliquely overlie the frontal, where it is visible in dorsal view (Fig. 37A). In the Thermopolis specimen, there is a similar, slender, posteriorly triangular slip of bone present at the anterior margin of the dorsal rim of the left orbit (Fig. 37B), which, according to its position and shape, corresponds closely with the elements in the Eichstätt and the new specimen. In general, the prefrontal seems to be similar in shape to the same element in the therizinosauroid Erlikosaurus (Lautenschlager et al., 2014), but more slender, resembling the condition in Sinornithosaurus (Xu & Wu, 2001).

The presence of a separate prefrontal ossification is unexpected in Archaeopteryx, as it is absent in the vast majority of pennaraptorans. However, a similar slip of bone above the orbital rim as seen in Archaeopteryx is present in Jinfengopteryx (Ji et al., 2005), and might represent a prefrontal as well, but the fossil is too poorly preserved to confirm this observation with certainty. Likewise, one of the few illustrated specimens of Anchiornis, in which this region is sufficiently preserved (Pei et al., 2017: fig. 5) shows an elongate triangular element that seems to be separate from the lacrimal above the orbit, and a similar element might be present in an undescribed specimen referred to Anchiornis (XHPM 1084; C. Foth, 2014, personal observation). Separate prefrontals have also been described for the dromaeosaurids Deinonychus (Maxwell & Witmer, 1996) and Sinornithosaurus (Xu & Wu, 2001). In other pennaraptorans, including oviraptorosaurs (Clark, Norell & Rowe, 2002; Balanoff & Norell, 2012), most dromaeosaurids (Barsbold & Osmólska, 1999; Norell et al., 2006; Novas et al., 2009; Pei et al., 2014), troodontids (Currie, 1985; Xu & Norell, 2004; Norell et al., 2009; Tsuihiji et al., 2014), and avialans (Zhou, Clarke & Zhang, 2008; Zhang et al., 2013), the lacrimal has a pronounced posterior process, resulting in a T-shaped outline of this bone. This process usually tapers posteriorly, thus mimicking the morphology of the prefrontal in Archaeopteryx and the few other paravians in which this bone is separate. This similarity and the fact that a separate prefrontal is sporadically present in paravians lends support to the hypothesis that the posterior process of the lacrimal in other taxa is indeed derived from a fusion of the lacrimal with the prefrontal (Lautenschlager et al., 2014). Currie (1985) interpreted a slender slip of bone along the lateral margin of the frontal as the prefrontal in Troodon, noting that this bone is fused with the frontal and the difference is mainly notable as a change in surface texture (Currie, 1985: 1645). However, given the absence of an expanded posterior process in paravians, the shape of the prefrontal in those taxa where it is present, and the fact that this bone does not exclude the frontal from the orbital margin as Currie (1985) argued to be the case in Troodon, we consider it to be more likely that the prefrontal fuses to the lacrimal. The thin slip of bone parallel to the orbital margin with different surface texture in the specimen described by Currie (1985) might represent a slightly offset margin of the frontal, as it is also present in Archaeopteryx, and several other paravians (e.g. Anchiornis; Hu et al., 2009).

Why the prefrontal and lacrimal fail to coossify in a few taxa of paravians, including Archaeopteryx, remains unknown. In those taxa where an enlarged posterior process of the lacrimal is present, no suture is visible; even in a perinate specimen of the troodontid Byronosaurus described by Bever & Norell (2009), the two bones are obviously already completely fused, without any visible suture. In oviraptorids that have a pronounced posterior process as adults, a separate prefrontal ossification might be present in early juvenile individuals: Although Wang et al. (2016) describe the lacrimal as having a large posterodorsal process in the embryonic oviraptorid skull of IVPP V20182, close inspection of their figure 5a actually indicates a thin line separating an elongate triangular supraorbital ossification from the posterodorsal boss of the lacrimal (also indicated in their drawing of the skull; Wang et al., 2016: fig. 5b), so that both the shape of the prefrontal (if it is this bone) as well as its connection with the lacrimal would coincide with the situation seen in therizinosaurids (Lautenschlager et al., 2014).

Presence of coronoid. As with the prefrontal, a separate coronoid ossification has been said to be absent in Archaeopteryx (Elzanowski & Wellnhofer, 1996), but the new specimen shows a slip-like coronoid on the internal side of the mandible (Figs. 7 and 12). This bone is absent in modern Aves, but present in most basal paravians, such as dromaeosaurids (Currie, 1995; Norell et al., 2006), troodontids (Tsuihiji et al., 2014), and also hesperornithiforms (Clarke, 2004). In non-paravian theropods, the coronoid seems to be absent in a few clades, such as ornithomimosaurs (Osmólska, Roniewicz & Barsbold, 1972), therizinosaurs (Clark, Perle & Norell, 1994; Lautenschlager et al., 2014), and at least some oviraptorosaurs (Balanoff & Norell, 2012). If the absence of the coronoid in the Munich specimen of Archaeopteryx is not an artifact of preservation (which does not seem to be the case, as no trace of the element can be seen in any of the two mandibular rami), this bone might have been lost or fused to another mandibular element independently of other theropod taxa during the evolution of this genus.

Postcranial skeleton

Shape of cervical vertebrae. The new specimen for the first time provides evidence that at least some of the mid- to posterior cervical vertebrae of Archaeopteryx were opisthocoelous (Fig. 17). This contrasts with the usually platycoelous or amphi-platycoelous vertebrae of other basal paravians (Ostrom, 1969; Novas et al., 2009; Tsuihiji et al., 2014), whereas Patagopteryx and more derived avialans have heterocoelous cervicals (Clarke, 2004). More basal avialans, such as Confuciusornis or enantiornithines, show an incipient heterocoely (Chiappe et al., 1999). Opisthocoelous cervical vertebrae are usually correlated with large size in basal tetanurans (Madsen, 1976; Britt, 1991; Currie & Zhao, 1993a), but are also found in a few small-bodied coelurosaurs, such as Compsognathus (Ostrom, 1978; Peyer, 2006) and alvarezsaurids (Chiappe, Norell & Clark, 2002; Xu et al., 2013). Thus, the morphology of the cervical vertebral centra seems to be quite variable in tetanuran theropods. However, whereas convex-concave articulation facets provide stability in large-bodied taxa (Fronimos, Wilson & Baumiller, 2016), their functional significance in small taxa, such as Archaeopteryx or derived alvarezsaurids, remains enigmatic. Given the general functional significance of these articulations in other taxa (Salisbury & Frey, 2000; Fronimos, Wilson & Baumiller, 2016), stabilisation of the cervical vertebral column during rapid head movements in pursuit of elusive prey may be one possible explanation, but further research of this topic is necessary. We should also note that the cervical vertebrae are rather poorly preserved, and the cervical column is incomplete in the new specimen, so further studies of the cervical vertebral column of Archaeopteryx, preferably using CT or Synchotron data would be necessary to confirm our findings and elucidate the extent of opisthocoely in the vertebral column of this taxon. If confirmed, the transformation to opisthocoelous cervicals might represent a first step towards heterocoely in more derived avialans, as this condition includes an aspect of opisthocoely (dorsoventrally convex anterior and concave posterior articular surfaces).

Absence/presence of a sternum. As in other Archaeopteryx specimens, no traces of an ossified sternum are preserved in the new specimen, which is similar to the condition in Anchiornis, Mei, Sapeornis (Zheng et al., 2014), Jianianhualong (Xu et al., 2017), and most non-pennaraptoran theropods (Weishampel, Dodson & Osmólska, 2004, see below). This has recently led to the conclusion that sternal elements were completely absent in these species, and that the m. pectoralis, responsible for the ventral movement of the forelimbs, was alternatively attached between the proximal end of the humerus and the anterior portion of the gastral basket (Zheng et al., 2014; O’Connor et al., 2015a). In contrast, most other Pennaraptora had large, ossified sternal plates (Norell & Makovicky, 1997; Clark, Norell & Chiappe, 1999; Hwang et al., 2002; Burnham, 2004; Godfrey & Currie, 2004; Xu et al., 2010; Pei et al., 2014), which were fused into a keeled sternum in Ornithothoraces (Chiappe et al., 1999; Zhou, 2002; Clarke, Zhou & Zhang, 2006; O’Connor et al., 2011; Hu et al., 2015). Based on the distribution of this character within Pennaraptora, Zheng et al. (2014) speculated further that the sternal elements of Oviraptorosauria, Dromaeosauridae and Pygostylia were not homologous with each other.

Apart from biomechanical difficulties related to forelimb movements and costal breathing (Lambertz & Perry, 2015), several specimens of Archaeopteryx (i.e. Munich, Thermopolis, 11th and 12th specimen) show distal cartilaginous expansions usually in four to five pairs of their anterior dorsal ribs (Fig. 21; Wellnhofer, 2008, 2009; Foth, 2014). When the Munich and the 11th specimen are compared, these expansions seems to be present between the third (11th specimen) and eighth dorsal rib pair (Munich specimen). Similar expansion can be also found in basal bird Sapeornis (Zhou & Zhang, 2003b; Provini, Zhou & Zhang, 2009). According to Wellnhofer (2008, 2009) these expansions were most likely articular facets for sternal ribs, which would indicate the presence of a sternum. In addition, in various specimens of Archaeopteryx the anterior end of the gastral basket reaches only until the seventh (see Eichstätt and Thermopolis specimen) to ninth (see 11th specimen) dorsal vertebrae, leaving a large gap between the posterior end of the coracoid and the gastral basket. A similar gap is also present e.g. in Anchiornis (BMNHC PH822; Pei et al., 2017), Mei (Xu & Norell, 2004), Jianianhualong (Xu et al., 2017), Sapeornis (Zhou & Zhang, 2003b; Provini, Zhou & Zhang, 2009) and in various non-pennaraptoran theropods (Currie & Chen, 2001; Ji et al., 2007; Chiappe & Göhlich, 2010; Dal Sasso & Maganuco, 2011; Rauhut et al., 2012). The anterior extension of the gastralia itself, however, is similar to Pennaraptora (except of Archaeopteryx, Anchiornis, Mei, Sapeornis), but in the latter the gap is absent due the development of ossified sternal plates (Xu et al., 2010; Pei et al., 2014; O’Connor et al., 2015a). As the gastralia do not reach the pectoral girdle anteriorly, this gap might be seen as an indication for the presence of cartilaginous sternal elements in Archaeopteryx and other non-pennaraptoran theropods.

Looking at early juvenile specimens of Enantiornithes from the Jehol Group, no plate-like sternum, but several small, isolated, ossified sternal loci are preserved, which were not connected to each other by bone tissue (Chiappe, Ji & Ji, 2007; Zheng et al., 2012; O’Connor et al., 2015b). Assuming a similar developmental mode as in recent birds, these ossified loci were connected with each other by cartilaginous tissue, which ossifies in later ontogenetic stages, forming the plate-like sternum (Parker, 1867). As no traces of cartilage can be found in the pectoral region of these juvenile enantiornithine specimens, a general bias in preservation between bone and cartilage seems to be present, which would explain the frequent absence of sternal elements in many non-avialan theropods, Archaeopteryx and Sapeornis (Foth, 2014). A triangular crystalline calcite structure found between the right humerus and the left coracoid, ventral to the glenoid fossa of the right scapula of the Berlin specimen of Archaeopteryx, however, may provide direct evidence for the presence of cartilaginous sternal elements, as in the Solnhofen limestones chondral elements are often replaced by crystalline calcite after their decay (Tischlinger & Unwin, 2004). Thus, following Wellnhofer’s interpretation, Archaeopteryx probably possessed cartilaginous sternal elements, which were articulated with four to six sternal ribs. For comparison, in oviraptorosaurs and dromaeosaurids the number of sternal ribs varies between three and four (Clark, Norell & Chiappe, 1999; Hwang et al., 2002; Godfrey & Currie, 2004), while Confuciusornithidae and Ornithothoraces possess at least four (Chiappe et al., 1999; Clarke, Zhou & Zhang, 2006; O’Connor et al., 2011; Hu et al., 2015). Although we cannot say anything about the morphology of the sternal elements in Archaeopteryx, the supposed number of sternal ribs indicates that the sternal elements were potentially larger than in non-avialan Pennaraptora, similar to the condition in other basal birds. Based on these arguments we conclude that cartilaginous sternal elements were also present in non-pennaraptoran theropods. This is further supported by single findings of bony remains in some theropods, such as in the ceratosaur Limusaurus (Xu et al., 2009b), the abelisaurid Carnotaurus (Bonaparte, Novas & Coria, 1990), the allosauroid Sinraptor (Currie & Zhao, 1993a), the tyrannosaurid Gorgosaurus (Lambe, 1917), and the alverezsaurid Mononykus (Perle et al., 1994), which are interpreted as sternal elements. In the latter two species the sternal elements are even fused and keeled, resembling the condition of Ornithothoraces. Given that the sternal elements of Carnotaurus, Gorgosaurus and Sinraptor are rather small and inconspicuous it is further possible that these elements have been previously overlooked during excavations. The absence of sternal elements in more than 200 specimens of Anchiornis studied by Zheng et al. (2014) might thus reflect the difficult preservation of cartilage in the sediments of the Tiaojishan Formation: Wang et al. (2017c) noted numerous soft tissues that showed up in specimens of Anchiornis under laser-stimulated fluorescence (LSF). Interestingly, however, the presence of a cartilagenous pad at the end of the pubis was indicated by an area that did not fluoresce in between the distal end of the pubis and the integument (Wang et al., 2017c: fig. 4b), indicating that cartilage might not be visible in these specimens under special lighting methods, such as LSF or UV. Other lighting techniques might be necessary to visualise cartilage in these specimens, or cartilage might simply note be preserved at all.

Position of gastralia in respect to pubis. In all specimens of Archaeopteryx, in which both the gastral basket and the pubis are adequately preserved, the posterior end of the gastral basket is placed at about half-length of the pubis (Figs. 38A–38C), not at its distal end, as might be expected. This unusual preservation can be seen in the Berlin, Eichstätt, Solnhofen, Munich, 11th, and the new specimen (Wellnhofer, 2008, 2009; Foth, Tischlinger & Rauhut, 2014); in the Thermopolis specimen, the gastralia do not reach the pubis, but their posterior end is placed approximately at the same relative level as in the other specimens (Mayr et al., 2007). As noted above, this elevated position of the gastralia in respect to the pubis is also seen in the troodontids Jinfengopteryx (Ji et al., 2005) and Jianianhualong (Xu et al., 2017), the avialans Anchiornis (Fig. 38D; Zheng et al., 2014; Pei et al., 2017), Eosinopteryx (Godefroit et al., 2013a), Sapeornis (Zhou & Zhang, 2003b), and Sinornis (Sereno, Rao & Li, 2002), and various dromaeosaurids (Hone et al., 2010; Xu et al., 2010; O’Connor, Zhou & Xu, 2011; Han et al., 2014; Lü & Brusatte, 2015); all of these taxa have opisthopubic, retroverted pubes. In contrast, the preservation is different in theropods with a pro/mesopubic anatomy, including megalosauroids (Fig. 38F; Rauhut et al., 2012), tyrannosauroids (Lambe, 1917; Xu et al., 2012), Scipionyx (Dal Sasso & Maganuco, 2011), compsognathids (Fig. 38E; e.g. Currie & Chen, 2001; Hwang et al., 2004; Peyer, 2006; Ji et al., 2007; Hu, Wang & Huang, 2016), ornithomimosaurs (Osborn, 1916; de Klerk et al., 2000; Varricchio et al., 2008), oviraptorosaurs (Zhou et al., 2000; Balanoff & Norell, 2012), and the troodontid Gobivenator (Tsuihiji et al., 2014), where the gastral basket reaches the distal pubic boot. This probably represents the natural association between these structures, as the gastralia are connected with the sternum and the pubis by midventral ligaments and are embedded in the M. rectus abdominis in recent reptiles (Carrier & Farmer, 2000; Claessens, 2004; Fechner & Schwarz-Wings, 2013). The arrangement found in Archaeopteryx and other paravians thus most likely represents a post-mortem displacement, which seems to be related to the opisthopubic anatomy, as evidenced from the lack of this displacement in the mesopubic Gobivenator (Tsuihiji et al., 2014). Although the change in pelvic anatomy probably led to a reconfiguration of pelvic and thoracal musculature (Carrier & Farmer, 2000), a post-mortem displacement of the gastral basket after death due to tension is unlikely, as muscle tones are triggered by nerve impulses, occurring only pre- and perimortem (Leisman & Koch, 2009; Reisdorf & Wuttke, 2012). However, it is possible that the retroversion of pubis in the taxa in question led to an increase or ventrally directed relocation of the abdominal air sac system, so that the reposition of the gastralia may result from a post-mortem collapse of the air sacs, causing a depression in the abdominal region. This might thus indicate that the opisthopubic morphology in paravians was related to a change in function and probably an increase of these abdominal air sacs.

Skeletal proportions

Previous studies proposed that all differences found in the skeletal proportions of Archaeopteryx represent allometric variation within a growth series (Houck, Gauthier & Strauss, 1990; Senter & Robins, 2003). However, the sample size of these two studies was only five specimens. Since 2003 five more specimens were discovered or described. Using a single value t-test, we compared the 12th specimen against a sample of eight other Archaeopteryx specimens (London, Berlin, Maxberg, Eichstätt, Solnhofen, Munich, Thermopolis, and 11th specimen). Because the Eichstätt specimen is a juvenile individual (Wellnhofer, 1974; Erickson et al., 2009), we excluded this specimen in a second analysis, to minimise the effect of potential ontogenetic variation. All data are based on Mayr et al. (2007), Wellnhofer (2008, 2009) and own measurements. The data were divided through femur length to minimise the impact of size.

According to the t-test, the new specimen shows some significant differences compared to the other specimens, including the presence of a relatively long scapula, a relatively short humerus, ulna (only when the Eichstätt specimen is excluded from the sample), metacarpal I, tibia and metatarsus (Table 3). In addition, some pedal digits were found to be relatively shorter, too. Thus, the test indicates that the new specimen has relatively short legs and arms when compared to size. In addition, when the Eichstätt specimen is excluded the skull of the 12th specimen is significantly longer than in the other Archaeopteryx.

Due to lack of data, it cannot currently be evaluated if the differences found are within the range of intraspecific variation (when compared to other non-avialan theropods and recent birds) or if they might justify the establishment of a new species. The latter option might be supported by a few osteological differences, such as the different shape and position of the promaxillary foramen and the short and blunt jugal process of the palatine (if compared to the much longer and more slender process in the Thermopolis specimen; Mayr et al., 2007), but little is known about the variability of such characters in paravian theropods. Furthermore, given its slightly older age and different provenance it also cannot be ruled out that the differences seen in the 12th specimen represent an anagenetic or biogeographic variation of the genus Archaeopteryx (see discussion below). Moreover, many of the specimens of Archaeopteryx deviate from the average seen in this genus in one or more proportions. A relative long scapula, for example, can also be found in the Berlin and 11th specimens, while a shortened humerus is present in the Eichstätt (subadult) and Thermopolis specimens. Further research into variability in both non-avialan and avialan theropods is necessary to better evaluate the significance of this variation.

Variation in the dentition

As discussed above, Archaeopteryx shows a very characteristic tooth shape in the premaxillary, often the anterior maxillary and anterior dentary teeth, which can be used to diagnose the genus. However, when the entire dentition is compared, the individual specimens show a lot of variation, as previously recognised by Howgate (1984a). Variation is seen in the number of teeth, the spacing of teeth in different regions, the shape of teeth in different positions within the dentition, and the inclination of individual teeth.

While the number of premaxillary teeth is constant at four in all specimens where this number can be established (Berlin, Eichstätt, Solnhofen, Thermopolis, 11th, and, probably, 12th specimen), the number of maxillary teeth varies between eight (Berlin and Eichstätt specimens) and nine (Daiting, Thermopolis and 12th specimens), and the number of dentary teeth between 11 (Eichstätt), 12 (Munich) and 13 (12th specimen). However, as the total number of teeth in the maxilla and the dentary are only determinable in a few specimens, the significance of these differences in tooth count is unclear. Individual variation in tooth number of one or more positions in the maxilla and dentary has been noted in several non-avialan theropods (Madsen, 1976; Colbert, 1989; Currie, 2003), whereas the number of premaxillary teeth generally seems to be more stable. Furthermore, different tooth counts might also be due to ontogenetic variation (see Rauhut & Fechner, 2005; Kundrát et al., 2008; Bever & Norell, 2009).

The spacing of individual teeth shows a high amount of variation between the different specimens of Archaeopteryx. As noted above, both the premaxillary and dentary dentitions are offset from the anterior tip of their respective bones in the 12th specimen. A similarly large offset in the premaxilla is present in the Solnhofen (Wellnhofer, 1992) and, probably, the London specimens (Howgate, 1984b), whereas it is present, but notably smaller in the Berlin, Eichstätt, Thermopolis, and 11th specimens (Dames, 1884; Wellnhofer, 1974, 1988b, 1992, 2008, 2009; Howgate, 1984a; Mayr et al., 2007; Foth, Tischlinger & Rauhut, 2014). An offset in the dentition of the dentaries is also found in the Eichstätt and Munich specimens. This small space anterior to the dentition might indicate the presence of an incipient rhamphotheca, although other indications for this (such as increased vascularity) cannot be established. In the premaxilla, tooth spacing in the Berlin, 11th and, probably, the 12th specimen differs from other specimens. In the Eichstätt, Solnhofen and Thermopolis specimens, the first two premaxillary teeth are closely spaced (half a tooth width or less in between the teeth), whereas there is a gap of at least one tooth width between the second and third and the third and fourth premaxillary tooth (Fig. 39B; Wellnhofer, 1974, 1988b, 2008, 2009; Howgate, 1984a; Mayr et al., 2007). In contrast, the Berlin specimen has premaxillary teeth one to three closely spaced, followed by a gap between the third and fourth tooth (Fig. 39A; Howgate, 1984a; Wellnhofer, 2008, 2009), and in the 11th specimen, all premaxillary teeth are equally and widely spaced, with approximately one tooth width distance between individual elements (Fig. 40; Foth, Tischlinger & Rauhut, 2014). If our interpretation that there is a further premaxillary tooth missing between the second and third preserved tooth in the new specimen (see above), this specimen would again differ from other specimens in that all teeth are equally, but closely spaced, with considerably less than one tooth width in between individual teeth (Fig. 14).

Variation in the spacing of individual teeth is also present in the maxillary and dentary dentitions. One extreme condition is seen in the Eichstätt specimen, in which the teeth are approximately evenly spaced, but individual teeth are considerably more than one tooth width apart (Fig. 39B; Wellnhofer, 1974, 2008, 2009; Howgate, 1984a). Furthermore, a diastema of more than two tooth widths is present between the premaxillary and maxillary teeth in the anterior end of the maxilla (Fig. 39B; Wellnhofer, 1974, 1988b, 1992, 2008, 2009; Howgate, 1984a). In the Berlin and Thermopolis specimens, the maxillary teeth are also rather widely spaced, with the distance between individual teeth being approximately one tooth width in the anterior part of the maxilla, but increasing in more posterior teeth (Fig. 39A; Howgate, 1984a; Mayr et al., 2007; Wellnhofer, 2008, 2009) to a maximum of almost two tooth widths between the seventh and the eighth tooth in the Thermopolis specimen (Mayr et al., 2007). The Solnhofen specimen also shows a diastema between the premaxillary and maxillary teeth of more than two tooth widths, but in contrast to the Eichstätt specimen, here the toothless portion is mainly formed by the posterior end of the premaxilla (Wellnhofer, 1988b, 1992, 2008, 2009). The first maxillary tooth is offset from both the anterior end of the maxilla and the second maxillary tooth by approximately one tooth width, but maxillary teeth two to four seem to be closely spaced. From the space between the fourth and fifth maxillary tooth onwards, the distance between individual teeth is again more than one tooth width. A similar pattern seems to be present in the Daiting specimen, in which maxillary teeth two to four are closely spaced, followed by more widely spaced posterior teeth (Fig. 41; Tischlinger, 2009). However, in this specimen, the premaxillary body and anterior end of the maxilla are missing, and the first maxillary tooth is only represented by its broken root. The 12th specimen described here also has a gap of approximately one tooth width between the first and second maxillary tooth, but here, maxillary teeth two to six are closely spaced, and only from the gap between maxillary tooth six and seven onwards, the spacing becomes wider again (Fig. 14). Furthermore, there is no marked diastema between the premaxillary and maxillary dentition; the distance between the last premaxillary and first maxillary tooth is approximately one tooth width and thus considerably less than in the Eichstätt and Solnhofen specimens. In the London specimen, finally, the preserved maxillary teeth are even up to two tooth widths apart (Wellnhofer, 2008: fig. 5.27). Howgate (1984b) argued that there were empty tooth positions between the preserved teeth in this specimen, mainly based on a UV image published by de Beer (1954: pl. 9, fig. 4). If this was the case, the spacing of maxillary teeth would be considerably less than one tooth width; however, close inspection of a high quality cast of the jaw remains of the London specimen kept at the BSPG did not reveal any evidence for the presence of additional alveoli in between the preserved teeth. Detailed observations on the actual specimen would be necessary to clarify this point.

As the Berlin, Eichstätt and Solnhofen specimens are preserved with the lower jaw in occlusion, the spacing of their dentary teeth is difficult to establish. However, the visible dentary teeth in both the Eichstätt and Solnhofen specimens are widely spaced, with the distance between teeth being apparently more than the width of the individual teeth (Wellnhofer, 1974, 1988b, 1992, 2008, 2009). In the Daiting specimen, only the posterior seven dentary teeth (or their roots) are preserved, but these teeth are widely spaced, at approximately one tooth width distance (Fig. 41). In contrast, the anterior dentary teeth in the Munich (SNSB-BSPG 1999 I 50) and 12th specimen are less than one tooth width apart, and the spacing of the teeth increases from the seventh or eighth dentary tooth onwards (Fig. 42). In the 11th specimen, the dentary teeth seem to be less than one tooth width apart throughout the series (Fig. 15).

Further variation affects the inclination of the teeth, which not only varies between the specimens, but sometimes also between different regions of the dentition within one specimen. While the teeth are usually perpendicular with respect to the long axis of the jaw bone, the premaxillary teeth of the Solnhofen and 11th specimen (Fig. 40), the maxillary teeth of the Thermopolis and 12th specimen (Fig. 14) and the dentary teeth of the Daiting (Fig. 41) and Munich specimen (Fig. 42) are notably procumbent. In the Eichstätt specimen, the last premaxillary tooth and the anterior maxillary teeth are perpendicular, but the anterior premaxillary teeth and posterior maxillary teeth are notably procumbent. Similarly, in the Thermopolis specimen, the first two premaxillary teeth are slightly procumbent, whereas the third and fourth premaxillary tooth are perpendicular (Mayr et al., 2007). Although teeth angulation might be influenced by taphonomic factors, as teeth may become disattached from their respective jaw bone and thus change orientation somewhat, it seems unlikely that this can account for all of this variation. Indeed, differences in angulation of the teeth that are probably preservational artefacts are present in a number of specimens, such as the Daiting specimen, where one maxillary tooth seems to be posteriorly inclined, whereas another is apparently procumbent, and yet another tooth is perpendicular (Tischlinger, 2009). However, the very regular anterior inclination of the teeth in the specimens (e.g. in the premaxillary of the 11th specimen and the dentary of the Munich specimen; Figs. 40, 42A and 42B) noted above indicate that this might really be a primary morphology.

Finally, some variation occurs in the tooth shape itself, both within the individual tooth rows, as well as between specimens, as already noted, based on the much smaller sample size then available, by Howgate (1984a). Although premaxillary teeth of all specimens in which the premaxilla is preserved show the common characteristics outlined above, there is some variation in the shape of premaxillary teeth. The anterior two premaxillary teeth of the Berlin, Solnhofen, and 11th specimen have a strongly convex mesial and straight (11th specimen) to notably convex (Solnhofen specimen) distal margin, so that the tooth tip is placed slightly mesial to or above the level of the distal margin of the crown base (see Wellnhofer, 1988b, fig. 4; 2008: fig. 5.50; Mayr et al., 2007: fig. 5c). The posterior two premaxillary teeth in these specimens have a slightly concave distal margin, but with the tip still being placed above the distal margin of the crown base. All of these teeth show a notable mesial bulge basally above the root, and a small distal bulge is present in the third and fourth premaxillary tooth. The premaxillary tooth preserved with the London specimen shows a slightly concave distal margin (Wellnhofer, 2008: fig. 5.27), indicating that this tooth belongs to premaxillary tooth position three or four, whereas the complete second preserved premaxillary tooth of the 12th specimen has a straight distal margin, supporting its identification as second premaxillary tooth. In contrast to all of these specimens, the more or less complete second to fourth premaxillary teeth of the Eichstätt specimen (the first tooth is missing the tip) are recurved, with only a very slight mesial bulge and no distal bulge, and a notably concave distal margin. Finally, in the Thermopolis specimen, the premaxillary teeth follow the pattern seen in the Berlin, Solnhofen and 11th specimen in having a convex mesial and straight (first two teeth) to concave (premaxillary teeth three and four) distal margin, but they have a reduced mesial bulge, as seen in the Eichstätt specimen.

The lateral teeth of Archaeopteryx change morphology in the course of the tooth row, usually from more slender, recurved mesial teeth to more robust, less curved distal teeth, but patterns and morphologies seen differ between different specimens. One extreme is the Eichstätt specimen, in which all maxillary teeth are slender and recurved, being only slightly more robust, but otherwise very similar to the premaxillary teeth in this specimen (Wellnhofer, 1974, 2008, 2009; Howgate, 1984a). A very similar situation is found in the Thermopolis specimen, in which the maxillary teeth are also recurved and pointed (Mayr et al., 2007). In the Berlin specimen, the anterior maxillary teeth are similar to the distal premaxillary teeth in having slight mesial and distal bulges above the root and a slightly concave distal margin (Wellnhofer, 2008: fig. 5.50). From the sixth maxillary tooth onwards, the teeth are more robust and have convex mesial and distal margins. In the Solnhofen, Daiting and 12th specimen, the anteriormost maxillary teeth have a convex mesial and straight distal margin, but from the second (12th specimen) to fourth (Daiting specimen) tooth position on, the teeth are bulbous, with strongly convex mesial and distal margins, and the tip of the crown becomes more centrally placed in distal teeth. This also seems to be the situation in the London specimen, as far as can be judged from the preserved maxillary fragment (Wellnhofer, 2008: fig. 5.27).

Similar differences are also found in the dentary teeth in those specimens where these teeth are exposed. In the Munich specimen, the dentary teeth are similar to the maxillary teeth in the Eichstätt and Thermopolis specimens in being slightly recurved throughout the series, with notably convex mesial and straight to slightly concave distal margins (Wellnhofer, 1993, 2008, 2009; Elzanowski & Wellnhofer, 1996). Such recurved teeth are only present in the anteriormost tooth positions in the dentary of the 12th specimen; more distal teeth are similar to the maxillary teeth in being bulbous and having convex mesial and distal margins. The latter morphology is also found in the middle and distal dentary teeth of the Daiting and 11th specimens.

In summary, the different specimens of Archaeopteryx show variation in several different aspects of tooth morphology, number, spacing, and inclination, and no two specimens show the exact same pattern in all of these aspects (Table 4). Howgate (1984a) proposed four possible explanations for differences in the dentition of the London, Berlin and Eichstätt specimen, including sexual dimorphism, ontogenetic variation, polymorphism (all different types of intraspecific variation) and interspecific variation. Indeed, based on the unusual, recurved teeth of the Eichstätt specimen, Howgate (1984a) proposed a new species, Archaeopteryx recurva, for this specimen, which he even referred to a separate genus, Jurapteryx, based on additional proposed osteological differences, in a later publication (Howgate, 1985). Unfortunately, with the increase of number of specimens the pattern of variation remains confusing, so that none of Howgate’s (1984a) hypotheses can currently be favoured. Due to the unsolved species taxonomy of Archaeopteryx (see above), both intra- and interspecific variation are possible, although some types of intraspecific variation are maybe more relevant than others. As, for instance, the sex of the different Archaeopteryx specimens is unknown, sexual dimorphism cannot be tested. Furthermore, although dental polymorphism is widespread throughout tetrapods, most studies are focused on urodeles or mammals (Pedersen, 1991; Hanken, Wake & Freeman, 1999; Szuma, 2002, 2011), and the phenomenon of dental polymorphism is not well understood in reptiles in general and possibilities to study this phenomenon in dinosaurs are restricted only to some key taxa with high number of specimens. Current information on dental variability mainly concerns variability in the number of teeth between different individuals (Madsen, 1976; Raath, 1977; Colbert, 1989, 1990) or the variabilty of tooth shape within a single dentition (Smith, 2005, 2007; Hendrickx, Mateus & Araújo, 2015b). Furthermore, various studies documented ontogenetic variation in both number and morphology of teeth within theropod dinosaurs (Colbert, 1989; Carr, 1999; Rauhut & Fechner, 2005; Samman et al., 2005; Kundrát et al., 2008; Bever & Norell, 2009; Buckley et al., 2010; Tsuihiji et al., 2011; Rauhut et al., 2012), which seems to be most extremely developed in the basal ceratosaur Limusaurus (Wang et al., 2017b). These changes probably correlate with ontogenetic shifts in the diet spectrum (Farlow, 1976; Rauhut et al., 2012) as documented in extant crocodylians (Cott, 1961; Hutton, 1987; Webb, Hollis & Manolis, 1991; Da Silveira & Magnusson, 1999; Horna, Cintra & Ruesta, 2001).

When compared with body size, some differences in the dentition of Archaeopteryx could be explained as ontogenetic differences, including the variation in the tooth shape of premaxillary and anterior dentary teeth, and the number of dentary teeth. Thus, assuming an ontogenetic trend, smaller specimens possess recurved premaxillary and maxillary (Eichstätt and Thermopolis specimen) and dentary teeth (Eichstätt and Munich specimen), changing to a more bulbous morphology with growth, while the number of dentary teeth increases from 11 to 13. However, the presence of bulbous maxillary and dentary teeth in the Daiting specimen, which is smaller than both the Thermopolis and Munich specimens in respect to its humeral length (OR, own data), casts doubt on this interpretation, at least in respect to tooth shape. Furthermore, the size differences between the Thermopolis and several other specimens with bulbous teeth are minimal. Thus, the 11th and 12th specimens are 10% and 5% larger than the Thermopolis specimen (based on femoral length), respectively, but both show well-developed bulbous teeth in both maxilla and dentary.

Current research has shown increasing evidence that the different basins of the Late Jurassic limestones of Bavaria are characterised by different ages and fossil assemblages (Röper, 2005; Schweigert, 2007, 2015; Viohl & Zapp, 2007; López-Arbarello & Schröder, 2014; Ebert, Kölbl-Ebert & Lane, 2015; Rauhut et al., 2017; Foth & Rauhut, 2017). Based on stratigraphic data, Archaeopteryx spans of the entire Hybonoticeras hybonotum zone of the Tithonian (from the lowermost Painten Formation to the Mörnsheim Formation), including five ammonite horizons (Schweigert, 2015). Given that Schweigert (2005) calculated an average duration of 165,000 years per ammonite horizon in the Late Jurassic, this thus represents approximately 700,000 to one million years (see also Viohl, 2015b). Furthermore, the finds of the Archaeopteryx spread from the Schamhaupten Basin in the east to the Langenaltheim–Solnhofen Basin and Rennertshofen Basin in the west, with the eastern basins being generally older (Schweigert, 2015) and showing a higher terrestrial influence, indicated by abundant plant remains (including tree trunks), lepidosaurs, freshwater turtles and the non-avialan theropod dinosaurs (Reisdorf & Wuttke, 2012; Joyce, 2015; Jung, 2015; Tischlinger & Rauhut, 2015; Tischlinger, Göhlich & Rauhut, 2015; Viohl, 2015b). Thus, both anagenesis and biogeography could be further factors that influence variation in Archaeopteryx. However, the order of specimens from east to west does not reveal any pattern in the dentition. As noted by Viohl (1998), the limited flight capabilities of Archaeopteryx and the significant distance to the larger landmasses of the Rhenian and Bohemian massifs strongly indicate that the habitat of Archaeopteryx were numerous small islands formed by emerged parts of the reef complexes surrounding the plattenkalk basins, which resulted from a notable transgression at around the Kimmeridgian–Tithonian boundary (Keupp et al., 2007; Viohl, 2015b). One possibility might thus be that, following the immigration of the common ancestor of all Archaeopteryx specimens into the Solnhofen Archipelago in the late Kimmerdigian, probably from the east (Foth & Rauhut, 2017), populations of the urvogel became isolated on different islands and evolved in divergent directions, possibly resulting in a ‘species flock’ of Archaeopteryx due to island speciation (Glor, Losos & Larson, 2005; Losos & Ricklefs, 2009). If so, the variation in the dentition of Archaeopteryx might represent to some degree a Jurassic equivalent to the famous Darwin Finches, in which species show a great variety in the shape of their beaks, correlating with respective diet spectra (Sato et al., 2001; Abzhanov et al., 2006; Lamichhaney et al., 2015). However, in the absence of more rigorous approaches to the taxonomy of Archaeopteryx (e.g. by summarising all morphological and morphometric variation in a phylogenetic analysis on specimen level; see e.g. Yates, 2003; Tschopp, Mateus & Benson, 2015), this hypothesis must remain speculative at the moment.