Cyclotron angelini

Cyclotron angelini (Linnarsson, 1875)

Figs. 2–6.

Material.—One specimen with preserved soft parts (ZPAL Cr.11/ph060), Furongian, Olenus Biozone, Dębki 2 borehole, northern Poland.

Description.—The incompletely preserved specimen ZPAL Cr.11/ph060 described herein possesses two small nodes in the antero-dorsal position on the left shield, a centrally located circular lobe and a shield outline suggesting that it belongs to Cyclotron angelini (Linnarsson, 1875). Centrally, the shield is drawn out into a shallow circular lobe. An interdorsum, a characteristic cuticular bar separating the left and right shields dorsally by two longitudinal furrows (not a hinge), is present (Fig. 6A 1).

The right valve is preservationally absent, and the left valve is slightly incomplete antero-ventrally and posteriorly. The shield length measures 1520 μm but is estimated to originally have been approximately 1600 μm.

This specimen is possibly the largest one of the hitherto known phosphatocopids with preserved soft parts. Yet, it still seems to represent an immature ontogenetic stage, as specimens of C. angelini from the Olenus Biozone of Sweden may have grown to a shield length of up to 3.5 mm in length (Williams et al. 1994). The morphology described below is based on observations of specimen ZPAL Cr.11/ph060.

Soft parts: Cyclotron angelini comprises a flat-tubular body proper with differentiated head and trunk appendages and a large shield comprising left and right valves that completely surround the rest of the body. The body proper is connected to the shield only along a very narrow dorsal area, approaching not more than almost the width of the interdorsum. The number of post-mandibular segments involved in the shield formation is not clear but might include at least four segments—as known from other late ontogenetic stages of phosphatocopids, i.e., two segments more than the head composition at the level of Labrophora (= a cephalothoracic shield). Accordingly, it is also unclear how much of the trunk is missing in the specimen at hand and how many segments are free from the shield.

In the antero-ventral part of the body, a tube-like hole, around 50 μm in diameter is located anterio-medially of the antennal insertions (Fig. 3A 2: arrow). Since it is located too far anteriorly for representing the actual mouth opening underneath the labrum, which should extend posteriorly to the level of the antennal spines, it might represent the turn of the esophagus into the head with the transition to the midgut running backward (a feature known from specimens of other Orsten taxa, such as the two species of Skara, see Müller and Walossek 1985).

The anterior ventral soft parts of C. angelini (Figs. 2–6), such as the hypostome-labrum complex, are not preserved. However, the void in the anterior part of the body suggests its previous location. The entire region at the antero-lateral edges of the hypostome including the insertions of the first pair of appendages, the antennulae, is not preserved, only phosphatic granules mark their original place, which means that the antennulae are lacking. Further posteriorly, the sternum with its pair of paragnaths (on the mandibular portion of the sternum), covered by fine setulae or denticles (Figs. 3A 2, 6A 5) is present, which also includes the fourth head segment with a pair of appendages similar to the more posterior ones. The body posterior of the sternum is undifferentiated. An opening is located posterior to the terminal limbs, presumed to represent the anus (Fig. 6A 4: arrow). In all, six pairs of post-antennular appendages are present: antennae and mandibles and four post-mandibular limbs.

Antennula: Not preserved in the specimen at hand.

Antenna: The second appendage (antenna) is uniramous, lacking an exopod and measures c. 220 μm from the basal joint to the beginning of the terminal endopodal spine (Fig. 3A 2 –A 5). The limb includes a prominent trapezoidal, antero-posteriorly compressed, uniform stem portion that slightly curves medially; its proximo-distally extending median margin is wedge-shaped and runs out into a row of spines, in which short and longer spines alternate. These spines point towards the mouth opening at the rear of the hypostome (Figs. 3A 2, A 3, 4A 2). Proximo-laterally, the limb stem exhibits a c. 17 μm long cuticular plate, interpreted as a sclerotized part of the arthrodial membrane that extends proximally into the body proper (Fig. 3A 3 –A 5)—this structure has not been previously observed in any other phosphatocopid species. A separation of the limb stem into a proximal coxa and a distal basipod is not obvious, hence the antennal limb stem is referred to as a syncoxa.

The two-divided endopod (Fig. 3A 3, A 7) arises latero-distally from the apex of the syncoxa. Its proximal podomere is elongated like an oblique shovel with a row of 6–7, proximo-distally extending and spine-like, setae (Fig. 3A 7), originally, they might have pointed around the caudal end of the labrum toward the mouth, which is not preserved in this specimen. The short hump-like distal endopodal portion arises far laterally and bears an antero-posteriorly extending set of three small setae, arranged at a right angle against the spine row of the proximal endite of the endopod (Fig. 3A 7). Here, the terminal spine is the central one and the smaller originally median and lateral setae are situated anteriorly and posteriorly (Fig. 3A 1, A 3, A 7). The exopod is apparently absent. At its expected position (as obtainable from other phosphatocopid species; see Maas et al. 2003), a small hump is present (Fig. 3A 4: arrow). The slight concavity of the anterior side of the antenna (Fig. 3A 1) is due to the oblique attachment of this limb at the sides of the labrum and it was presumably used for pushing food into the mouth area. Accordingly, the arthrodial membrane, which articulates the appendage with the body, is ample, recognizable by the largely poorly preserved area there (Fig. 3A 4).

Mandible: The third appendage (mandible) measures around 445 μm, thus being twice as long as the antenna, again excluding the distal endopodal spine (Figs. 3A 1 –A 6). The mandible inserts laterally to the paragnaths and bears a large coxa, c. 280 μm long in medio-lateral extension (Fig. 3A 6). Medially, the coxa is drawn out into a wide, flattened and slightly obliquely tilted gnathobase pointing towards the midline (Fig. 4A 1, A 6). The median extension of the gnathobase is largest proximally (anteriorly), decreasing towards its distal end (posteriorly). The proximal margin of the gnathobase is armed with a prominent spine proximally flanked by a seta. A row of five tuft-like short serrated spinules with a main tip and a smaller adjacent tip arise from the gnathobasic edge between the proximal and distal end of the gnathobase (Fig. 4A 2, A 4, A 6). The anterior ends of the opposing gnathal edges of the right and left mandibles approach each other medially. They are located close to each other and parallel to the surface of the paragnaths and appear to be fairly symmetrical (Fig. 3A 3).

Distal to the coxal gnathobase, a wedge-shaped setose endite with a circular cross section more distally, extends medially (Fig. 4A 5). Its tip bears a prominent spine, which is flanked by a set of 5–6 irregularly arranged setae anteriorly and posteriorly, respectively (Fig. 4A 5). On the right mandible this element is broken off, however, the empty space between the coxa and the endopod suggests its previous location (Fig. 4A 6). In the terminology of Maas et al. (2003), this structure is the “basipodal endite”, or the strongly reduced basipod (Haug et al. 2013), characteristic of the morphogenesis of phosphatocopids, where a well-developed basipod is present only in the youngest ontogenetic stages (Maas et al. 2003).

The mandibular endopod is subdivided into two portions and arises latero-distally on the apex of the coxa (Fig. 4A 5, A 6). The proximal portion/podomere is club-shaped and drawn out medially into a robust, medially pointing tooth slightly dentate at its distal margin. The spine is slightly directed towards the oral region. Distal to the spine, two setae are located, one medio-distally, the second one weakly inclined anteriorly (Fig. 4A 5). Additional small setae occur also on the club of the proximal portion of the endopod (Fig. 4A 5). The smaller distal portion of the endopod arises latero-distally on the proximal portion of the endopod and bears an anterior retention seta, a strong median spine. A posterior, possibly sieving seta is broken off close to its base (Fig. 4A 2, A 5, A 6). The boundary between the limb stem and the endopod on the lateral side of the limb is well defined (Fig. 4A 5, A 6).

Laterally on the mandibular limb stem (= on the outer side), the exopod arises about half way between the transition to the arthrodial membrane and the insertion of the endopod (Figs. 3A 1, A 3, 4A 1). The exopod arises lateromedially on the coxa and is a less than 100 μm long and 30 μm wide (Figs. 3A 5, 4A 3). It is unusually small and short compared to that of most known phosphatocopid species (see Maas et al. 2003). The arthrodial membrane of the exopod is slightly bent up into a ring-like structure with a diameter of around 6 μm (Fig. 4A 3: arrow). Its free margin, i.e., the median, lateral and proximal, is armed with a row of more than 12 setae, of which two proximal ones seem to be longest, measuring around 60 μm (Figs. 3A 5, 4A 3). The marginal row of at least ten long setae is interrupted by setae less than half as long, irregularly inserting and very slightly displaced from the marginal ridge (Fig. 3A 5).

Antenna and mandible are located closely together in an oblique antero-lateral to postero-median position at the flanks of the supposed position of the hypostome-labrum complex, their median armature with setae and spines pointing orally (Figs. 3, 4).

The mandible is followed by four appendages, which appear serial in design; only the endopod seems to be smaller in more posterior limbs compared to the anterior ones. Since the first post-mandibular limb is much better preserved than the succeeding ones we give details on the morphology of the remaining trunk limbs when they differ from the first post-mandibular limb.

First post-mandibular limb: The fourth appendage, i.e., the first post-mandibular limb (Fig. 5A 1 –A 4, A 8), is approximately 490 μm long, from its insertion to the tip of the endopod, excluding the distal endopodal spines. Since it is morphologically similar to the following appendages, it is not regarded as a maxillula. The fourth limb appears slightly compressed in antero-posterior aspect (Fig. 5A 1) and inserts almost laterally at the narrow body proper. Its stem portion consists of the separate proximal endite and the basipod, which carries the endopod and exopod.

As in many other phosphatocopids, the basipod is subtriangular with a wide excavation basally in its posterior cuticular surface, which is covered by membranous cuticle (Fig. 5A 2), permitting a wide posterior flexure of the limb. Laterally, the basipod has a curved, narrow and possibly pointed extension toward the body surface, which seems to be rather rigid (Fig. 5A 2). As based on other phosphatocopids (Maas et al. 2003), this extension anchors the limb far dorsally at the body as a pivot, so that the limb can be turned around this pivot while the ample soft median arthrodial membrane (Fig. 5A 1, A 3, A 4) allows for a wide array of swinging motion anteriorly and posteriorly. The weakly humped oval median surface of the single basipodal endite bears a group of 5–6 robust spines and a few finer marginal spines. The median surface is surrounded by a set of around 20 setae, which almost form a complete circle, only the distal two are much thinner than the others (Fig. 5A 1, A 8 and 5A 6, A 7 for second post-mandibular limb). This is noteworthy because on enditic surfaces of the limbs of other phosphatocopids and Cambrian eucrustaceans the setae are more clearly subdivided into two distinct sets, in which the setae are arranged in vertical or crescentic rows around the central group. The posterior set is almost vertical and appears to rise from small sockets (Fig. 5A 8), whether these were movable remains unclear. The anterior set is arranged in a more crescentic row. Three pores possibly occur on the posterior side of the basipodal endite (Fig. 5A 8: arrow) and on the proximal endite. They are located in a row just below the circularly arranged setae. On the anterior side, they are absent or not preserved. Similar pores have also been discovered in several other Orsten eucrustaceans (e.g., see Walossek 1993 for Rehbachiella kinnekullensis on appendages and the furcal rami) and are most likely of sensory function. Accordingly, these setae might also have served this function to monitor the motion of the posterior raking setae.

The proximal endite, inserting clearly separately below the endite of the basipod, extends above the surface of the sternum (Fig. 5A 1, A 2). Both the proximal endite and the basipodal endite are clearly separated from each other. The median surface of the proximal endite is two-partite, the proximal part is a rounded hump with a few shorter spines or setae, whereas the distal oval surface bears several robust and smaller spines. Around the two enditic parts an oval circle of at least 24 setae is present around the outer margin of the enditic protrusion (Fig. 5A 1, A 2). These setae are very thin and acute around the proximal surface and slightly longer distally. In all, the arrangement though similar to that of the basipodal endite, appears much more fragile and brush-like.

The two-segmented endopod, around 110 μm in length, inserts medio-distally on the basipod (Fig. 5A 1 –A 4). Its proximal podomere is rod-like and medially extended into a medio-distally pointing endite armed with numerous setae (Fig. 5A 1). Here the enditic armature is distinctly divided into three sets, the anterior and posterior rows of setae, 5 in the posterior row, and a few median spine-like setae on the central hump. The cuticle below the endite is also membranous so that the joint membrane is made of two parts, one from the basipod and one from the endopodal podomere. The distal segment is of similar length as the proximal one, but smaller in diameter and designed as a rounded cone or tube with the setation arising from an oval mediodistal area. The arrangement is again circular, with fine setae around the proximal curve, progressively more rigid setae towards the tip and the most robust spine-like seta terminally (Fig. 5A 1 –A 3). Laterally on the outer slope there seems to be another short spine. Only two or three finer setae arise from the area between the setal circle. The two endopodal podomeres are separated by membranous cuticle, preserved as granular-like surfaces (Fig. 5A 1, A 3, A 4).

The outer edge of the basipod is rather narrow and steeply sloping. From this ridge the leaf- or paddle-shaped exopod, c. 110 μm in length, arises at the level between the joint of the first endopodal podomere and the proximal setae of the enditic armature (Fig. 5A 1, A 2). The basipod-exopod joint is rather poorly developed anteriorly, but sharply defined posteriorly. In anterior or posterior aspect, the shape of the exopod is elongated and triangular, as the surface narrows toward the distal end of the exopod, receiving more of a characteristic design with short annuli, but the joints between them are effaced. Only the median setae with their sockets reflect a subdivision into approximately 4 annuli. Due to this uniform surface, the marginal setation forms a continuous row from medially around the tip and distally to the joint with the basipod. It is further noteworthy that the length of the limb is almost the same from the body to the tip of the endopod and the exopod.

Enditic setae of the anterior and posterior sets as well as the exopod setae bear fine setules densely arranged in two rows (Fig. 5A 1, A 3). This arrangement is known also from other phosphatocopids (Maas et al. 2003), and eucrustacean taxa, but it is unknown from taxa derived from the earlier evolutionary lineage of Crustacea (e.g., Walossek 1993, see also Haug et al. 2013 on the evolution of crustacean appendages).

Second post-mandibular limb: This limb (Fig. 5A 6, A 7) is rather poorly preserved and details of its morphology remain unclear in specimen ZPAL Cr.11/ph060. The second post-mandibular limb looks similar to the first one. Preserved details include the setiferous proximal endite and fragments of the basipod together with the basipodal endite (Figs. 3A 1, 5A 6, A 7). On the lateral basipodal margin of the left limb (Fig. 4A 5) and the right limb (Fig. 5A 6), three tiny slender setulose setae rise in a row, the distal one is the longest and measures c. 85 μm, the two proximal setae are c. 35 μm long (Fig. 5A 5). By contrast, no such setae occur on the first post-mandibular limb.

The fifth appendage, or second post-mandibular limb, is the last cephalic appendage in the taxon Labrophora, which includes Phosphatocopida and Eucrustacea. Yet, in phosphatocopids there is no clear cephalic boundary, and at least two more segments are associated with the shield, forming a larger unit; the cephalothorax, convergent to several taxa within Eucrustacea and Euarthropoda.

Third post-mandibular limb: The sixth appendage, third post-mandibular limb, is known only from the poorly visible external part of the basipod with the basipodal endite of the left limb and part of the basipod of the right limb (Fig. 3A 1). It is presumed that the general morphology largely mimics that of the preceding limbs.

Fourth post-mandibular limb: The seventh appendage, fourth post-mandibular limb, is located at the posterior end of the body (Fig. 6A 1 –A 3), and is ventrally directed. The right limb is well preserved, whereas the left is mostly masked by matrix, except for its setae, which are partly visible. The right limb is around 230 μm long, excluding the terminal setae on the exopod. The stem is formed by an undivided, sub-rectangular basipod with a few medio-distal setae. The proximal endite could not be observed in the specimen at hand.

The endopod rises medio-distally on the basipod, as in the preceding limbs, but it is undivided and carries one terminal long and two short anterior and posterior setae Fig. 6A 1 –A 3). The exopod rises on the distal margin of the basipod. It is paddle-shaped, but not annulated, and directed ventrally. The exopod bears 21 setulose setae around its entire free margin, most setae at the outer margin are broken off close to their base. The folded arthrodial membrane, which links left and right limbs with the ventral body proper, is well-developed (Fig. 6A 3).

Stratigraphical and geographical range.—Furongian, Olenus Biozone, Sweden (see Williams et al. 1994 and Hinz-Schallreuter 2000) and Poland.