On the value of Burmese amber for understanding insect evolution: Insights from †Heterobathmilla – an exceptional stem group genus of Strepsiptera (Insecta)

Burmese amber and amber from other periods and regions became a rich source of new extinct insect species and yielded important insights in insect evolution in the dimension of time. Amber fossils have contributed to the understanding of the phylogeny, biology, and biogeography of insects and other groups, and have also gained great importance for dating molecular trees. Another major potential is the documentation of faunal, floral and climatic shifts. Evolutionary transitions can be well‐documented in amber fossils and can reveal anatomical transformations and the age of appearance of structural features. Here, using a new stem group species of Strepsiptera from Burmite, we evaluate this potential of amber insect fossils to assess the current phylogeny of Strepsiptera, with the main emphasis on the early splitting events in the stem group. Amber fossils have greatly contributed to the understanding of the evolution of Strepsiptera in the late Mesozoic and the Cenozoic. †Heterobathmilla kakopoios Pohl and Beutel gen. et sp. n. described here is placed in the stem group of the order, in a clade with †Kinzelbachilla (†Kinzelbachillidae) and †Phthanoxenos (†Phthanoxenidae). †Phthanoxenidae has priority over †Kinzelbachillidae, and the latter is synonymised. The superb details available from this new fossil allowed us to explore unique features of the antennae, mouthparts, and male copulatory apparatus, and to provide a phylogenetic hypothesis for the order. The younger †Protoxenos from Eocene Baltic amber was confirmed as sister to all remaining extinct and extant groups of Strepsiptera, whereas the position of the Cretaceous †Cretostylops in the stem group remains ambivalent. While the value of Burmite and amber from other periods has a recognized impact on our knowledge of the evolution in various lineages, this new fossil does not fundamentally change our picture of the phylogeny and evolution of early Strepsiptera. However, it offers new insights into the morphological diversity in the early evolution of the group.

Occasionally amber fossils provide insights in features linked to life habits, for instance feeding, prey capture, or reproduction. Cretaceous amber fossils of †Archipsyllidae ( †Permopsocida) with well-preserved skeletal features helped to bridge the gap between chewing and sucking feeding habits in Paraneoptera Yoshizawa and Lienhard, 2016).
Long-proboscid scorpionflies from Cretaceous amber give an indirect evidence of uptake of liquid from gymno-and angiosperm reproductive organs, and the detection of pollen grains adjacent to the fossils support pollinator activity of these species (Lin et al., 2019). Direct evidence of angiosperm pollinivory in Coleoptera has been recently recorded for two families, Mordellidae (Bao et al., 2019) and Kateretidae , while two records of stem Anthophila (Poinar and Danforth, 2006;Danforth and Poinar, 2011;Poinar, 2020) and an unplaced aculeate, †Prosphex , provide evidence for increasing angiosperm-dependence in Mesozoic Hymenoptera. Locomotion and climbing on broad leaves, as of angiosperms, or wet branches is indicated by the specialized and enlarged tarsal pads of Timematodea (Phasmatodea) from Cretaceous Burmese amber . Social behaviours are also observed, as in brood care by a 100-million-year-old scale insect documented by Wang et al. (2015), and possible inter-colonial combat in the stem ant genus †Gerontoformica by Barden and Grimaldi (2016). Specialized predatory habits are amply evinced by stem Formicidae (e.g. Perrichot, 2014;Perrichot et al., 2020), and even by the dictyopteroid †Alienoptera, for which a unique type of cephalo-prothoracic preycatching mechanism was recently described Ko c arek, 2019). Debris-carrying camouflage among predaceous larvae is reported from Cretaceous Burmese, French, and Lebanese ambers, including Chrysopoidea, Myrmeleontoidea, and Reduviidae (Wang et al., 2016a; see also P erez de la Fuente et al., 2012). Moreover, ectoparasitism has been confirmed via preservation of a rhopalosomatid larva in its host gryllid (Lohrmann and Engel, 2017), and feather-feeding has been suggested by preservation of partially damaged dinosaur feathers with fossil nymphs of †Mesophthiridae . Endoparasitism is indicated by the presence of a minute strepsipteran primary larva in Burmite, which is extremely similar to extant first-instar larvae .
Amber inclusions (and impression fossils) of different periods and regions have gained great importance for dating molecular trees (e.g. Ronquist et al., 2012;Wahlberg et al., 2013;Misof et al., 2014;Espeland et al., 2018;McKenna et al., 2019;Matsumura et al., 2019). The importance of a reliable identification of extinct species has been recently underlined in a study on a Triassic impression fossil, †Leehermania prorova Chatzimanolis, Grimaldi and Engel, which was originally placed in the polyphagan beetle family Staphylinidae (Chatzimanolis et al., 2012). It was transferred to the suborder Myxophaga based on indepth phylogenetic analyses with cladistic and Bayesian methods (Fik a cek et al., 2020). This transfer has a strong impact on future dating of the megadiverse beetle suborder Polyphaga and the series Staphyliniformia (see e.g. Misof et al., 2014). The "peril of dating beetles" was also emphasized by Toussaint et al. (2017).
Amber fossils can provide crucial insights on the emergence of biogeographic patterns (e.g. Rust et al., 2010;Abellan et al., 2011;Jałoszy nski, 2012;Barden and Ware, 2017;Cai et al., 2017;Poinar, 2018;Gimmel et al., 2019;Mashimo et al., 2019). For example, re-evaluation of taxonomic assumptions for fossils attributed to or near the ant genus Leptomyrmex resolved decades of speculation about the biogeographic origin of the Australasian radiation of the genus, clearly demonstrating a Neotropical origin for the clade with probable Oligocene trans-Antarctic dispersal (Boudinot et al., 2016;Barden et al., 2017). More recently, a Baeomorpha (Hymenoptera, Rotoitidae) from mid-Cretaceous Burmese amber, extends the distribution of fossil Rotoitidae from northern Laurasia to the southern Hemisphere, where the two extant genera are restricted to Chile and New Zealand (Huber et al., 2019). A Gondwanan origin of Zoraptera was suggested based on fossil material embedded in Burmese amber (Mashimo et al., 2019). Whereas Myanmar was traditionally considered as part of Laurasia (Mitchell, 1993;Boucot et al., 2013), it is now assumed that the West Burma Block, which includes the early Cretaceous fossil site of Burmese amber, was originally attached to Gondwana (e.g. Metcalfe, 2017;Poinar, 2018).
A major potential of Mesozoic and Tertiary amber fossils is the documentation of faunal, floral and climatic shifts. A brief comparison of the late Cretaceous flora and fauna preserved in Tilin amber (central Myanmar) with early Cretaceous and Eocene biota was presented by Zheng at al. (2018), describing for instance a pre-Cenozoic transition from stem group to crown group Formicidae. A snapshot of the Middle Eocene northern European staphylinine rove beetle diversity was recently presented in Brunke et al. (2019) and compared to the extant fauna of this area. However, comprehensive comparisons between biota documented by amber fossils of different world regions and geological periods are still missing, although taxonomically comprehensive work has been published (e.g. Rasnitsyn and Quicke, 2002). Systematic reviews are accumulating for different groups, such as for the ants (e.g. LaPolla et al., 2013;Barden, 2017) and various beetle taxa (e.g. Krell, 2006;Abell an et al., 2011;Legalov, 2012Legalov, , 2020Peris, 2020), and also studies on the palaeobiology of predators, parasitoids and parasites, plant-arthropod associations, and the diversification of insects based on disparity of mouthparts (e.g. Labandeira, 2002Labandeira, , 2006aLabandeira and Currano, 2013;Ponomarenko and Prokin, 2015;Nel et al., 2018). Because of the rapid accumulation of new fossil taxa in recent years (e.g. Ross, 2019aRoss, , b, 2020, the need for comprehensive reviews is acute. Anatomical insights can be gained from amber fossils on principle. However, preservation of internal soft parts of fossil insects is very rare, and Cretaceous amber fossils with well-preserved internal organs have not been described yet. An exception are mummified tissues in amber documented by Grimaldi et al. (1994), and indirect flight and leg muscles observed in some aculeate fossils in Burmite (B. Boudinot pers. obs.). Specimens of insect Tertiary fossils with preserved internal organs are known but also extremely rare. Soft parts of cantharid and nitidulid beetles embedded in Miocene Dominican amber were reported by Henwood (1992a), and the same author found exceptionally well-preserved flight muscles in fossil dipterans extending even to cell ultrastructure (Henwood, 1992b). The almost complete anatomical reconstruction of the Eocene stem group strepsipteran †Mengea tertiara (Menge) (Pohl et al., 2010;H€ unefeld et al., 2011) remains a unique exception. It was shown that the internal structures of this species, as documented with µ-computed tomography (µ-CT), do not differ distinctly from those of extant strepsipteran males of Mengenillidae. If turbidity is present in the amber, e.g. in the case of the larva of †Mengea , or if important syninclusions prohibit grinding of the amber for better examination of the fossils, µ-CT provides an opportunity to examine amber fossils. This was emphasized in Soriano et al. (2010), although the potentially destructive effects of the procedure were also noted in that study. Very good results were repeatedly obtained at DESY (Deutsches Elektronen-Synchrotron, Hamburg, Germany) (e.g. Pohl et al., 2010Pohl et al., , 2019 and other facilities (e.g. Bai et al., 2016Bai et al., , 2018, usually without recognizable negative effects on the specimens. However, a discoloration such as a darker stripe in the amber can sometimes be caused by Xradiation during scanning . Methods for preparing small-sized 3D amber samples were discussed in detail by Sidorchuk and Vorontsov (2018). These techniques distinctly facilitate and improve the study of very small insects or other minute organisms.
In the case of the species described here, the use of µ-CT was not necessary because the amber was very clear, and all morphological details of the new fossil were visible without µ-CT scanning.
Burmese amber has provided new insights into a number of different areas, and our ability to describe morphological details of the fossils is constantly improving. This raises the question whether accumulating information necessarily improves our phylogenetic understanding of the investigated group, such as Strepsiptera, which is increasingly well represented in Cretaceous amber (e.g. Pohl and Beutel, 2016). The phylogenetic placement of this small and highly specialized holometabolous order (c. 600 spp.) (e.g. Beutel, 2008, 2013) has been strongly disputed over a long time (e.g. Pohl and Beutel, 2013), but was reliably settled recently. A systematic position in a clade Coleopterida, together with the megadiverse Coleoptera, is supported by large morphological  and molecular data sets, including transcriptomes and genomes (Wiegmann et al., 2009;Niehuis et al., 2012;Boussau et al., 2014;Misof et al., 2014;Peters et al., 2014). Strepsiptera is mainly characterized by the endoparasitic ecology of the larvae and usually also of females (Stylopidia, ca. 97% of all species). Linked with this specialized lifestyle are the extremely miniaturized 1 st instar larvae and extreme sexual dimorphism (e.g. Pohl and Beutel, 2008).
†Protoxenos janzeni Pohl, Beutel et Kinzelbach, also embedded in Baltic amber, was identified as the sister group of all the remaining Strepsiptera Pohl and Beutel, 2016), even though it is much younger than †Cretostylops engeli Grimaldi and Kathirithamby from Burmese amber .
The new fossil strepsipteran is described and its morphology documented. The phylogenetic placement is evaluated by adding the observed structural features to previously published data matrices Beutel, 2005, 2016;Bravo et al., 2009). The character evolution linked with early splitting events in the order is discussed, with special emphasis on the unique condition of the copulatory apparatus.

Material
The single specimen (holotype) of †Heterobathmilla kakopoios Pohl and Beutel, gen. et sp. n. in Burmese amber is from the wellknown deposit of the southwest corner of the Hukawng Valley (26°20 0 N, 96°36 0 E) in Myanmar, dated as 98.79 AE 0.62 Ma (e.g. Shi et al., 2012;Poinar, 2018). It is part of the collection of the Zoologisches Forschungsmuseum Alexander Koenig in Bonn, Germany (accession number ZFMK-STR-00000104). In order to document the head of the fossil in frontal view, the amber was trimmed with a razor blade and then polished with emery papers and mud chalk.

Specimen imaging
The piece of amber was temporarily mounted on a glass microscope slide using glycerine and covered with a glass coverslip. A Leica MZ 12.5 stereomicroscope with magnifications up to 1009 was used for observations (Leica Microsystems GmbH). The images of the entire specimen were taken with a Canon EOS 7D equipped with a Canon MP-E 65 mm macro lens fitted with a StackShot macro rail (Cognisys). Images of the head and the terminal segments were taken with a Canon EOS 7D equipped with a Nikon M Plan 20 ELWD microscopic lens, plus an adjustable extensions bellow and illuminated with two flashlights. An Axio Zoom.V16 with a Plan NeoFluar Z 1.09 (Carl Zeiss Microscopy GmbH) was used for the tarsi and the images were saved as CZI files. An Axiovert S100 inverted fluorescence microscope, equipped with a Spot CCD camera (Visitron Systems GmbH), was used for autofluorescence images of the terminal segments of the fossil.
Stacks of several partially focused images were recorded for overcoming limited depth of field. Single images of the Axio Zoom.V16 were exported with ZEN 2.3 lite. All images were stacked with Zerene Stacker (Zerene Systems LLC). They were processed using Adobe Photoshop CS6 (Adobe System Incorporated) and arranged as plates. Adobe Illustrator CS6 (Adobe Systems Incorporated) was used for lettering. Drawings are based on the micrographs and observations made with the Leica MZ 12.5 stereomicroscope.

Phylogenetic analyses
The characters of †Heterobathmilla were added to the data matrix published by  with Mesquite (Maddison and Maddison, 2018). In addition, the characters of later described extant and fossil taxa such as Bahiaxenos, †Kinzelbachilla, †Phthanoxenos, the larvae of †Mengea and †Eocenoxenos were added to the matrix Henderickx et al., 2013;Engel et al., 2016;Pohl and Beutel, 2016;Pohl et al., 2019). Parsimony analyses were carried out with NONA version 2.0 (ratchet, 1000 replicates) (Goloboff, 1999) and TNT (Goloboff et al., 2008)  Diagnosis. Male. Distinguished from all other strepsipteran families on the basis of the following features: Size less than 6 mm; antennal foramen distinctly widened, about twice as wide as diameter of scapus; labrum free, with paired anterolateral processes; galea present.
Nomenclature. In their original descriptions, †Kinzelbachilla and †Phthanoxenos were placed in separate families, †Kinzelbachillidae and †Phthanoxenidae by Pohl and Beutel (2016) and Engel et al. (2016), respectively. Considering the combination of a unique apomorphy (dorsal antennal processes) with a unique plesiomorphy (presence of parameres), it appears tempting to erect a new family for †Heterobathmilla. The monophyly of other groups would not be affected by this taxonomic rank. However, as the results of the analysis (see below) suggest a very close relationship of †Heterobathmilla with †Kinzelbachilla and †Phthanoxenos, we prefer to place the three described species in a single family.
The question of priority for †Kinzelbachillidae and †Phthanoxenidae was complicated by an online-early article distribution that was compounded by a misleading publication date. Specifically, the name †Phthanoxenidae was provided in the online advance copy of Engel et al., with the date given as "Available online 13 November 2015". However, this version cannot be considered as published because the work does not contain evidence of registration in ZooBank (Articles 8.5.3 and 9.9, International Commission on Zoological Nomenclature, 2012). Consequently, as specified by Article 21.9 of the code, the nomenclatural acts of Engel et al. are to be considered valid and available based on the publication date of the print version, which is recorded as March 2016 in the text of Volume 58 of Cretaceous Research. Although this implies priority for †Kinzelbachillidae, validly published on 4 January 2016 in an online-early version registered in ZooBank, the managing editor of Cretaceous Research confirmed that Volume 58 was mailed on 1 December 2015, conferring availability of †Phthanoxenos at that time by article 21.4 addressing incorrect dates. For these reasons, we recognize †Phthanoxenidae as the senior synonym of †Kinzelbachillidae syn. n.
Key to adult males with main focus on fossil families Etymology. The name is derived from "Heterobathmie", used by W. Hennig (1974) to characterize a mosaic pattern of plesiomorphic and apomorphic features ("Heterobathmie der Merkmale"). The endingilla is used in several generic names in the order, for instance in Mengenilla or Bohartilla.
Diagnosis. Differs from all other known extant or extinct Strepsiptera by the bipectinate antenna and the presence of parameres, the latter subdivided into a proximal and a distal portion.
Compound eyes with small ommatidia not separated by chitinous bars, without microtrichia between them. Labrum well-developed and free, with distinct paired distolateral processes. Mouthfield sclerite not developed. Anterolateral antennal foramina large, almost one third as wide as head anterior to compound eyes. Eight-segmented antenna bipectinate; antennomeres 3-8 with lateral flabellae, and 4-8 with additional dorsal processes; Mandibles strongly developed, with distinct primary and secondary articulation; with apically pointed distal part almost forming right angle with robust base. Maxilla not divided into cardo and stipes; lacinia absent; galea distinct, maxillary palp one-segmented, with oviform sensillum. No individual labial elements recognizable; labial palp absent.
Anterior pronotal border strongly convex (Fig. 2). Mesonotum with distinct posterolateral process; anterior and posterior border concave (Fig. 2). Discrimen of metaventrite reaching about half length of sclerite (Fig. 4); distinct anteriorly converging furrows reach beyond anterior end of discrimen; additional oblique lines originate from furrow and extend anterolateral towards lateral edge of ventrite. Metatrochanter with oblique distal edge and apical excavation for insertion of metafemoral base (Fig 4); femora with slightly convex edges; tibiae slightly curved; tarsi very slender, with diameter very slightly decreasing distally; basitarsomere of pro-and middle legs about twice as long as following segment; tarsomeres 2 and 3 about equally long; tarsomere 4 shortest; tarsomere 5 about twice as long as 4 (Fig. 4). R 1 reaches distal 3/4 th of wing and then merges with anterior margin (Fig. 2); r 2 short, restricted to distal region, nearly reaching tip of wing; r 3 short; r 4 longer; r 5 reaching posterior margin, strongly pigmented basally; ma 1 and ma 2 long, almost reaching wing margin; ma 3 extending from base to hind margin or at least ending very close to it; cua 1 extending from base to hind margin.
Abdomen slightly less than half as long as entire body, subparallel (Figs 3, 4). Sternites I-VII between 3 and 3.5 times as wide as long. Sternum VIII rounded posteriorly. Penis with very slender distal part, apically pointed (Figs 8A,B, 9); parameres appear divided into proximal and distal portion by transverse membranous zone; apically strongly sclerotized and pointed (Fig. 9).
with the same number of steps were obtained with TNT (traditional search). The strict consensus tree in both analyses was identical (Fig. 10).

Discussion
As in several other groups, the knowledge of the past diversity of Strepsiptera has been distinctly improved by amber fossils. A total of 41 fossil strepsipteran species are presently described (see Kogan and Poinar, [2019] for a current checklist). Of these, 20 species are from Miocene Dominican amber, 13 species from Eocene Baltic amber, two compression fossils from the Eocene Green River formation (USA), one species from Eocene Fushun Amber (China), one from Eocene Brown coal Geisel Valley (Germany), and one from Colombian copal (Holocene -Pleistocene). It is noteworthy that all known strepsipterans from the Cretaceous belong to the stem group. Members of extant groups, like representatives of Stylopidia, are completely absent in late Mesozoic deposits. In contrast, various representatives of this large strepsipteran subunit that have endoparasitic adult females are recorded from the Eocene, including Corioxenidae, Elenchidae, Myrmecolacidae and Stylopidae (see below). This suggests that the transition of the females to permanent endoparasitism did not occur before the Paleogene.
Usually, adult males are discovered as amber inclusions, for instance †Mengea tertiara (Menge, 1866; see also Kinzelbach and Pohl, 1994) and †Protoxenos  from Baltic amber, †Cretostylops Grimaldi et Kathirithamby, †Kinzelbachilla and †Phthanoxenos Engel and Huang from Burmese amber Kathirithamby and Engel, 2014;Pohl and Beutel, 2016;Engel et al., 2016),  (Kinzelbach and Pohl, 1994), or a species of the extant family Myrmecolacidae from Eocene amber from north-eastern China (Wang et al., 2016b). Amber fossils of adult females of the basal groups of Strepsiptera remain unknown (in contrast to Kathirithamby, [2018]). A tiny primary larva from Burmese amber was discovered recently , and also a freeliving late instar, very likely an immature stage of †Mengea tertiara from Baltic amber .
Amber fossils have greatly contributed to the understanding of the evolution of Strepsiptera in the late Mesozoic and the Cenozoic. In our analyses, the relatively young †Protoxenos ( †Protoxenidae) from Eocene Baltic amber was clearly confirmed as sister to all remaining extinct and extant groups. A slender distal mandibular part (char. 42.1) and broad fan-shaped hind wing (char. 62.1) are unambiguous apomorphies of Strepsiptera excl. †Protoxenidae. An additional potential apomorphy of this clade is the distinctly reduced size and less robust body . A large, free and bi-lobed labrum (chars. 38.0, 39.1), very robust mandibles (char. 42.0), eight-segmented antennae (char. 21.1) with lateral flabellae on antennomeres 3-7, maxillae with a galea (char. 47.0), and hind wings longer than wide (char. 62.0) are strepsipteran groundplan features preserved in †Protoxenos. It is conceivable that males of †Protoxenos were still taking up food, in contrast to extant strepsipterans. However, gut contents could not be reliably identified in the Eocene species . Despite the clarified first split in Strepsiptera, the fossil evidence concerning the area of origin remains ambiguous.
†Protoxenos  was discovered in Baltic amber, whereas the oldest known species are preserved in Cretaceous Burmese amber.
representatives of Strepsiptera. †Heterobathmilla is unambiguously placed in a clade with †Kinzelbachilla and †Phthanoxenos, supported by a distinctly widened antennal foramen (19.0). Another potential synapomorphy is the position of the oviform sensillum near the tip of the maxillary palp (56.1). However, this feature, like many others, is not recognizable in †Phthanoxenos  and is thus ambiguous at this node. Another shared feature is the presence of an anterolateral labral process (39.1). However, this structure is also present in †Protoxenos, and thus likely a groundplan feature of the order. The monophyly of †Phthanoxenidae and the absence of "parameres" in all hitherto known fossil or extant members of the order (including †Protoxenos), raises the question of the evolutionary background of these lateral lobes of the penis ("aedeagus").
The term "paramere" has been contentious for generations (e.g. Snodgrass, 1935Snodgrass, , 1957Crampton, 1938), and its evolutionary identity as a distinct structure without homologs in hemimetabolous groups is further called into question by †Heterobathmilla. Two anatomical observations support the interpretation that these paired structures are gonopods, or genital appendages of other Hexapoda (Boudinot, 2018). First, the "parameres" of †Heterobathmilla correspond positionally to the gonopods of other Holometabolaincluding Hymenoptera, Raphidioptera, and Mecopterida. Second, the apparent transverse line of the structure matches the coxa-stylus pattern of abdominal appendages IX across the winged insects (Boudinot, 2018), a pattern also consistent among Paleozoic Pterygota Prokop et al., 2020). The primary point of uncertainty is that the penis hinges on abdominal sternum IX, similar to other Strepsiptera, rather than within the gonopods as observed in those Holometabola that retain these appendages (Boudinot, 2018). Despite this, it remains plausible that the apparent "parameres" of †Heterobathmilla and "lateral lobes" of Coleoptera are at least partially homologous with gonopods, albeit integrated developmentally. Although re-expression of genital appendages in the case of †Heterobathmilla would be more parsimonious, multiple independent losses cannot be ruled out considering the presently known stem group taxa. Moreover, parallel losses of structures across the phylogeny, such as wing vein abscissae and muscles, are known to occur based on rigorous statistical modeling (e.g. Klopfstein et al., 2015). Further resolution on this issue of genital homologies and functional morphology may be provided by µ-CT scanning, as in Pohl et al. (2010), if the specimen is fine enough to have preserved muscular tissue. The position of †Cretostylops in the stem group remains uncertain. The genus is placed in a trichotomy with the †Phthanoxenos - †Heterobathmilla - †Kinzelbachillaclade and the monophylum comprising †Mengea and extant Strepsiptera. Presently no features are available to solve this phylogenetic ambiguity. The placement of the extant Bahiaxenos from Brazil (known only from the male holotype) in an unresolved trichotomy with †Mengea and crown group Strepsiptera, is likely due to the lack of any characters of immature stages or females. As a whole, fossils have not fundamentally changed our picture of the phylogeny and evolution of Strepsiptera but have provided tantalizing clues to ancient morphological transformations. Relationships in crown group Strepsiptera are not affected at all by extinct taxa (e.g. Henderickx et al., 2013;see also Fig. 10 and Pohl and Beutel, 2005: Fig. 28).
The age of origin of Strepsiptera, the late Carboniferous (or earliest Permian), can be assessed based on the well-established sister group relationship with Coleoptera and the fossil record of beetles (e.g. Ponomarenko, 1969;Niehuis et al., 2012;Boussau et al., 2014;Beutel et al., 2018; see also ). The splitting event was dated as ca. 300 Mya in Misof et al. (2014) and as ca. 350 Mya in McKenna et al. (2019). Unambiguous beetle fossils from the Lower Permian are usually comparatively large and well-sclerotized species (e.g. Ponomarenko, 1969;Kirejtshuk et al., 2014). In contrast to this, strepsipterans, except for the minute primary larvae (Pohl, 2000(Pohl, , 2002Pohl et al., 2018) or the cephalothorax of endoparasitic females (e.g. Richter et al., 2017) and male puparia (e.g. , are weakly sclerotized and fragile. Consequently, the chances to be preserved as impression fossils are low. The vast majority of the fossils are amber inclusions, even though specimens in Eocene oil shale or Eocene limestone are also known (Kinzelbach and Pohl, 1994;Antell and Kathirithamby, 2016), although a single primary larva was found in Eocene brown coal from the Geisel valley in Germany (Kinzelbach and Lutz, 1985;Pohl, 2009). As already pointed out in Pohl and Beutel (2016) early evolutionary history of the order in the late Palaeozoic and earlier Mesozoic remains completely in the dark. The Carboniferous genus †Stephanastus, represented by the incompletely preserved holotype, was assigned to a new extinct order †Skleroptera by Kirejtshuk and Nel (2013). This taxon was interpreted as a subgroup of Coleopterida, supposedly the sister group of Strepsiptera + Coleoptera (Kirejtshuk and Nel, 2013). However, this hypothesized placement of the fossil was rejected as unfounded and very unlikely by Beutel et al. (2019). A recently discovered Cretaceous primary larva  is nearly identical with recent first instars. It underlines a remarkable evolutionary stasis in the order over ca. 100 million years, and it discards earlier alleged findings of strepsipteran immatures (e.g. , which very likely belong to the beetle family Ripiphoridae Batelka et al., 2018). As far as adults are concerned, the oldest known fossils differ only in details from the extant forms (e.g., Pohl and Beutel, 2016). Unless well-preserved impression fossils from the early Mesozoic or Permian are discovered, the morphological gap between Strepsiptera and Coleoptera will remain large .
Burmese Cretaceous amber has turned out to be a rich source of new extinct species of Strepsiptera and other insects. Since the description of the first fossil strepsipteran from Burmese amber in 2005, three new species have been discovered. However, except for one primary larva, only males have been found. The minute 1 st instar larva has revealed a Mesozoic origin of endoparasitism. Moreover, the amber fossils have contributed greatly to the understanding of the evolution and morphological diversity of the group in the late Mesozoic and Cenozoic. With the clarified position of †Protoxenos from Eocene Baltic amber as sister to all remaining strepsipteran groups, the area of origin of Strepsiptera remains still ambiguous. Terminal segments with external genitalia, dorsal view. Abbreviations: dpa, distal portion of paramere; ppa, proximal portion of paramere; pe, penis; pr, process; sVII-sIX, sternites VII-IX; tVII-tIX, tergites VII-IX; tl, transverse line; X, abdominal segment X. Scale bar 100 µm. Fig. 10. Strepsipteran phylogeny, strict consensus tree of 39 minimum length trees obtained with NONA; Bremer indices are given as bold numbers beneath branches; (1) Baltic amber (minimum age approx. 42-49 Ma, probable maximum age 54 Ma [Odin and Luterbacher, 1992;Ritzkowski, 1997]), (2) Burmese amber (98.79 AE 0.62 Ma [Shi et al., 2012;Poinar, 2018]), (3) Dominican amber (15-20 Ma [Iturralde-Vinent and MacPhee, 1996;Iturralde-Vinent, 2001]). [Colour figure can be viewed at wileyonlinelibrary.com]