Revisiting the morphology of the Cretaceous ommatid beetle Clessidromma palmeri (Coleoptera: Archostemata: Ommatidae)

The morphology of Clessidromma palmeri Jarzembowski et al. from mid-Cretaceous Burmese amber is revised based on a reexamination of the type specimen. Contrary to previous studies, our observation confirms that Clessidromma has open procoxal cavities. The characters such as platform on the ventral side of head, modified metacoxae, and ridges on head and pronotum are suggested to be diagnostic characters for Clessidromma. Clessidromma zengi Kirejtshuk from the same deposit is transferred into a new genus, as Kirejtomma zengi comb. nov.


INTRODUCTION
Archostemata is a small suborder of Coleoptera, represented by only four extant families (excluding Jurodidae; Lawrence, 2016). Many Permian/ Triassic coleopterans (e.g., Permocupedidae, Tshekardocoleidae) were once placed in Archostemata due to their somewhat similar elytral structure (e.g., Ponomarenko, 1969;Kirejtshuk et al., 2014). A recent cladistic analysis, however, suggested that they are indeed stem-group beetles and should not be placed in Archostemata (Beutel et al., 2008). Ommatidae is a small family of Archostemata (McKenna et al., 2019), with three extant genera known to date (Hörnschemeyer & Beutel, 2016;Escalona et al., 2020). Numerous Mesozoic ommatids have been found across Gondwana and Laurasia (e.g., Ponomarenko, 1969;Tan & Ren, 2009;Ashman et al., 2015). However, many characters in these compression-impression fossils are either not preserved or hard to interpret. Based on our preliminary examination, the morphological cladistic analysis of Ommatidae by Tan et al. (2012) seems to contain many coding errors, which was probably at least partly caused by the comparatively strong taphonomic artefacts in compression-impression fossils. The recent findings of relatively well-preserved ommatids in mid-Cretaceous Burmese amber have greatly increased our knowledge on the morpho-logical disparity of this family (e.g., Liu et al., 2017;Jarzembowski et al., 2019;Li et al., 2020aLi et al., , 2021aTihelka et al., 2020). The morphological characters of the amber inclusions are less affected by taphonomic artefacts and could be crucial for further phylogenetic studies of Ommatidae. Jarzembowski et al. (2018) reported a new ommatid species in a new genus, Clessidromma palmeri Jarzembowski et al., from mid-Cretaceous Burmese amber. They mentioned that C. palmeri is unusual in Ommatidae in having tshekardocoleid-like fore legs. However, since the type specimen is strongly carbonized, it is extremely difficult to assess many of the characters of this species under incident light. After a detailed examination under fluorescence, it turns out that many structures were mistakenly illustrated in the original drawings, and some other important features were neglected in the original description by Jarzembowski et al. (2018). The clarified morphology of C. palmeri also permits a re-evaluation of the generic placement of an addition species previously assigned to Clessidromma (Kirejtshuk, 2020).
Photographs under incident light were taken with a Zeiss Discovery V20 stereo microscope. Widefield fluorescence images were captured with a Zeiss Axio Imager 2 light microscope or a Zeiss Lumar V12 stereo microscope combined with a fluorescence imaging system. In some cases, the amber piece was immersed in mineral oil (CAS 8020-83-5) to reduce refraction (Fu et al., 2021). Confocal images were obtained with a Zeiss LSM710 confocal laser scanning microscope, using 488 nm Argon laser excitation line. Images under incident light and widefield fluorescence were stacked in Helicon Focus 7.0.2 or Zerene Stacker 1.04. Confocal images were stacked in Helicon Focus 7.0.2. Images were further processed in Adobe Photoshop CC to enhance contrast.  Revised diagnosis: Head roughly hexagonal; dorsal surface with a transverse carina separating vertex and frons; medial portion of ventral surface forming a distinctly elevated platform. Pronotal disc with three longitudinal ridges; lateral edges dentate. Pro-and mesocoxae oblong. Metacoxae with a longitudinal carina medially.

Systematic
Remarks: Kirejtshuk (2020) merged the genus Lepidomma into Clessidromma. However, Lepidomma actually differs distinctly from Clessidromma (Li et al., 2020b). In C. palmeri, the medial portion of the ventral surface of the head forms a distinctly elevated platform, extending to the posterior end of the neck (Fig. 4A). This elevated platform is unknown in any other ommatids, and could be regarded as a diagnostic character of the genus. Such a structure is obviously absent in Lepidomma ( fig. 6A in Li et al., 2020b). The metacoxa of C. palmeri is somewhat aberrant (Figs. 4D, 5F). Though the exact structure of the metacoxa is hard to determine due to the poor preservation of the specimen, it surely lacks the posterior excavation for reception of metafemur, and therefore differs from most other ommatids and cupedids, including Lepidomma. Lepidomma is characterized by a special type of scales with distinctive ridges running down the sides ( fig. 4D in Li et al., 2020b), which is absent in C. palmeri. Besides, there are three longitudinal ridges on the pronotal disc of C. palmeri (Fig. 3B), while no such ridges are present in Lepidomma ( fig. 4C in Li et al., 2020b).  Head (Figs. 3A, 4A) prognathous, roughly hexagonal, constricted posteriorly to form a neck. Vertex and frons separated by a somewhat v-shaped transverse carina. Ventral surface of head forming a distinctly elevated platform medially, extending to posterior end of neck (Figs. 4A, 5A). Compound eyes entire and finely facetted, without interfacetal setae. Antennal insertion area located anteriorly, anteromesad anterior margin of compound eyes, separated by more than two but less than three diameters of antennomere 1. Antennae (Figs. 3A, B, 4A) 11-segmented, fili-moniliform, with thin setae on all segments, extending beyond anterior prothoracic margin, but not reaching posterior prothoracic margin; antennomere 3 about twice as long as 4. Mandibles (Fig. 5A) long, with three vertically aligned teeth. Maxillary and labial palps seemingly short (Fig. 5A). Posteromedian pit on prementum not observed. Separate mentum probably absent.
Remarks: Kirejtshuk (2020) reported a new species, Clessidromma zengi Kirejtshuk, from Burmese amber, and assigned it to genus Clessidromma. Clessidromma zengi, however, differs dramatically from C. palmeri, the type species of Clessidromma, and therefore is not a true member of Clessidromma. The head of C. zengi does not possess an elevated platform on the ventral side ( fig. 9C in Kirejtshuk, 2020), and its pronotum and elytra lack any distinct longitudinal ridges (figs. 9D, 10A in Kirejtshuk, 2020). Clessidromma zengi has typical metacoxae for ommatid and cupedid archostematans ( fig. 10D in Kirejtshuk, 2020). In addition, the lateral pronotal edges are dentate in C. palmeri (Fig. 3B), while they are relatively smooth in C. zengi. The pronotum of C. zengi is somewhat more similar to Lepidomma. However, it differs  Avulsos Zool., 2021;v.61: e20216195 6/8 from Lepidomma in lacking the ridged scales and elytral carinae. The character combination of C. zengi does not fit into any other known ommatid genera as well. Thus we establish a new genus here to accommodate C. zengi. Thus C. zengi is here transferred into the new genus as Kirejtomma zengi comb. nov.

Etymology:
The generic name is formed based on the generic name "Omma" and the surname of the Russian entomologist Alexander G. Kirejtshuk. The name is neuter in gender. Jarzembowski et al. (2018) placed Clessidromma in a newly established tribe, Clessidrommatini, mainly based on its prothoracic structures. According to Jarzembowski et al. (2018), the foreleg insertion of Clessidromma is similar to tshekardocoleids, where the insertions are near the middle of the prothorax. Escalona et al. (2020) further explicitly stated that Clessidromma has externally closed procoxal cavities. However, a detailed examination reveals that the prothoracic structure of Clessidromma is actually similar to other ommatids, rather than tshekardocoleids. The procoxal cavities of tshekardocoleids are externally closed and indeed located well away from the posterior edge of prothorax ( fig. 29 in Ponomarenko, 1969), whereas the mesoventrite of Clessidromma possesses well-developed procoxal rests (Fig. 4C), indicating that its procoxal cavities are in fact externally open. Besides, in Tshekardocoleidae, the procoxae are widely separated by the broad prosternal process, and the prosternal process extends far beyond the posterior end of procoxae. By contrast, Clessidromma has adjacent procoxae, and its prosternal process is reduced (Fig. 4B), which is typical in Ommatidae (Hörnschemeyer & Beutel, 2016).

DISCUSSION
Clessidromma superficially resembles Tetraphalerus Waterhouse in having a rather elongate body shape. The sharp edge of the ventral platform on head may possibly indicate the presence of a Tetraphalerus-like ventrolateral antennal groove. Clessidromma is additionally similar to Tetraphalerus in having longitudinal ridges on pronotum. However, Tetraphalerus is unique in Ommatidae (and differs from Clessidromma) in having antennomeres 5-11 without setae and mushroom-shaped tubercles (Beutel et al., 2008). The separate mentum present in Tetraphalerus and related fossils (Li et al., 2021b) is also not observed in Clessidromma. Considering the high morphological diversity found in fossil Ommatidae, the similarity between Clessidromma and Tetraphalerus might be merely homoplasious. The exact position of Clessidromma within Ommatidae requires further studies.