New species of Omma Newman from mid-Cretaceous Burmese amber (Coleoptera, Archostemata, Ommatidae)

A new fossil species of the extant archostematan genus Omma Newman, O. forte sp. nov., is reported from mid-Cretaceous amber from northern Myanmar. The extinct ommatid genus, Cionocups Kirejtshuk from the same deposit, is considered as a junior synonym of Omma, and C. manukyani is transferred to Omma, as O. manukyani comb. nov. A key to species of Omma from Burmese amber is also provided.


Introduction
Ommatidae is a small family in the beetle suborder Archostemata (Hörnschemeyer and Beutel 2016). In a recent phylotranscriptomic study, this family has been recovered as the sister group of Micromalthidae, rather than the superficially similar-looking Cupedidae (McKenna et al. 2019), so it cannot be included in Cupedidae as a subfamily (for alternative interpretations, see Kirejtshuk 2021). Traditionally, two extant genera were recognized in Ommatidae, i.e., Omma Newman from Australia and Tetraphalerus Waterhouse from South America (Lawrence 1999). Recently, the comparatively diverse genus Omma was split into Omma sensu stricto and Beutelius Escalona et al. (Escalona et al. 2020), partly based on the cladistic analysis in Hörnschemeyer (2009). Omma has relatively short maxillary and labial palps, and the gulamentum is not depressed, while Beutelius has longer maxillary and labial palps, and the gulamentum is depressed anteriorly.
The fossil record of Omma is relatively abundant (as listed by Kirejtshuk 2020). The earliest putative Omma fossils date back to the Late Triassic (Crowson 1962;Fig. 1). Jurassic and Cretaceous Omma fossils have been reported from numerous localities across the continents of Europe and Asia (e.g., Ponomarenko 1966Ponomarenko , 1969Tan et al. 2012;Cai and Huang 2017). Recently, well-preserved Omma fossils have also been reported from the mid-Cretaceous Burmese amber. Based on our observation, Omma represents a relatively common genus in Burmese amber (compared with other ommatid genera). To date, three Omma species have been established based on fossil material from Burmese amber (Jarzembowski et al. 2017(Jarzembowski et al. , 2020Kirejtshuk 2020).
In this study, we describe a new species of Omma in mid-Cretaceous Burmese amber. The placement of some other previously published Omma and Omma-related fossils are also reviewed based on our new observations.

Materials and methods
The Burmese amber specimens studied herein (Figs 2-6) originated from amber mines near Noije Bum (26°20'N, 96°36'E), Hukawng Valley, Kachin State, northern Myanmar. The specimens are deposited in the Nanjing Institute of Geology and Palaeontology (NIGP), Chinese Academy of Sciences, Nanjing, China. The amber pieces were trimmed with a small table saw, ground with emery paper of different grit sizes, and finally, polished with polishing powder.
Photographs under incident light were mainly taken with a Zeiss Discovery V20 stereo microscope. Widefield fluorescence images were mainly captured with a Zeiss Axio Imager 2 light microscope combined with a fluorescence imaging system. Confocal images were obtained with a Zeiss LSM710 confocal laser scanning microscope, using the 488 nm Argon laser excitation line (Fu et al. 2021 (Kirejtshuk, 2020) comb. nov.
Key to Omma species in mid-Cretaceous amber from northern Myanmar  Description. Body comparatively wide, about 9.3 mm long and 3.8 mm wide, tuberculate, with thin setae and scales.
wing venation (especially the branching pattern of CuA) is another important character differentiating the two genera. In most beetle fossils preserved in amber, the hind wings are hidden by the elytra, and are thus not available for taxonomic purpose. Fortunately, the hind wings are partly exposed in the holotype of O. forte. In O. forte, the posterior branch of CuA (CuA 3+4 ) is fused with CuP+AA 3 , closing the wedge cell, and the anterior branch of CuA (CuA 1 ) is fused with MP 3+4 (Fig. 3G)

Discussion
Extant Omma stanleyi and Omma fossils from Burmese amber are characterized by the dentate anterior margin of labrum (Escalona et al. 2020;Figs 4A, 6C). Such a character is not detected in any other ommatids in Burmese amber we examined (though it would be hard to confirm the state in other compression fossils). Thus, this character could possibly be an autapomorphy of Omma and the closely related Beutelius. The presence of scales (ribbed scale-like setae) has been suggested as a diagnostic feature separating Beutelius from Omma (Escalona et al. 2020). In the newly discovered fossil O. forte, the setae on elytra are relatively slender, which is kind of similar to that of extant O. stanleyi. However, under confocal microscopy, the ribs could be clearly seen on the flat setae of O. forte (Fig. 4G), suggesting the interrelationships among Omma and Omma-like species might be more complicated.
Numerous Mesozoic fossils have been assigned to the genus Omma. Unfortunately, as noted by Escalona et al. (2020), some key diagnostic characters (e.g., anterior de-pression in the gulamentum) are not easily available for many compression/impression fossils, making it hard to validate their generic attribution. Escalona et al. (2020), nevertheless, confirmed the placement of several fossil Omma species, including the Late Triassic O. liassicum Crowson. However, the key characters such as mouthparts and gulamentum are not well-preserved in O. liassicum as well. Besides, in its holotype, the propleuron is likely to be separated from the prosternum by a distinct suture (Fig. 1B). As such, we think O. liassicum cannot be confidently differentiated from Beutelius (and even some more distantly related genera, e.g., Bukhkalius Kirejtshuk & Jarzembowski;Li et al. 2021). Kirejtshuk (2020) created the genus Cionocups to accommodate a new species from Burmese amber, Cionocups manukyani. He noted that Cionocups is similar  to Cionocoleus, an extinct genus found in the Cretaceous of Eurasia. Cionocoleus shares a similar morphology with extant Omma, except for the lack of window punctures on the elytra (Tan et al. 2007). Cionocups, however, is clearly different from Cionocoleus in having clear window punctures on the elytra, though somewhat smaller than that of Omma. Thus, Cionocups is more similar to extant Omma, rather than Cionocoleus. Cionocups manukyani differs from extant O. stanleyi in antennae longer and with serration (teeth) along the lateral sides of pronotum and elytra (Kirejtshuk 2020). However, in some fossil Omma species, the antennae can also be elongated. For example, the antennae of O. lii and O. forte also reach the elytral base when posteriorly directed (Fig. 2). Besides, our newly reported O. forte also possesses teeth along the sides of pronotum and elytra (Fig. 3B, E, F). Detailed observations under confocal microscopy suggest that the difference between teeth and rounded tubercles is not always distinct, with a set of transitional states between the two extremes (Figs 4B, 6E). Therefore, we suggest that the character combination of C. manukyani is not unique enough for a separate genus. Here Cionocups is considered as a junior synonym of Omma and C. manukyani is transferred to the genus Omma, as Omma manukyani comb. nov.