New species of Paraodontomma from mid-Cretaceous Burmese amber with muscle tissue preservation (Coleoptera: Archostemata: Ommatidae)

The third member of the extinct ommatid genus Paraodontomma is reported from mid-Cretaceous Burmese amber. Our observation confirms the transverse ridges on elytra as a diagnostic character for the genus. Paraodontomma leptocristatum sp.  nov. differs from previously reported congeners mainly in head subquadrate and without prominent protuberances, pronotal disc without prominent ridges, elytral ridges indistinct, and teeth along elytral margins not forming a wavy pattern. Musculature is preserved in the newly discovered specimen of P. leptocristatum, which further demonstrates the preservation potential and irreplaceable value of amber fossils.


INTRODUCTION
Ommatidae today is a small family in the beetle suborder Archostemata, represented by only three genera in the extant fauna (Hörnschemeyer & Beutel, 2016;Escalona et al., 2020). Recent transcriptomic analyses placed Ommatidae sister to the monospecific Micromalthidae, rather than of Cupedidae (McKenna et al., 2019), even though the two small families differ dramatically in their adult morphology. In contrast to the very low recent diversity, Ommatidae were much more diverse during the Mesozoic. Numerous fossil genera discovered across Europe and Asia (e.g., Tan & Ren, 2009) are listed in Kirejtshuk (2020). Among them, Paraodontomma Yamamoto was first reported by Yamamoto (2017) from mid-Cretaceous Burmese amber, represented by the type species P. burmiticum. Later Jarzembowski et al. (2018) described the second species in this genus, P. szwedoi. Yamamoto (2017) suggested that Paraodontomma is most similar to Odontomma Ren et al., a brochocolein beetle known from Cretaceous Yixian Formation in China (Ren et al., 2006).
Well preserved muscle tissues have been identified from insects in Eocene/Oligocene Baltic amber (Poinar-Jr. & Hess, 1982, 1985Grimaldi et al., 1994;Pohl et al., 2010), Miocene Dominican amber (Henwood, 1992a, b;Grimaldi et al., 1994), and Eocene Oise amber (Van de Kamp et al., 2014) (as discussed in Pohl et al., 2021). Recently, flight muscles were also reported from a wasp in the older Cretaceous Burmese amber (Grimaldi et al., 2019). In earlier studies of internal structures of extinct species, amber pieces with insects were cut open and examined with an electron microscope (SEM or TEM) (e.g., Poinar-Jr. & Hess, 1982, 1985Grimaldi et al., 1994). Even though this method revealed exceptional details, even at the subcellular level, the specimens were irreversibly destroyed. The development of micro-computed tomography (micro-CT) offers a great chance to investigate fossils non-destructively. The first nearly complete anatomical reconstruction of an insect fossil was conducted with the strepsipteran Mengea tertiana Grote from Baltic amber (Pohl et al., 2010). Later Van de Kamp et al. (2014) also successfully recovered the brain and flight muscles of a fossil wasp in Oise amber. ISSN On-Line: 1807-0205 ISSN Printed: 0031-1049ISNI: 0000-0004-0384-1825 In the present study we report the third species of Paraodontomma from Burmese amber. With the aid of micro-CT, fossilized musculature was discovered inside the beetle, which further demonstrates the potential information content of amber fossils.

MATERIAL AND METHODS
The holotype of Paraodontomma leptocristatum sp. nov. (Figs. 1-8) studied herein originated from amber mines near Noije Bum (26°20′N, 96°36′E), Hukawng Valley, Kachin State, northern Myanmar. The specimen is deposited in the Nanjing Institute of Geology and Palaeontology (NIGP), Chinese Academy of Sciences, Nanjing, China. The amber piece was trimmed with a small table saw, ground with emery paper of different grit sizes, and finally polished with polishing powder. The previously published holotype of P. szwedoi (Figs. 9-11; NIGP169926, incorrectly labeled as NIGP130418 in Jarzembowski et al., 2018) was also re-examined for comparison.
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 combined with a fluorescence imaging system. In some cases, the amber pieces were immersed in mineral oil . 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 with colour coding for depth in ZEN 2.3 (Blue Edition), or without colour coding in Helicon Focus 7.0.2. Microtomographic data were obtained with a Zeiss Xradia 520 Versa 3D X-ray microscope at the micro-CT laboratory of NIGP, and analyzed in VGStudio MAX 3.0. Scanning parameters were as follows: isotropic voxel size, 9.4355 μm; power, 4 W; acceleration voltage, 50 kV; exposure time, 1.5 s; projections, 2201. Images were further processed in Adobe Photoshop CC to enhance contrast.

Etymology:
The specific name is derived from Greek "leptos", thin, and Latin "crista", crest, referring to the weak ridges on the elytra.   Li, Y-D., et al.: New Paraodontomma fossil with muscle preservation Pap. Avulsos Zool., 2021;v.61: e20216153 7/13 The genus name Paraodontomma is an extended version of the genus name Omma Newman, which is a neuter noun in Ancient Greek, meaning eye. Thus, according to Article 30.1.2 of ICZN (1999), the gender of Paraodontomma should also be neuter, rather than feminine as originally claimed by Yamamoto (2017). Yamamoto (2017) placed Paraodontomma in the tribe Brochocoleini, and further suggested a close relationship between Paraodontomma and Odontomma.

DISCUSSION
Brochocoleini defined by Tan et al. (2012) is characterized by more than one row of window punctures on the elytral epipleura (e.g., fig. 73 in Hong, 1982; fig. 1 in Liu et al., 2017). However, our new observation revealed that window punctures are absent on the epipleural rim of Paraodontomma (Figs. 2E, 3D, 10F, 11C). Therefore, its tribal placement should be questioned and investigated in the future. Yamamoto (2017) ruled out a close relationship of the extinct genus to fossils with wide epipleural rims assigned to Tetraphalerus, based on the lack of characteristic antennal grooves in Paraodontomma. However, no antennal grooves have been validly observed in any of the Mesozoic "Tetraphalerus" fossils ,   leptocristatum sp. nov., holotype, NIGP174676, based on X-ray microtomographic reconstruction. Scale bar: 1.5 mm. Li, Y-D., et al.: New Paraodontomma fossil with muscle preservation Pap. Avulsos Zool., 2021;v.61: e20216153 9/13 and many of them have been transferred into genus Allophalerus Kirejtshuk just recently (Kirejtshuk, 2020). The overall shape of some Allophalerus is also similar to Paraodontomma. Therefore, it is ambivalent whether Paraodontomma is closer to Odontomma or Allophalerus based on currently available evidence. An eye-catching character of Paraodontomma is the presence of transverse ridges on the elytra, which are rarely seen in other Ommatidae and in Cupedidae. The transverse ridges, together with longitudinal ridges (elytral veins), partition the elytra into 2 × 2 squares, each containing four window punctures. These transverse ridges are rather prominent in P. burmiticum and P. szwedoi ( fig. 1 in Yamamoto, 2017;figs. 1-2 in Jarzembowski et al., 2018). Although these ridges are quite indistinct in P. leptocristatum sp. nov., the 2 × 2 squares are still discernable through the micro-CT slices (Fig. 6D). We thus suggest that the transverse ridges on elytra could potentially be an autapomorphy of genus Paraodontomma. The body and elytra of some Cupedidae and Ommatidae are decorated with different scales. Here confocal microscopy revealed a new type of small and rounded scales on the elytra of Paraodontomma (Figs. 3I, 11F). The outer elytral margins of P. burmiticum and P. szwedoi are armed with somewhat wavily arranged teeth (serration) (Figs. 10F, 11C; fig. 1A in Yamamoto, 2017), while in P. leptocristatum sp. nov. the arrangement of elytral teeth does not form a wave pattern (Figs. 2E, 4D). The head of P. burmiticum and P. szwedoi is relatively elongate, with the portion in front of eyes longer than the eye (Fig. 10A; fig. 2A in Yamamoto, 2017), while the head of P. leptocristatum sp. nov. is subquadrate, with portion in front of eyes shorter than eye (Fig. 5A).
Under micro-CT, the beetle from Burmese amber often appears as a subhomogenous highly-absorbing solid substrate or a void cavity (e.g., Jałoszyński et al., 2020). In contrast, micro-CT revealed fine internal softparts of our P. leptocristatum specimen (Figs. 6,8). The striated musculature could be clearly seen in the metathorax of the beetle (Figs. 6B, C), and correspond well to that of ex-  Li, Y-D., et al.: New Paraodontomma fossil with muscle preservation Pap. Avulsos Zool., 2021; v.61: e20216153 10/13 tant ommatid Tetraphalerus Waterhouse (Friedrich et al., 2009). The muscles showed no apparent shrinkage (similar to Grimaldi et al., 1994;but different from Henwood, 1992b), possibly suggesting a very rapid mummification. Unfortunately, due to the presence of taphonomic artefact and the limitation of resolution, we were only able to identify a few very large muscles. For example, Fig. 6C shows the largest muscle of metathorax, Musculus metanoto-sternalis, with one end attached on metaventrite. This muscle is present in Archostemata as a plesiomorphy but missing in many other groups. The minor muscles with more phylogenetic information were hard to discern in our specimen.

ACKNOWLEDGMENTS
We are grateful to Rolf G. Beutel for detailed and helpful discussion, Su-Ping Wu for technical help in micro-CT reconstruction, Yan Fang for technical help in confocal imaging, and Dao-Jun Yuan for help in inspecting the holotype of Paraodontomma szwedoi deposited at NIGP.