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.

Archostemata, Burmese amber, Cretaceous, Omma, Ommatidae

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 1966, 1969; Tan 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, 2020; Kirejtshuk 2020).

Figure 1. 

General habitus of Omma liassicum Crowson deposited at British Museum of Natural History, under incident light. A. Holotype, I.11095a; B. Holotype, I.11095b; C. Paratype, I.11070a; D. Paratype, I.11070b; The body length is about 15 mm in both specimens.

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.

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). 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 Helicon Focus 7.0.2 and Adobe Photoshop CC. Images were further processed in Adobe Photoshop CC to enhance contrast.

Order Coleoptera Linnaeus, 1758

Suborder Archostemata Kolbe, 1908

Family Ommatidae Sharp & Muir, 1912

Omma forte Li & Cai, sp. nov.

http://zoobank.org/5ECF739A-7BF6-4114-A95C-4B151973BFD1

Figures 2, 3, 4

Material examined

Holotype , NIGP176634.

Etymology

The specific name is from the Latin ‘fortis’, meaning strong, referring to the robust appearance of the species.

Locality and horizon

Amber mine located near Noije Bum Village, Tanai Township, Myitkyina District, Kachin State, Myanmar; unnamed horizon, mid-Cretaceous, Upper Albian to Lower Cenomanian.

Figure 2. 

General habitus of Omma forte sp. nov., holotype, NIGP176634, under incident light. A. Dorsal view; B. Ventral view; Scale bars: 2 mm.

Description

Body comparatively wide, about 9.3 mm long and 3.8 mm wide, tuberculate, with thin setae and scales.

Head (Fig. 3A, C) prognathous, seemingly wider than long. Neck region constricted, broad. Compound eyes comparatively large, protuberant. Antennal insertions located at anterior corners of head, separated by more than three diameters of antennomere 1. Subantennal grooves absent. Frontoclypeal suture absent. Labrum (Fig. 4A) transverse, with dentate anterior margin. Antenna (Fig. 3A) long, extending beyond base of prothorax when posteriorly directed, 11-segmented and filiform. Mandibles (Fig. 4B) tridentate with vertically aligned teeth. Maxillary and labial palps short, not reaching the level of the eyes when extending posteriorly (Figs 3C, 4C); apical maxillary and labial palpomeres probably expanded and securiform.

Figure 3. 

Details of Omma forte sp. nov., holotype, NIGP176634, under widefield fluorescence. A. Head, dorsal view; B. Prothorax, dorsal view; C. Head, ventral view; D. Hind leg, ventral view; E. Elytral base, dorsal view; F. Elytral apex, dorsal view; G. Hind wing, dorsal view; H. Hind wing, ventral view. Abbreviations: an1–3, antennomeres 1–3; el, elytron; ey, compound eye; md, mandible; mtf, metafemur; mttb, metatibia; mtts1, metatarsomere 1; pc, procoxa; pf, profemur; pn, pronotum; wc, wedge cell. Scale bars: 500 μm.

Pronotal disc (Fig. 3B) widest in the anterior third, slight narrowing posteriorly; anterior corners rounded; lateral edges with distinct tooth-like tubercles (dentate) (Fig. 4D).

Figure 4. 

Details of Omma forte sp. nov., holotype, NIGP176634, under confocal microscopy, with the relative positions of the images labeled in Suppl. material 1. A. Mouthparts, posterodorsal view; B. Mouthparts, anterior view; C. Mouthparts, ventral view; D. Prothorax, dorsal view, showing the gradual change from rounded tubercles to teeth (arrowheads); E. Scutellum, dorsal view; F. Elytron, dorsal view; G. Scales, tubercles, and window punctures on elytron, dorsal view; H. Metacoxa, ventral view; I. Abdomen, ventral view. Abbreviations: el, elytron; lb, labrum; lbp, labial palp; md, mandible; mtc, metacoxa; mxp, maxillary palp; pn, pronotum; sc, scutellum; v4,5, ventrites 4,5. Scale bars: 300 μm.

Scutellar shield (Fig. 4E) subtrapozoidal, wider posteriorly. Elytra (Figs 2, 3E, F) with ten rows of window punctures; lateral edges with distinct tooth-like tubercles (dentate); surface with ribbed slender scales (Fig. 4G); punctures in the ninth row distinctly transverse; interspace between eighth and ninth puncture rows thickened (Fig. 2). Metaventrite narrowed anteriorly; discrimen present; metakatepisternal suture well developed (Fig. 4H). Metacoxae (Fig. 4H) strongly transverse with subconical mesal projections, contiguous, extending laterally to meet elytra, with weak coxal plates.

Hind wings (Fig. 3G, H) well developed. CuA forked; CuA₁ fused with MP₃₊₄; CuA₃₊₄ fused with CuP+AA₃; wedge cell present.

Legs long and slender. Tibiae with at least one well-developed spur. Tarsi long and slender; metatarsus especially long; metatarsomere 1 about as long as metatibia (Fig. 3D). Pretarsal claws simple.

Abdomen with five coplanar ventrites, separated by distinct grooves (Fig. 4I).

Remarks

The new species can be confidently assigned to Omma. The length of maxillary and labial palps is a key diagnostic character separating Beutelius from Omma (Escalona et al. 2020). Omma forte sp. nov. has short maxillary and labial palps, not reaching the level of the eyes, which is in accordance with Omma. The hind 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 (CuA₃₊₄) is fused with CuP+AA₃, closing the wedge cell, and the anterior branch of CuA (CuA₁) is fused with MP₃₊₄ (Fig. 3G), which matches perfectly with the venation of Omma stanleyi (fig. 64 in Escalona et al. 2020; fig. 4B in Lawrence et al. 2021), while the CuA of Beutelius is unforked (figs 67–68 in Escalona et al. 2020) or with an incomplete anterior branch only (fig. 4A in Lawrence et al. 2021).

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 depression 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).

Figure 5. 

General habitus of Omma cf. manukyani, NIGP176635, under incident light. A. Dorsal view; B. Ventral view. Scale bars: 1.5 mm.

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.

Figure 6. 

Details of Omma cf. manukyani, NIGP176635, under widefield fluorescence (A, D, G) or confocal microscopy (B, C, E, F, H, I), with the relative positions of the confocal images labeled in Suppl. material 1. A–C. Head, dorsal view (A, B) or ventral view (C); D–F. Prothorax, dorsal view (D, E) or ventral view (F); G–I. Elytra, dorsal view. Abbreviations: an1, antennomere 1; el, elytron; ey, compound eye; lb, labrum; lbp, labial palp; md, mandible; mxp, maxillary palp; pc, procoxa; pn, pronotum; ps, prosternum; v4,5, ventrites 4,5. Scale bars: 600 μm (A, D, G); 300 μm (B, C, E, F, H, I).

The original confocal data are available in the Zenodo repository (https://doi.org/10.5281/zenodo.5693059).

We are grateful to Rong Huang for technical help in confocal imaging. We also thank Thomas Hörnschemeyer and André Nel for their valuable comments on an earlier version of this paper. Financial support was provided by the Second Tibetan Plateau Scientific Expedition and Research project (2019QZKK0706), the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB26000000 and XDB18000000), and the National Natural Science Foundation of China (41688103).