Paraodontomma leptocristatum
Creators: name: Li, Yan-Da
name: Yamamoto, Shûhei
name: Huang, Di-Ying
name: Cai, Chen-Yang
Publisher: name: Zenodo
Year: 2021
Subjects: Biodiversity
Taxonomy
Animalia
Arthropoda
Insecta
Coleoptera
Ommatidae
Paraodontomma

description: Paraodontommaleptocristatum Li,Yamamoto & Cai sp.nov. (Figs. 1-8)
Material: Holotype, NIGP174676, adult male, covered with numerous bubbles.
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.
Diagnosis: Head (Figs. 2A, B, 5) subquadrate (elongate in P. burmiticum and P. szwedoi), without prominent posterior protuberances (with prominent protuberances above eyes in P. szwedoi, and also possibly in P. burmiticum; Fig. 10A). Pronotal disc without prominent ridges (Fig. 2C) (with longitudinal ridges in P. szwedoi; Fig. 10C). Elytra with indistinct longitudinal and transverse ridges (Figs. 2F, 6D) (strongly developed in P. burmiticum and P. szwedoi; Fig. 10I). Teeth along outer elytral margins not forming a wave pattern (Figs. 2E, 4D) (wavily arranged in P. burmiticum and P.szwedoi; Figs. 10F, 11C).
Description: Body length about 8.1 mm; maximum width about 3.6 mm between elytral margins in posterior ⅓ of body. Head and prothorax slender compared to compact unit formed by pterothorax and abdomen; shoulders pronounced. Integument covered with rounded tubercles and setae.
Head (Figs. 2A, B, 5, 6A) prognathous, constricted posteriorly to form a neck (though not well visible on the specimen); maximum width about 1.4 mm (including eyes); dorsal surface without prominent posterior protuberances. Compound eyes relatively large, protruding laterally. Antennal insertion area located anteriorly, anteromesad anterior margin of compound eyes, separated by more than one but less than two diameters of scapus. Antennal grooves absent on dorsal or ventral side. Postocular temples short.
Labrum transverse, with tuberculate dorsal surface and anterior margin densely set with setae (Fig. 3A). Antenna (Figs. 2A, 5) short, extending beyond anterior prothoracic margin when posteriorly directed, but not reaching posterior prothoracic margin, 11-segmented and submoniliform, with thin and short setae; antennomere 1 distinctly wider than other antennomeres, somewhat globular; antennomere 2 distinctly smaller, subquadrate, about as wide as long; antennomere 3 elongate,slightly widening distally;antennomeres 4-10 short, subequal in length; antennomere 11 spindle-shaped, tapering apically. Ratio of antennomere lengths: ∼ 3.4: 2.3: 3.4: 2.1: 2.1: 1.9: 1.8: 1.8: 1.9: 1.8: 2.9. Mandibles tridentate with vertically arranged teeth (Fig. 3A). Apical maxillary and labial palpomeres slender, not expanded, with a small cavity near apex (Fig. 3A). Prementum with deep posteromedian pit. Separate mentum present, transverse. Gular sutures not visible.
Pronotal disc (Fig. 2C) anteriorly distinctly narrower than head, posteriorly about as wide, roughly hexagonal, with rounded lateral edges; length about 1.3 mm; maximum width about 1.6 mm; lateral margin strongly serrated; anterolateral and posterolateral pronotal corners angles not projecting and rounded; surface without protuberances or furrows, evenly convex. Pronotal hypomeron narrow, separated from exposed propleuron by slightly curved suture (Fig. 1B). Pleurosternal suture straight (Fig. 1B). Prosternum (Fig. 2D) quadrangular, about as wide as long, very slightly narrowing anteriorly, with slightly convex anterior margin; prosternal process short and narrow, not reaching middle region of procoxae (Fig. 3B). Procoxal cavities contiguous posterior to prosternal process, posteriorly open (Fig. 3B). Procoxae globular to cone-shaped.
Shoulder region pronounced. Mesoscutellar shield not visible (hidden by incrustations). Mesopleura and mesoventrite not clearly visible, partly concealed by legs and partly blurred. Mesocoxal cavities contiguous (Fig. 3C). Mesocoxae similar to procoxae in size and shape. Metanepisternum large, broad anteriorly (Fig. 3E); exposed part of metepimeron moderately sized, visible between elytral epipleura and concave posterolateral metanepisternal edge (Fig. 3E). Metaventrite trapezoidal, strongly narrowing anteriorly, with distinct anteromedian projection; discrimen not visible, probably missing; metakatepisternal suture distinct and complete; metatrochantin distinctly exposed. Metacoxae medially contiguous, transverse, distinctly reaching beyond lateral margin of metaventrite and reaching elytral epipleura laterally; metacoxal plates absent but excavation for metafemur at rest present.
Elytra about 5.5 mm long, each 1.8 mm wide in middle region, with setae rooted on tubercles along longitudinal ridges (elytral veins) and small rounded scales on surface (Figs. 3F, 4I). Elytral disc separated into 2 × 2 squares by very indistinct longitudinal and transverse ridges (Fig. 6D). Epipleural rim moderately wide, about ⅓ of elytral width anteriorly, evenly narrowing posteriorly, without window punctures (Figs. 2E, 3D). Teeth along outer elytral margins present, but not forming a wave pattern.
Legs (Figs. 2D, E, G, 3F) slender; all three pairs similar in size and shape. Femora robust, widest in middle region. Tibiae about as long as femur, slender. Tarsi 5-segmented, cylindrical; with simple tarsomeres, without distinct brushes of adhesive microtrichia. Length ratio of protarsomeres: ∼ 1.8: 1.3: 1.0: 1.0: 1.1. Length ratio of mesotarsomeres:∼ 5.3: 1.5: 1.4: 1.1: 3.4. Length ratio of metatarsomeres: ∼ 6.7: 2.2: 1.4: 1.2: 3.8.
Abdomen broad, with five coplanar ventrites, separated by distinct grooves (Fig. 2H); ventrites 2-4 subequal in length; ventrites 5 about 1.3 times as long as ventrite 4, with broadly rounded apex (Fig. 3G). Aedeagus possibly trilobate, as shown in Fig. 6E.
Etymology: The specific name is derived from Greek “leptos”, thin, and Latin “crista”, crest, referring to the weak ridges on the elytra.
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).
DISCUSSION
Yamamoto (2017) placed Paraodontomma in the tribe Brochocoleini, and further suggested a close relationship between Paraodontomma and Odontomma. 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 (Li et al., 2021), 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 extant 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.
description: Published as part of Li, Yan-Da, Yamamoto, Shûhei, Huang, Di-Ying & Cai, Chen-Yang, 2021, New species of Paraodontomma from mid-Cretaceous Burmese amber with muscle tissue preservation (Coleoptera: Archostemata: Ommatidae), pp. 1-13 in Papéis Avulsos de Zoologia 61 on pages 3-10, DOI: 10.11606/1807-0205/2021.61.53, http://zenodo.org/record/5213117