Ceratognathini gen. et sp. indet. 2, 2023

Yamamoto, Shuhei, 2023, The smallest stag beetles (Coleoptera, Lucanidae): hidden paleodiversity in mid-Cretaceous Kachin amber from northern Myanmar, Evolutionary Systematics 7 (2), pp. 211-235 : 211

Ceratognathini gen. et sp. indet. 2

Figs 1E , 13

Material examined.

1 ex. (sex undertemined), a nearly complete adult preserved in a flattened semicircular yellowish amber, approximately 16.7 mm × 6.6 mm × 2.5 mm in size (Fig. 1E ); specimen accession number SEHU-0000121208, housed in HUM. Although the specimen appears well-preserved (only slightly modified) with nearly a complete body, the left metatarsomeres beyond the second segment were lost (Fig. 13C ). More importantly, a thin air layer on the ventral surface of the body makes it difficult to observe the ventral body parts, such as the abdomen and antennae.

Locality and horizon.

Hukawng Valley (26°20'N, 96°36'E), Kachin State, northern Myanmar; unnamed horizon, mid-Cretaceous, Upper Albian to Lower Cenomanian.

Description.

Details of ventral side not uniformly observable. Body (Fig. 13A, B ) small, narrowly elongate, subparallel sided, 2.72 mm long (measured from anterior margin of head to apex of elytra). Color uniformly dark brown. Dorsal surface somewhat modified, probably due to the fossilization process, seemingly densely punctured, without neither apparent setae nor modified scales. Head (Fig. 13A-D ) small, transverse, 0.60 mm wide across eyes; vertex flattened. Compound eye (Fig. 13A-D , ey) large, entire, not divided by ocular canthus. Antennae and mouthparts not visible. Pronotum (Fig. 13A, D ) moderately transverse oval, with arcuate and very weakly crenulate lateral margins, 0.92 mm wide and 0.70 mm long (along midline). Elytra (Fig. 13A, C, E ) complete, narrowly elongate, nearly subparallel-sided, with left elytron 1.87 mm long (measured from pronotal posterior end to apex of elytron) and 0.45 mm wide; outer margins, particularly those of posterolateral ones, narrowly explanate. Legs (Fig. 13A-C, F, G ) relatively short, slender. Protibia (Fig. 13A, B, F ) slender, flattened, only weakly curved, gradually slightly widened towards apex, 0.60 mm (left) in length, with one large apical spur (see Fig. 13F , as); external margin armed with three strong teeth in apical half, i.e., apical tooth (at) and mid-dorsal teeth 1-2 (mdt1 - 2) (sensu Holloway 2007: fig. 1), with 5 short and subcontiguous tooth-like spines before mid-dorsal tooth 1, 3 between mid-dorsal tooth 1 and mid-dorsal tooth 2, and finally 4 between apical tooth and mid-dorsal tooth 2 (Fig. 13F , arrow). All tarsi (Fig. 13A-C, F, G ) 5-segmented, each slender and rather long. Arolium (Fig. 13F, G , ra, sa) developed, with rather long rod, bearing 2 bristles on its apex. Abdomen not well visible. Genitalia not visible.

Systematic placement and comparison.

An accurate definitive assessment of the systematic position of this fossil is difficult; there is no clear view of the ventral side due to a thin whitish air layer, which makes several important features unobservable, such as the abdominal segmentation, details of the antennae, and prosternal process (Fig. 13B ). Nevertheless, its putative placement in the lucanid tribe Ceratognathini is supported by the protibial structures, namely the protibial edges have a peculiar arrangement of spine-like teeth (Fig. 13F ). Overall, the other features, including the general habitus, head, pronotum, and 5-5-5 tarsal formula, generally agree well with the ceratognathins, particularly the extinct ones described above. Unlike Oncelytris , there is no distinct tubercles found on the elytra (see Li Y-D et al. 2023). Based on the structures of the protibial edges, this specimen cannot be assigned to Cretognathus gen. nov., " Ceratognathini gen. et sp. indet. 1," or any extant ceratognathin genus. However, this taxonomic consideration as a lucanid is provisional; further visualization and verification is needed for a definitive assignment.

Figures

Figure 1. General view of amber pieces with lucanid inclusions. A, B. Protonicagus mandibularis sp. nov., holotype, SEHU- 0000121205, showing dorsal (A) or lateral (B) views of the holotype; C. Cretognathus minutissimus gen. et sp. nov., holotype, SEHU- 0000121206; D. Ceratognathini gen. et sp. indet. 1, SEHU- 0000121207; E. Ceratognathini gen. et sp. indet. 2, SEHU- 0000121208. Scale bars: 3.0 mm (A, B); 1.0 cm (C); 5.0 mm (D, E).

Figure 13. Details of Ceratognathini gen. et sp. indet. 2, SEHU- 0000121208. A. General habitus, dorsal view; B. General habitus, ventral view; C. General habitus, lateral view; D. Head and pronotum, dorsal view; E. Left elytron, dorsolateral view; F. Left protibia and protarsus, ventral view; G. Left mesotibia and mesotarsus, dorsal view. Abbreviations: as, apical spur; at, apical tooth of protibia; cl, claw; ey, compound eye; mdt 1 - 2, mid-dorsal teeth 1 - 2 of protibia; mst 5, mesotarsomere 5; msti, mesotibia; pn, pronotum; pt 1 - 5, protarsomeres 1 - 5; pti, protibia; sa, setae on rod of arolium. Scale bars: 1.0 mm (A - C); 0.3 mm (D); 0.5 mm (E); 0.2 mm (F, G).