Ceratognathini gen. et sp. indet. 1, 1843

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. 1

Figs 1D , 11 , 12.

Material examined.

1 ex. (sex undertemined), a complete adult preserved in a narrowly elongate yellowish amber, approximately 17.1 mm × 6.9 mm × 3.8 mm in size (Fig. 1D ); specimen accession number SEHU-0000121207, housed in HUM. The specimen is poorly preserved. The body is noticeably deformed and squashed, although the amber is transparent without pigmentation (Fig. 11 ). Due to the heavily modified condition of the specimen, it was impossible to make a detailed comparison or accurate measurements of each body part; consequently, only the body length was measured.

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 well observable. Body (Fig. 11 ) small, narrowly elongate, subparallel sided, 2.78 mm long (measured from anterior margin of head to apex of elytra). Color uniformly dark brown. Dorsal and ventral surfaces heavily modified, probably due to the fossilization process, without neither setae nor modified scales. Head (Fig. 12A ) very small, transverse. Antenna (Fig. 12B-D ) seemingly 10-segmented (provisional interpretation shown in Fig. 12B, D ) with three-segmented small club (Fig. 3A ). Pronotum (Fig. 11A ) slightly wider than long, with arcuate lateral margins. Procoxal cavities (Fig. 11B ) probably subcontiguous. Elytra (Fig. 11A ) complete, narrowly elongate, nearly subparallel-sided. Legs (Figs 11 , 12B, E-G ) relatively short. Protibia (Figs 11 , 12B, E, F ) slender, flattened, moderately curved, gradually widened apically, with one large apical spur; 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 about 5 short and subcontiguous tooth-like spines before mid-dorsal tooth 1, 5 between mid-dorsal tooth 1 and mid-dorsal tooth 2, and finally 5 between apical tooth and mid-dorsal tooth 2 (Fig. 12B , arrow). All tarsi (Fig. 12B, E, G ) 5-segmented, each slender and rather long. Arolium (Fig. 12E, G , ra, sa) developed, with short rod, bearing 2 bristles on its apex. Abdomen not well observable. Genitalia not visible.

Systematic placement and comparison.

It is challenging to assign the new material to Lucanidae unambiguously based on the available morphology. I could not observe the details of the abdomen to clarify if it has five-free ventrites or clear antennal segmentation. These are important diagnostic features that define the family. However, it is best placed in Lucanidae , more specifically the tribe Ceratognathini , based on the general habitus, 5-5-5 tarsal formula, antennae appearing 10-segmented (if correct) with three-segmented apical club, and the structures of legs including protibial outer edges with well-developed arolium between the pretarsal claws (e.g., Ratcliffe 2002; Holloway 2007; Reid 2019). The new specimen lacks modified scales and setae on the dorsal surface, as in Oncelytris and the two other Kachin amber ceratognathins recorded here. The extinct taxon is treated provisionally herein as " Ceratognathini gen. et sp. indet. 1" and can be differentiated from Cretognathus gen. nov. by having a much smaller head and antennal club, more slender tarsi, and different protibial edges (slightly different arrangement of subcontiguous tooth-like spines and much wider mid-dorsal teeth). Moreover, this new specimen does not seem to be congeneric with " Ceratognathini gen. et sp. indet. 2" by the distinctly smaller head and protibial morphology (shape, size, location, and number of the subequal tooth-like spines). Nevertheless, it is difficult to distinguish it from Oncelytris , due to the poor state of preservation of the fossil. In fact, the dorsal and ventral surfaces of " Ceratognathini gen. et sp. indet. 1" are greatly distorted, preventing the accuate observation of the head and elytra whether it possesses tubercles and protuberances on the dorsum or not (cf. Li Y-D et al. 2023). However, it is also noted that the protibiae of " Ceratognathini gen. et sp. indet. 1" is evidently arcuate, whereas those of Oncelytris are linear, suggesting that they may belong to different genera.

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 3. Details of Protonicagus mandibularis sp. nov., holotype, SEHU- 0000121205. A. Head with left antenna, dorsal view; B. Head, dorsolateral view; C. Pronotum, left lateral half, dorsal view; D. Pronotum, dorsolateral view; E. Elytral base with shoulder, dorsolateral view. Abbreviations: a 1 - 10, antennomeres 1 - 10; ey, compound eye; lb, labrum; lp 3, labial palpomere 3; md, mandible; mp 3 - 4, maxillary palpomeres 3 - 4; pn, pronotum; pom, preocular margin of head; rsp, row of setae along outer margin of pronotum. Scale bars: 0.3 mm (A, B); 0.5 mm (C, D, E).

Figure 11. General habitus of Ceratognathini gen. et sp. indet. 1, SEHU- 0000121207. A. Dorsal view; B. Ventral view. Scale bars: 1.0 mm.

Figure 12. Details of Ceratognathini gen. et sp. indet. 1, SEHU- 0000121207. A. Head, dorsal view; B. Right antenna and apical part of right protibia with protarsus, ventral view; C. Right antenna, ventrolateral view; D. Left antenna, lateral view; E. Right protibia and tarsus, ventrolateral view; F. Right protibia, dorsal view; G. Mesotarsi and metatarsi, ventrolateral view. Abbreviations: a 1 - 10, antennomeres 1 - 10 (putative interpretation); as, apical spur; at, apical tooth of protibia; cl, claw; hd, head; mdt 1 - 2, mid-dorsal teeth 1 - 2 of protibia; mst 5, mesotarsomere 5; msti, mesotibia; mtt 1 - 5, metatarsomeres 1 - 5; mtti, metatibia; pn, pronotum; pt 1 - 5, protarsomeres 1 - 5; pti, protibia; ra, rod of arolium; sa, setae on rod of arolium. Scale bars: 0.2 mm (A - F); 0.3 mm (G).