Cretognathus, Yamamoto, 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

Genus Cretognathus gen. nov.

Figs 1C , 6 , 7 , 8 , 9 , 10

Type species.

Cretognathus minutissimus sp. nov., here designated.

Differential diagnosis.

Cretognathus gen. nov. is easily separated from all known extant ceratognathin genera from the Australian Region, except the extant Holloceratognathus passaliformis (Holloway), by the complete lack of modified scales on the dorsal surface (Figs 6A , 8 ). It can be distinguished from H. passaliformis by the absence of several derived features, such as widely explanate pronotal and elytral margins and distinctly paddle-shaped tibiae ( Holloway 2007). Cretognathus gen. nov. is somewhat similar to the recently described ceratognathin genus Oncelytris Li & Cai, 2023 (in Li Y-D et al. 2023) from Kachin amber in the absence of dosal scales (Li Y-D et al. 2023). However, the distinctly tuberculate elytra of Oncelytris are unusual in the family (Li Y-D et al. 2023), and Cretognathus gen. nov. apparently lacks such distinct tubercles on the dorsum of elytra.

Diagnosis.

Body very small, moderately elongate oval, rather flattened; body length well below 3.0 mm. Dorsal surface lacks neither modified scales nor setae (Figs 6A , 8 ). Head with broadly rounded anterior margin (Fig. 7A ); vertex simple, without a pair of protuberances (Fig. 7A ). Antenna non-geniculate, with three-segmented, rather strong club (Fig. 7A, B, D ). Mandibles small, inconspicuous, bearly visible dorsally, with sharply pointed apices (Fig. 7A-C ). Pronotum densely strongly punctate (Fig. 8A ); lateral pronotal margins smooth, with very narrow gutter along whole margins (Fig. 8A, D ). Prosternum biconcaved; prosternal process strongly narrowed and laminate between procoxae, resulting in subcontiguous procoxae (Fig. 10A ). Elytra smooth without distinct tubercles and evident rows of punctures (Fig. 8B, C, E ); lateral margin with narrow epipleural gutter. Outer protibial margin with one large, curved, and slender apical tooth (at), one large, curved, and slender mid-dorsal tooth 2 (mdt2), one smaller mid-dorsal tooth 1 (mdt1), and about 16 subequal smaller teeth (Fig. 9A, B ).

Included species.

Only Cretognathus minutissimus sp. nov.

Etymology.

The generic name is composed of the prefix Creto - from “Cretaceus” referring to the "Cretaceous age" of the extinct genus and the type genus Ceratognathus Westwood, 1838 of Ceratognathini . It is masculine in gender.

Systematic placement and comparison.

Cretognathus gen. nov. can be unambiguously assigned to the scarabaeoid family Lucanidae based on the moderately elongate and rather dorsoventrally flattened body shape, 10-segmented antennae with a relatively long antennomere 1 and three-segmented, non-coherent and lamellate apical club, 5-5-5 tarsal formula, protibial structures with characteristic outer teeth, strongly developed bisetose arolium between the pretarsal claws, and five visible abdominal ventrites (e.g., Ratcliffe 2002; Holloway 2007; Cai et al. 2017; Reid 2019). Furthermore, the new genus can be safely attributed to the plesiomorphic extant subfamily Aesalinae based on its non-geniculate antennae with a three-segmented club, complete eyes (not divided by ocular canthus), small mandibles, non-striate elytra, and strongly narrowed prosternal process with subcontiguous procoxae (e.g., Holloway 2007; Reid 2019). Among the three tribes of Aesalinae , Cretognathus gen. nov. is separated from Nicagini by a combination of the following character states (Paulsen 2005): complete absence of evident ground scales (setae) on the dorsum, much longer setae of the arolium, and particularly the features of the outer protibial edge (i.e., apical tooth (at) much narrower and pointed, mid-dorsal tooth (mdt2) much longer and slenderer, located anteriorly, mid-dorsal tooth (mdt1) narrower with acute apex, and the presence of subequal smaller teeth between apical tooth and mid-dorsal tooth 2) (see Fig. 15 for the comparison). Furthermore, the new genus is easily distinguished from Aesalini in having a strongly narrowed prosternal process and subcontiguous procoxae (see Huang et al. 2009). Overall, the general morphology of Cretognathus gen. nov. matches the remaining tribe Ceratognathini , particularly based on the structures of the protibial edges. In fact, the new genus has the typical pattern of protibial spines. Consequently, it is best placed within Ceratognathini .

Ceratognathini is a small tribe with only four extant genera (Table 1 ): Ceratognathus Westwood, 1838; Hilophyllus Paulsen & Mondaca, 2006; Holloceratognathus Nikolajev, 1998; and Mitophyllus Parry, 1843 ( Holloway 1998, 2007; Paulsen and Mondaca 2006; Paulsen 2013). In addition, the extinct genus Oncelytris from Kachin amber was very recently described (Li Y-D et al. 2023). Although these extant genera share externally similar habitus and body parts, they are mainly identified by the different structures and arrangements of the smaller protibial outer teeth-like spines ( Holloway 1998, 2007; Paulsen and Mondaca 2006). Unlike Holloceratognathus which has numerous variably sized small spines, the new genus has numerous small, equal spines elsewhere along the outer edges of the protibiae (cf. Holloway 1998, 2007; Paulsen and Mondaca 2006). It is somewhat difficult to compare the new taxon with the three other extant ceratognathinin genera in further detail based on the structures and arrangements of the protibial teeth, but Cretognathus gen. nov. has relatively large teeth-like spines between the base and mid-dorsal tooth 2 (i.e., mid-dorsal tooth 1, mdt1, in this paper) along with the similar condition in Ceratognathus . Nevertheless, the new genus cannot be placed in Ceratognathus due to the lack of a pair of protuberances or tubercles (raised shiny patches) on the dorsal side of the head ( Holloway 2007). More importantly, no ground setation or modified scales are found on the dorsal surface of Cretognathus gen. nov. (Fig. 8 ). This condition has never been observed in any recent ceratognathinin (Fig. 15 ), including the Ceratognathus species, with the only exception H. passaliformis (see also Bartolozzi et al. 2017). Interestingly, however, the other Kachin amber ceratognathinin Oncelytris also lacks such modified scales on the elytra (Li Y-D et al. 2023).

From all of the lucanid fossils so far described from Kachin amber, Cretognathus gen. nov. can be readily separated from Anisoodontus Wu, Tang & Peng, 2022 (in Wu et al. 2022) ( Lucaninae ) in having a much smaller body (11.80-17.36 mm, including mandibles, in Anisoodontus ), non-geniculate antennae, and short, inconspicuous mandibles (cf. Wu et al. 2022); from Protonicagus ( Aesalinae : Nicagini ) by non-crenulate pronotal lateral margins without a row of setae, larger head with non-enlarged preocular margin of the eyes, absence of any dorsal scales or ground setae, and presence of small subcontiguous teeth-like spines between the apical tooth and mid-dorsal tooth 2 (cf. Cai et al. 2017; this study); from Electraesalopsis Bai, Zhang & Qiu, 2017 (in Qiu et al. 2017) (subfamily incertae sedis) by the presence of much shorter, unmodified mandibles, much strongly developed antennal club, broadly rounded clypeal margin, distinctly shorter metatarsi, smaller body (5.6 mm, including mandibles, in Electraesalopsis ), and structures of the protibiae with their strongly developed apical teeth (cf. Qiu et al. 2017); from Oncelytris by the lack of a pair of not well-separated protuberances and tuberculate elytra. Consequently, the new genus can easily be distinguished from any fossil genera described from Kachin amber. Furthermore, Cretognathus gen. nov. is separated from the two undetermined provisional ceratognathins reported below, mainly by the shape of the protibiae and antennae (the latter can be applied only for Ceratognathini gen. et sp. indet. 1).

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 6. General habitus of Cretognathus minutissimus gen. et sp. nov., holotype, SEHU- 0000121206. A. Dorsal view; B. Ventral view. Scale bars: 1.0 mm.

Figure 7. Details of Cretognathus minutissimus gen. et sp. nov., holotype, SEHU- 0000121206. A. Head with right antenna, dorsal view; B. Head with right antenna, ventral view; C. Enlargement of A; D. Right antenna, ventral view. Abbreviations: a 1 - 10, antennomeres 1 - 10; ey, compound eye; lp 3, labial palpomere 3; md, mandible; mp 3 - 4, maxillary palpomeres 3 - 4; pn, pronotum; pom, preocular margin of head; prt, protarsus. Scale bars: 0.3 mm (A, B); 0.2 mm (C, D).

Figure 8. Details of Cretognathus minutissimus gen. et sp. nov., holotype, SEHU- 0000121206. A. Pronotum and elytral base, dorsal view; B. Left elytron, dorsolateral view; C. Punctation on elytra, pronotum, and scutellar shield; D. Left lateral margin of pronotum, dorsal view; E. Left elytral margin with elytral shoulder, dorsal view. Abbreviations: epg, epipleural gutter along outer margin of elytron; pg, gutter along outer margin of pronotum; pn, pronotum; sc, scutellar shield. Scale bars: 0.5 mm (A, B, E); 0.3 mm (D).

Figure 9. Details of Cretognathus minutissimus gen. et sp. nov., holotype, SEHU- 0000121206. A. Right protibia and protarsus with arrow showing row of small teeth between large spines along outer edge of protibia, dorsal view; B. Right protibia with arrow showing row of small teeth between large spines along outer edge of protibia, ventral view; C. Left mesotibia and mesotarsus, dorsolateral view; D. Left metatibia and metatarsus, dorsolateral view; E. Left mesotarsus, ventrolateral view; F. Left mesotarsus, dorsolateral view; G. Left metatarsus, dorsolateral view; H. Left metatarsus, ventrolateral view. Abbreviations: as, apical spur; at, apical tooth of protibia; cl, claw; el, elytron epg, epipleural gutter along outer margin of elytron; mds, mid-dorsal spines of meso- and metatibia; mdt 1 - 2, mid-dorsal teeth 1 - 2 of protibia; mst 1 - 5, mesotarsomeres 1 - 5; msti, mesotibia; mtt 1 - 5, metatarsomeres 1 - 5; mtti, metatibia; pt 1 - 5, protarsomeres 1 - 5; pti, protibia; ra, rod of arolium; sa, setae on rod of arolium. Scale bars: 0.3 mm (A, C, D); 0.2 mm (B, E - H).

Figure 10. Details of Cretognathus minutissimus gen. et sp. nov., holotype, SEHU- 0000121206. A. Prosternum with arrow showing subcontiguous procoxae, ventral view; B. Meso- and metathorax, ventral view; C. Abdomen, ventral view; D. Meso- and metathorax with arrow showing discrimen (median longitudinal sulcus) on metaventrite, ventral view. Abbreviations: msf, mesofemur; msv, mesoventrite; mtc, metacoxal cavity; mtf, metafemur; mtti, metatibia; mtv, metaventrite; pf, profemur; ps, prosternum; v 1 - 5, ventrites 1 - 5. Scale bars: 0.3 mm (A); 0.2 mm (B); 0.5 mm (C, D).

Figure 15. Details of extant species of Aesalinae. A, D, G, K. Nicagus japonicus (tribe Nicagini), male, showing head with left antenna in dorsal view (A), prosternal process (intercoxal process of procoxae) with subcontiguous procoxae in ventral view (D), right protibia in dorsal view (G), or right metatarsus in ventral view (K) based on two specimens; B, E, H, J, L. Ceratognathus cf. niger (tribe Ceratognathini), female, showing head with left antenna in dorsal view (B), prosternal process (intercoxal process of procoxae) with subcontiguous procoxae in ventral view (E), right protibia in dorsal view (H), pronotum and left elytron with scales (J), or left metatarsus in ventral view (L) based on same specimen; C, F, I. Aesalus asiaticus asiaticus (tribe Aesalini), female, showing left antenna in dorsal view (C), prosternal process (intercoxal process of procoxae) with separated procoxae in ventral view (F), or right protibia in dorsal view (I) based on two specimens. Arrows show either smooth (G) or serrate outer edge (H) between mid-dorsal tooth 1 and mid-dorsal tooth 2 of each protibia. Abbreviations: a 1, a 2, a 8 - 10, antennomeres 1, 2, 8 - 10; as, apical spur; at, apical tooth of protibia; cl, claw; ey, compound eye; md, mandible; mdt 1 - 2, mid-dorsal teeth 1 - 2 of protibia; mtt 5, metatarsomere 5; psp, prosternal process between procoxae; ra, rod of arolium; scl, scales on head, pronotum, and elytra; sa, setae on rod of arolium.