Protonicagus mandibularis, 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

publication ID	https://dx.doi.org/10.3897/evolsyst.7.104597
publication LSID	lsid:zoobank.org:pub:921F39DA-40BB-4796-8E36-2194D1E9A138
persistent identifier	https://treatment.plazi.org/id/9689A395-8B60-46ED-BFC0-9E87158D2CA6
taxon LSID	lsid:zoobank.org:act:9689A395-8B60-46ED-BFC0-9E87158D2CA6
treatment provided by	Evolutionary Systematics by Pensoft
scientific name	Protonicagus mandibularis
status	sp. nov.

Treatment

Protonicagus mandibularis sp. nov.

Figs 1A, B View Figure 1 , 2 View Figure 2 , 3 View Figure 3 , 4 View Figure 4 , 5 View Figure 5

Material examined.

Holotype (sex undertemined), a complete adult preserved in a somewhat cuboid yellowish amber with spherical upper surface, approximately 7.2 mm × 5.4 mm × 3.1 mm in size (Fig. 1A, B View Figure 1 ); specimen accession number SEHU-0000121205, housed in HUM. The holotype specimen is well preserved, but noticeable pigmentation inside the amber makes detailed observation and photography difficult. In addition, the ventral surface of the lucanid is covered with a thin whitish layer, which makes detailed observation even more difficult (Fig. 2B View Figure 2 ).

Differential diagnosis.

Protonicagus mandibularis sp. nov. is most similar to Protonicagus tani Cai, Yin, Liu & Huang, 2017, also from Kachin amber, based on external morphological features in having a similar body size (3.62 versus 3.71 mm in P. tani ), preocular margin (sensu Holloway 2007: fig. 3) of the eyes strongly enlarged and produced laterally, pronotal lateral margin rounded and strongly crenulate with a row of setae, similar structures and arrangements of outer teeth along the protibiae with strong spines in basal half, legs with short pretarsal claw, and each posterior margin of abdominal ventrites 2-4 armed with a row of villiform teeth. However, the new species is easily distinguished from the latter by its general habitus (narrowly elongate versus rounded and oval in P. tani ), much larger non-acute apices of mandibles (acute in P. tani ), dorsal surface densely covered with thin and long ground setae (rather than short scales as in P. tani ; Cai et al. 2017: fig. 2B-D), much wider antennomeres 2-7, and obviously longer narrower metatarsi (see further details in Cai et al. 2017).

Diagnosis.

Body small (ca. 3.6 mm), moderately elongate, subparallel sided (Fig. 2A, B View Figure 2 ). Dorsal surface covered with thin and long setae, lacking modified scales (Fig. 3 View Figure 3 ). Antenna with spherical to transverse antennomeres 2-7 and three-segmented small club (Fig. 3A View Figure 3 ). Mandibles shorter than head, mostly visible dorsally, rather strongly curved and flattened, with rounded apices and nearly truncate apico-inner margins (Fig. 3A, B View Figure 3 ). Pronotum strongly and very densely punctate (Fig. 3C, D View Figure 3 ); lateral pronotal margins weakly crenulate, each with a row of setae (Fig. 3C, D View Figure 3 , rsp). Outer protibial edge with one large apical tooth (at), one large and one smaller mid-dorsal teeth (mdt1 - 2), and less than ten generally subequal smaller tooth-like spines (Fig. 4A, B View Figure 4 ). Metatarsi long and slender, but shorter than metatibiae (Fig. 2 View Figure 2 ).

Description.

Body (Fig. 2 View Figure 2 ) elongate oval, small, 3.62 mm long (measured from apex of clypeus to apex of elytra), 1.45 mm wide, moderately covex dorsoventrally. Color uniformly dark brown; antennae and mouthparts slightly paler. Dorsum (Figs 2A View Figure 2 , 3 View Figure 3 ) without modified scales, densely deeply punctate, setiferous punctures on pronotum and elytra large, each bearing suberect thin seta.

Head (Fig. 3A, B View Figure 3 ) small, rather strongly transverse, shorter than half pronotal length and narrower than half pronotal width, 0.44 mm long (measured from apex of clypeus to anterior margin of pronotum) and 0.57 mm wide across eyes; ventral side not well visible; vertex (Fig. 3A View Figure 3 ) densely punctate; preocular margins (Fig. 3A View Figure 3 , pom) conspicuous, strongly enlarged, protruding anterolaterally. Compound eye (Fig. 3A, B View Figure 3 , ey) relatively large, entire, not divided by ocular canthus. Antenna (Fig. 3A View Figure 3 , a1-10) 10-segmented, non-geniculate, with three-segmented weakly lamellate club, 0.66 mm long (left antenna); antennomere 1 (scape) elongate, approximately 2.7 times longer than wide, sparsely bearing nine long and thin bristles, not covered with many setae; antennomere 2 (pedicel) very small, spherical, wider than long, with probably only a single bristle, attached to tip of scape; antennomeres 3-6 evenly weakly widened apicad; antennomere 3 small, as long as wide, narrower than preceding antennomere, weakly dilated apically; antennomere 4 moderately transverse, slightly wider than preceding antennomere; antennomere 5 rather strongly transverse, wider than preceding antennomere; antennomere 6 strongly transverse, moderately wider than preceding antennomere; antennomere 7 strongly transverse, slightly shorter than preceding antennomere; antennomeres 8-10 forming small and relatively loose club, strongly asymmetrical, each not coherent. Mandibles (Fig. 3A, B View Figure 3 , md) mostly visible dorsally, slightly asymmetrical, small, shorter than head, bent strongly inward, dorsoventrally flattened and widened, not extending beyond maxillary palps, with rounded, non-acute apex and nearly truncate apico-inner margin. Labrum (Fig. 3A View Figure 3 , lb) small, transverse, with nearly truncate apex. Clypeus with produced anterior margin. Maxillary palpus (Fig. 3A, B View Figure 3 , mp3-4) 4-segmented, moderately long, slender; palpomere 2 small; palpomere 3 spindle-shaped, widest in middle, about half length as terminal palpomere, with a long bristle near apex on outer margin; palpomere 4 very long, slender, fusiform, slightly bent inward near base, narrower than preceding palpomere. Labial palpus (Fig. 3B View Figure 3 , lp3) probably three-segmented, long; palpomere 3 very long, slender, fusiform, slightly curved.

Pronotum (Figs 2A View Figure 2 , 3C, D View Figure 3 ) transverse, widest in middle, 1.39 mm wide and 1.06 mm long (along midline), dorsally convex, with weak basolateral constriction; lateral margin (Fig. 3C, D View Figure 3 ) broadly rounded, modetately crenulate along whole margin, equipped with a row of ventrolaterally-directed long setae; surface simple and even; anterior corners slightly produced with obtuse angles, whereas those of posterior ones right-angled. Procoxal cavity externally closed behind. Prosternum short, seemingly strongly transverse; prosternal process between procoxae very narrow, resulting in subcontiguous procoxae (Fig. 5A, C View Figure 5 , arrow).

Elytra (Figs 2A, C View Figure 2 , 3E View Figure 3 ) complete, narrowly elongate, nearly subparallel-sided in anterior three-quarters, with left elytron 2.24 mm long (measured from pronotal posterior end to apex of elytron) and 0.70 mm wide; lateral margins serrulate, each with a row of setae; surface smooth without tubercles, but densely covered with deep dense setiferous punctures uniformly, seemingly not forming longitudinal rows (Fig. 3A View Figure 3 ). Mesocoxal cavities narrowly separated. Metaventrite (Fig. 5 View Figure 5 , mtv) transverse, much longer than mesoventrite; surface generally densely punctured, with short but prominent discrimen (median longitudinal sulcus; see Fig. 5 View Figure 5 , arrow).

Legs (Figs 2 View Figure 2 - 5 View Figure 5 ) relatively short. Protibia (Fig. 2A, B View Figure 2 ) robust, flattened, weakly curved, gradually widened apically, with one large apical spur; external margin armed with three strong teeth in apical half, i.e., one large, broadly pointed apical tooth (at), one large, broadly pointed mid-dorsal tooth 2 (mdt2), and one smaller but still prominent mid-dorsal tooth 1 (mdt1) (sensu Holloway 2007: fig. 1), with 9 short, acute, and generally subequal smaller tooth-like spines (8 subcontiguous ones between base and mdt1, 1 between mdt1 and mdt2, and 0 between mdt2 and at, see arrows on Fig. 4A, B, E View Figure 4 for the latter condition). Procoxa widely transverse. Mesotibia with three small mid-dorsal spines at middle (Fig. 4H View Figure 4 , mds), with paired apical spurs (Fig. 4C, G, H View Figure 4 , as). Metacoxae (Fig. 5 View Figure 5 , mtc) transverse, subcontiguous. Metatibia (Fig. 4B View Figure 4 ) slender, only slightly longer than metatarsus, with three small mid-dorsal spines located at slightly beyond middle apically (Fig. 4D, F View Figure 4 , mds) and paired prominent apical spurs (Fig. 4D, F View Figure 4 , as). All tarsi 5-segmented, each slender and rather long; tarsomere 1 longer than tarsomere 2, tarsomeres 2-4 subequal in length, tarsomere 5 longest, as long as tarsomeres 2-4 combined (Fig. 4A, C-H View Figure 4 ). Pretarsal claw (Fig. 4A, C-H View Figure 4 , cl) short, simple, moderately curved, shorter than tarsomeres 3 and 4 combined. Arolium (Fig. 4C, D, G, H View Figure 4 ) developed, with short rod (Fig. 4D View Figure 4 , ra), bearing 2 bristles on its apex (Fig. 4D View Figure 4 , sa).

Abdomen (Figs 2B View Figure 2 , 5B View Figure 5 ) with 5 visible ventrites (v1 - 5); central part rather swelled above; each ventrite strongly transverse; ventrites 2-4, at least, each armed with a row of large villiform teeth along posterior margin; ventrite 5 with posterior margin broadly arcuate.

Genitalia not visible.

Etymology.

The specific name " mandibularis " (Latin mandibulum + - aris) refers to its unusual shape of the mandibles.

Locality and horizon.

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

Systematic placement and comparison.

Protonicagus mandibularis sp. nov. can be assigned to the scarabaeoid family Lucanidae based on the narrowly elongate body shape, 10-segmented antennae with three-segmented, non-coherent, lamellate apical club, anteriorly projecting and rather developed mandibles, 5-5-5 tarsal formula, protibial structures with characteristic outer teeth, 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 fossil taxon is assigned to the ancestral extant subfamily Aesalinae based on its non-geniculate antennae, complete eyes (not divided by ocular canthus), small mandibles, non-striate elytra, and strongly narrowed prosternal process with subcontiguous procoxae. It is easily separated from the New Zealand endemic Lampriminae , Dendroblax earlii White with a Nicagus -like body and short mandibles in both sexes, by the considerably different locations and shapes of the protibial outer teeth, distinctly smaller body size, glabrous metaventrite, and much longer linear antennomere 1 (cf. Fujita 2010; Bartolozzi et al. 2017). Within Aesalinae , the new species is similar externally to the extinct genus Protonicagus , also from Kachin amber (tribe Nicagini ), including the similar body size, notably enlarged lateral margins of preocular margin of the eyes, rounded lateral margin of the pronotum with strong crenulation bearing a row of setae, similar structures of the protibiae, and abdominal ventrites 2-4 each armed with a row of villiform teeth ( Cai et al. 2017). However, I found several distinct features that do not match P. tani , the sole member of Protonicagus : 1) much narrower, elongate body; 2) clearly larger non-acute mandibles; 3) absence of dense, inclined, linear scales on the pronotum and elytra, but covered with thin, long setae; 4) much wider antennomeres 2-7; and 5) obviously longer and narrower metatarsi with longer tarsomeres 1 (cf. Cai et al. 2017). From the sole extant genus Nicagus in Nicagini , P. mandibularis sp. nov. is ruled out mainly by the characters mentioned in Cai et al. (2017) and described above. Therefore, I prefer to tentatively include it in Protonicagus because of the general morphological similarity.

Given the distinct morphological differences in lucanid species due to sexual dimorphism, it is difficult to determine whether such differences are interspecific characters within the genus or just individual variation between the different sexes of the same species. I have considered these morphological differences to be interspecific rather than sexual dimorphism and it is hereby described as a new species. This conclusion is supported by the marked morphological differences. For example, the extant sole nicagin genus Nicagus has generally similar body shapes in both sexes ( Paulsen and Smith 2005), unlike the great differences seen in the fossil presented here and P. tani . Similarly, the body surface of P. mandibularis sp. nov. lacks short distinct scales, which should be considered an interspecific difference rather than a sexual difference.