Palaeabraeus glabrus, Jan & ºimon-Pražák & Yamamoto & Lackner & Fikáček & Prokop & Michael & Caterino, 2024

Simon-Pražák, Jan, Yamamoto, Shûhei, Lackner, Tomáš, Fikáček, Martin, Prokop, Jakub & Caterino, Michael S, 2024, A bonanza of Cretaceous fossils provides insights into the evolution of antennal protection in clown beetles (Coleoptera: Histeridae), Zoological Journal of the Linnean Society (zlae 137) 202 (3), pp. 1-28 : 10-13

Palaeabraeus glabrus sp. nov.

( Fig. 8 )

Material: Kachin amber (c. 99 Mya). Amber piece dimensions: 5 × 2 × 2 mm. Amber clear with several insect fragments and air bubbles. Deposited at HUM ( SEHU-0000121213 ) .

Measurements: Head width: 0.23 mm; width between anterior pronotal angles: 0.25 mm; width between posterior pronotal angles: 0.5 mm; pronotal length: 0.23 mm; sutural elytral length: 0.6 mm; elytral width (in pair, across widest point): 0.56 mm.— Body: Oval, tapered anteriorly and posteriorly, strongly convex, impunctate, glabrous.— Head: Frons convex, weakly and sparsely punctate; frontal stria absent; narrow epistoma with fine, sparse setae; eye convex, coarsely faceted, anterior portion exposed laterally; labrum short, convex, bearing two dominant setae curving ventrad and several secondary setae; mandibles largely concealed, with a few setae on lateral margins; galeae exposed, densely setose; maxillary palps long, penultimate palpomere about third length of terminal; terminal palpomere swollen at base, tapered apically, shallowly concave on inner surface; labial palpi with apical palpomere long, slender, penultimate palpomere about half its length; antennal insertions slightly open anteriorly, under weak frontolateral bulge; scape short, weakly widening apically, curved posteriad below eye; pedicel cylindrical, almost as long as antennomeres 3 and 4 combined; antennomeres 3–8 widening slightly toward club, each with peripheral setae, 3–6 moniliform, 7 and 8 with sharper basal margins, each narrowed to apex; antennal club round, slightly tapered to apex, tomentose, without annuli.— Thorax: Pronotal disk convex, almost twice as wide as long, sides weakly curved, evenly tapered from base to apex; anterior emargination deep; anterior corners strongly downturned, acute; marginal pronotal stria present, distant from anterior margin, closer to lateral margins. Elytra strongly convex, apically truncate, glabrous, and impunctate, without distinct dorsal striae; epipleuron with single weak submarginal carina. Wings fully developed (protruding beyond elytra). Prosternum short, lacking distinct lobe; keel shallowly emarginate at base, with lateral striae narrowed at middle, widened to base and apex; prosternum incised on either side for reception of the antennal funicle; antennal cavities located anterolaterally, well defined by marginal ridge posteriorly and laterally. Mesoventrite rather long, more than half as long as intercoxal width, strongly developed anteriorly, with broadly acute median process inserting into base of prosternal keel; marginal mesoventral stria complete, broadly rounded; mesometaventral suture well developed; metaventrite slightly impressed along midline; postmesocoxal stria complete, strongly recurved behind coxa to mesepimeron; lateral metaventral stria not evident; metepisternum broadly exposed, tapered beneath anteriorly produced metepimeron.— Legs: Profemur slightly convex along inner margin, both inner and outer margins setose; protibia weakly curved, widened slightly to apex, setose and with one subapical denticle along outer margin; protarsal groove well developed; protibial spurs absent; mesofemur similar to profemur; mesotibia slender, widened slightly to apex, with fine setae along inner and outer margins, few stronger spines present near inner apical corners; metafemur and metatibia similar to those of mesothorax; tarsi 5-5-5, tarsomeres 1–4 similar in length, ultimate tarsomeres as long as tarsomeres 2–4 combined; protarsomeres 1–4 with expanded and flattened ventral setae (probable sexual dimorphism); all tarsomeres with numerous ventral setae (not just one apicoventral pair); tarsal claws simple.— Abdomen: Abdominal ventrite 1 flat between coxae, with slightly curved parallel striae from inner corner of metacoxa to ventrite 2; ventrites 2–5 short; propygidium shorter than pygidium, glabrous; pygidium slightly longer than basal width, weakly convex, apically rounded, impunctate.

Taxonomic assignment: The species can be assigned to the subfamily Abraeinae based on the lack of distinctive prosternal lobe ( Fig. 8B, D ), convex eyes ( Fig. 8E, F ) and fused antennal club ( Fig. 8A, B ). Abraeinae contain the following tribes: Abraeini , Acritini , Acritomorphini , Plegaderini , Teretriini , Trypanaeini , Trypeticini , and the extinct Pantostictini . Teretriini , Trypanaeini , and Trypeticini can be ruled out as they all represent groups with elongate cylindrical body shapes ( Wenzel 1944). Acritomorphini have an elongate body shape, long prosternum, anteriorly positioned antennal cavities, and annulate antennal club ( Wenzel 1944, Mazur 1997). Plegaderini have a flattened body shape and pronotum with a pair of longitudinal grooves laterally, often with a transverse pronotal stria connecting the grooves ( Wenzel 1944). Pantostictini likewise have a slightly flattened and rugosely punctuate body surface ( Poinar and Brown 2009). Acritini have their tarsal formula 5-5-4 ( Kryzhanovskij and Reichardt 1976). Therefore, based on the round and convex body shape, fused antennal clubs and the 5-5-5 tarsal formula ( Fig. 8B, F ) the species may be placed in the Abraeini tribe. Other characters supporting the position of the species within Abraeini are the presence and position of antennal cavities ( Fig. 8F ), emarginate basal pronotal margin receiving anterior projection of mesoventrite ( Fig. 8B, D ). Close relation to Abraeini was also indicated by all performed phylogenetic analyses. The main differences setting this species apart from recent Abraeini genera ( Abraeus Leach , Spelaeabraeus Moro , and Chaetabraeus Portevin ) are: oval body shape, relatively long prosternal process, absence of punctuation, and only slightly dilated protibia.

Diagnosis: Absence of dorsal striae and punctuation. Marginal mesoventral stria complete, broadly rounded; mesometaventral suture well developed; metaventrite slightly impressed along midline; postmesocoxal stria complete, strongly recurved behind coxa to mesepimeron; lateral metaventral stria not evident. Only Pseudacritus extinctus , described in this study, is similar in general body shape and some characters to this species. Palaeabraeus glabrus can be distinguished by the position of antennal cavities (mediolaterally as opposed to P. extinctus with anteriorly positioned cavities), absence of antescutellar stria and complete absence of punctuation.

Etymology: Latin ‘glabrus’ refers to the smooth cuticle of the species.

Figures

Figure 8. Palaeabraeus glabrus gen.et sp. nov.. A, dorsal view.B, ventral view. C, mouthparts lateral view.D, head, prosternal process and mesoventrite ventral view.E, frontal view.F, lateral view.Abbreviations:ac, antennal cavity; apm, anterior margin of prosternal process; ees, elytral epipleural stria; ga, galea; lp, labial palpus; lps, lateral stria on prosternal process; ls, labral setae; mms, marginal mesoventral stria; mp, maxillary palpus; pf, prosternal fissures for receiving antennal funicle; pms, postmesocoxal stria; ppm, posterior margin of prosternal process.