Prodigister tricostatus, 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 : 16-18

Prodigister tricostatus sp. nov.

( Fig. 11 )

Material: Kachin amber (c. 99 Mya). Amber piece dimensions: 15 × 8 × 2 mm. Amber clear with several insect fragments and air bubbles. Deposited at HUM ( SEHU-0000121220 ) .

Dimensions. Head width: 0.4 mm; width between anterior pronotal angles: 0.42 mm; width between posterior pronotal angles: 0.7 mm; pronotal length: 0.5 mm; sutural elytral length: 0.95 mm; elytral width (in pair, across widest point): 0.75 mm.— Body: Elongate, subparallel-sided, almost rectangular in shape. Pronotum irregularly sculptured. Elytra with several very prominent costae. Dorsal cuticle with fine setation. Thorax and abdomen strongly compressed (deformed) laterally.— Head: Frons and clypeus with sparse setae. Frontoclypeal suture absent. Frontal stria complete, weakly carinate. Labrum long (width: length ratio is 3: 2), plurisetose and punctuate. Mandibles with several prominent setae on outer margin and a blunt subapical tooth, prostheca well developed. Terminal maxillary palpomere elongate and pointed, as long as palpomeres 2–3 combined. Scape straight and parallel-sided with several setae apically. Pedicel short and bulbous, as long as antennomeres 3–4 combined, wider basally than distally. Antennomere 3 short (about half of antennomere 4 length). Antennomeres 3–8 becoming wider apically. Antennal club round and flattened, setose, without annuli.— Thorax: Pronotum with irregular sculpturing, mostly convex medially, with a pair of large depressions posterolaterally and one mediobasally, elevations on either side aligned with longitudinal elytral ridges; pronotal sides margined, sinuate, widening posterad, narrowed anterad; marginal pronotal striae not evident. Basal pronotal margin rounded. Marginal prosternal stria complete. Pronotum sparsely covered with fine, short setae. Scutellar shield not visible. Elytra with fine punctuation and vestiture (setae 5× as long as in pronotum). Dorsal elytral striation not visible. Epipleural stria complete. Outer elytral margin carinate and weakly elevated. Medioanterior part strongly depressed. Sutural area strongly elevated forming a keel that is sharp on anterior two-thirds, thence gradually diminishing posterad; each elytron with strong lateral longitudinal costa, rather sharp in anterior half of elytra, becoming lower and broader apically; elytral apices obscured by cracks and evidently distorted. Elytral epipleural stria complete. Ventral body surface strongly compressed, creating a (probably artificial) keel. Prosternal lobe short and broad, punctuate. The presence and character of antennal cavity cannot be determined due to the ventral deformation. Prosternal keel without apparent striation or punctuation. Mesoventrite strongly compressed ventrally, striation not visible. Metaventrite finely punctuate, lateral metaventral stria visible on anterior third.— Legs: Profemur very wide, flattened. Protibia strongly dilated with denticles along its outer margin. Protarsal groove present. Protibial spur not developed. Terminal protarsomere as long as protarsomeres 1–4 combined. Mesofemur flattened, broad basally and becoming narrower apically. Mesotibia approximately half of the mesofemoral length, slightly dilated with thick setae along the outer margin, diminishing in size basally. Terminal mesotarsomere as long as mesotarsomeres 2–4 combined. Metafemur broad. Metatibia narrow with only two to three thick setae apically. Metatarsus identical to mesotarsus.— Abdomen: First visible abdominal sternite long and wide, finely punctuate. Rest of the abdomen hardly observable due to cracks and contamination. Pygidium weakly costate.

Taxonomic assignment: Few characters that would serve as diagnostic at higher levels are clearly visible in this specimen. Furthermore, it is considerably deformed by lateral compression, especially ventrally, and sternal characters are severely altered. The strongly convex pronotum and costate elytra, being reasonably bilaterally symmetrical, show no signs of being artefacts of deformation, and distinguish this taxon from any known histerids, extinct or extant. These are not similar to costae in onthophiline genera, and this genus could not be placed there. The antennal club is not easily visible, but it is sufficiently clear that it lacks distinct annuli, as are seen in a number of other Cretaceous ‘basal grade’ histeroids. The strongly expanded protibiae are a remarkably common feature among Cretaceous taxa, seen in Olexum , Yethiha , Druantia , Platycretus , and Promyrmister ( Zhou et al. 2019, Caterino 2021, Simon-Pražák et al. 2023b). Of these, the species bears some characters linking it to Platycretus : very wide and flat profemur, and mesofemur longer than mesotibia (Simon-Pražák et al. 2023b). However, the plurisetose labrum, as well as fused antennal club, would suggest a closer relationship to Dendrophilinae taxa— Olexum , Yethiha , or Druantia ( Caterino 2021, Simon-Pražák et al. 2023b). The position of this genus could not be sufficiently resolved via phylogenetic analyses either, as it nested in various positions in Histeridae outside the Antigracilinae and Abraeinae clades. As some of the key characters for higher taxonomic assignment are not visible due to deformation, we leave this genus unplaced to a higher taxon for now, and hope that better preserved specimens materialize to reveal more phylogenetically significant characters.

Diagnosis: Prolonged, parallel-sided body shape with several elytral dorsal costae. Dorsal cuticle setose. Labrum long and multisetose. Head without a frontoclypeal suture, frontal stria complete. Antennal club fused, without annuli. Basal pronotal margin widely rounded. Wide and flat profemur. Protibia strongly dilated with denticles. Mesotibia narrow and spinose, metatibia narrow and almost smooth. The species is unlike any known recent or extinct Histeridae species and is easily, unequivocally identifiable.

Etymology: The species name ‘tricostatus’ refers to the three dorsal costae on the elytra.

Figures

Figure 11. Prodigister tricostatus sp. nov.. A, dorsal view.B, lateroventral view.C, lateral view. D, elytra.E, frontal view.F, mouthparts. Abbreviations:C1–C3, costae 1–3; ds, dorsal setae; gal, galea; lab, labrum; man, mandible; mp, maxillary palpus; mt, mesotibia; pt, protibia.