Placatister cascus, 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 : 18-20

Placatister cascus sp. nov.

( Fig. 12 )

Material: Kachin amber (c. 99 Mya). Amber piece dimensions: 9 × 5 × 1 mm. Amber clear with several insect fragments and air bubbles. Deposited at MEBHK ( P1425/E ) .

Measurements: Head width: 0.36 mm; width between anterior pronotal angles: 0.56 mm; width between posterior pronotal angles: 0.92 mm; pronotal length: 0.58 mm; sutural elytral length: 0.8 mm; elytral width (in pair, across widest point): 1.2 mm.— Body: Round, strongly dorsoventrally flattened (probably partially compressed). Cuticle regularly punctuate. Colour reddish brown to black.— Head: Frons flat, wide and punctuate, strongly, squarely margined along inner edge of each eye, with a fine stria along their inner edges continuous with a complete, transverse frontal stria. Frontoclypeal suture not visible. Epistoma constricted, medially ridged, sides receding below broad frontal shelf. Labrum trapezoidal (width: length ratio 5: 1), emarginate anteriorly, with a pair of short setae at anterolateral corners, lacking punctuation. Mandibles wide and thin, basally with weakly dentate incisor edge, and a long, acute tip. Terminal maxillary palpomere wide and round; other ventral mouthparts damaged and/or missing. Antenna inserted under frontal shelf, strongly dorsoventrally flattened, possibly partially compressed. Scape short, widened, and curved to apex. Pedicel wide, subcylindrical, as long as antennomeres 3–5 combined. Antennomeres 3–7 similar in shape and size, short, wider than long (width: length ratio 3: 1). Antennomere 8 adjacent to the antennal club, 2.5× wider than antennomere 7. Antennomeres 9–11 form a wide, flat, and round antennal club with well-defined sutures.— Thorax: Pronotal disc flat. Anterior margin widely emarginate, with fine marginal pronotal stria; not evident on lateral or basal margins. Basal margin broadly rounded, almost angularly projecting. Surface of pronotal disc densely punctuate, punctures separated by 1–2× their diameter. Scutellar shield not visible, obscured by bubbles. Elytra truncate, without dorsal striation, epipleural stria complete; elytral disc appears weakly depressed in humeral corners (possibly due to damage). Elytral punctuation with the same pattern as in pronotum. Prosternal lobe rather short, subtruncate anteriorly, set off from keel by pair of deep, setose foveae, weakly conjoined at their bases; prosternal keel flat, short, abruptly descending at front into two large approximate prosternal foveae, with fine longitudinal striae; finely punctate between; sides of prosternum widely incised for passage of antennal funicle, with deep channel reaching middle of base of prosternal keel; sides of prosternal keel appear concave. Mesoventrite wide and short, obscured by bubbles, probably finely punctuated. Meso-metaventral suture visible as a fine line. Metaventrite punctuate, with lateral metaventral stria directed posterolaterally to hind coxae. Other striation not visible. Metepisternum broadly exposed, rectangular.— Legs: Profemur flatted, slightly widened. Protibia narrow and smooth, without dentation, few longer setae present along inner apical edge, without tibial spur. Tarsal groove present as an impressed line on the outer edge (along the entire protibial length). Tarsi 5-5-5; protarsomere 5 as long as protarsomeres 2–4 combined. Claws well developed, about half the length of protarsomere 5. Mesofemur swollen, mesotibia narrow. Three prominent setae can be seen in the median part of the left mesotibial outer margin. Mesotarsus identical to protarsus. Metafemur slender, slightly curved and swollen in the median part. Metatibia slender, with a couple stronger spines at outer apical corner; metatarsus identical to protarsus.— Abdomen: First visible abdominal sternite without visible striation. All visible abdominal sclerites finely punctuated. Propygidium very short, almost entirely covered by elytra. Pygidium weakly convex, about two-thirds as long as wide.

Taxonomic assignment: The species displays several highly plesiomorphic characters, such as the absence of a prosternal lobe and antennal cavity, as well as in the shape of the antennal club with distinct segments ( Fig. 12D ). The flattened antenna with wide and short antennomeres 3–7 is especially unique among the entire Histeridae family. Prosternal structure, and particularly the prosternal foveae ( Fig. 12B, G ), are plausibly similar to those of some Saprininae (e.g. Tomogenius ) ( Lackner 2010). Nevertheless, the homology of these structures is highly unlikely. The presence (if true) of an antennal receptacle along the sides of the prosternal keel would also be similar to that of Saprininae. However, other characters, such as the distinctly sutured antennal club ( Fig. 12D ), flattened body shape ( Fig. 12C ), and simple leg shape with slender tibiae without denticles ( Fig. 12B ), would be incompatible with this placement. The phylogenetic analyses did not resolve the position of this taxon, as it nested in various positions throughout the Histeridae tree (but outside the Saprininae and Histeromorphae clades). This species most likely represents a primitive stem lineage of Histeridae , but a subfamily or tribal assignment is impossible based on the observable characters.

Diagnosis: Head with a complete frontal stria. Antennomeres short and wide, antennae dorsoventrally flattened. Prosternal process with a pair of setose foveae in apical part and with concave sides. Cuticle dorsally and ventrally punctuate. Legs narrow and smooth. The species may remotely resemble the flat Olexum complanatum or the round-bodied species of Anapleus . It is distinguished by the wide and segmented antennal club and the wide and short segmented flagellum.

Etymology: This species represents a plesiomorphic set of characters, and the species’ name ‘cascus’ means ancient/primitive.

Figures

Figure 12. Placatister cascus gen.et sp. nov.. A, dorsal view.B, ventral view. C, lateral view.D, antennal club. E, mouthparts. F, frontal view.G, prosternal keel.H, antenna.Abbreviations:ees, elytral epipleural stria; fs, frontal stria; pf, prosternal foveae.