Platycretus muscularis Simon Prazak & Lackner, 2023

Simon Prazak, Jan, Fikacek, Martin, Prokop, Jakub & Lackner, Tomas, 2023, Under the Cretaceous bark: Fossil evidence for the ancient origin of subcortical lifestyle of clown beetles (Coleoptera: Histeridae), Arthropod Systematics & amp; Phylogeny 81, pp. 439-453 : 439

Platycretus muscularis Simon Prazak & Lackner sp. nov.

Type material.

Holotype specimen (1407/E), unknown sex, Northern Myanmar, inclusion in burmite, Cenomanian (ca. 99 Ma), amber piece somewhat opaque, with significant amount of debris and bubbles, ca. 6 ×4× 2 mm, synclusions: plant material, minute seeds, unidentified insect fragments.

Description.

Measurements. Head width: 0.2 mm, width between anterior pronotal angles: 0.41 mm, width between pronotal prosternal angles: 0.93 mm, pronotal length: 0.75 mm, elytral length: 1.11 mm, elytral width (across widest point): 0.93 mm. - Body. Rectangular, flattened, dorsal cuticle wholly covered with irregular prominent tubercles and depressions. Cuticle pitch black, legs chestnut brown. - Head. Frons with deep anteromedian depression, punctate. Supraorbital stria present, carinate. Frontal stria carinate, widely interrupted medially, extending to the clypeolabral suture, convergent anteriorly. Mandibles stout, massive, outer margin almost perpendicular, carinate. Labrum long, almost square-shaped, asetose, without punctures, appears to be fused with clypeus. Left mandible with acute prominent tip, rest of the mandibles invisible. Terminal maxillary palpomere thin and long, flattened. Remaining mouthparts unobservable. Scape curved, as long as antennomeres 2-8 combined, outer margin distinctly carinate. Pedicel slightly longer than antennomere 3. Antennomeres 3-8 short, becoming wider apically. Antennal club large, round, covered with a velvet-like white layer (short setation). Two intersegmental sutures of antennal club visible, straight. Eyes flattened. - Thorax. Pronotum rectangular, very wide, flat, with large depressions and irregular large shallow punctures of various sizes mostly in the anterolateral region. Anterior pronotal angles prominent, obtuse. Basal margin of pronotum significantly curved posteriorly. Lateral edges parallel with and subsequent to the lateral edges of elytra. Marginal pronotal stria complete. Outer lateral stria present as a short thin basal fragment reaching approximately 1/3 of pronotal length apically. Inner lateral pronotal stria deeply impressed, complete laterally, absent anteriorly (interrupted at the anterolateral angles of the pronotum). Surface between marginal and lateral pronotal striae convex. Scutellar shield small, triangular. Elytra without punctuation, with extensive depressions covering its entire dorsal area. Marginal elytral stria deeply impressed, complete, wider anteriorly. Dorsal striae I and II faintly present on posterior half of elytra. Elytral epipleural stria complete, carinate. Prosternal lobe large, broad, delicately punctuate, with two thin transverse lines. Anterior prosternal margin almost straight medially. Marginal prosternal stria not distinguishable. Prosternal process obfuscated by debris. Antennal cavity present, partially closed by prosternal alae. Mesoventrite with irregular depressions, surface structures unclear. Meso-metaventral area with large depression, most likely caused by external factors (and hence likely representing an artefact), making the striation on mesoventrite and metaventrite indiscernible. Metaventrite with shallow punctures. Metepisternum depressed and irregularly sculptured. - Abdomen. First visible abdominal ventrite square-shaped, with deep punctures separated by their diameter. Rest of abdomen missing. - Legs. Profemur flat. Protibia flat and very broad, with 6 short and thick denticles on outer margin. Protarsal groove present, faintly S-shaped. Protibial spur well developed. Protarsus short. Mesofemur massively thickened, triangular in cross section. Mesotibia relatively short (shorter than mesofemur), flat and very broad (broader than protibia), with short thick denticles along its outer margin. Mesotarsus not visible. Metafemur very thickened, triangular in cross section. Metatibia largely similar to mesotibia but not as wide, with short thick denticles and numerous shorter setae along the outer margin. Large accumulation of short slender denticles present apically around the metatibia-metatarsal articulation. Metatarsus long (about 3/4 of the metatibial length).

Taxonomic assignment.

The present species undoubtedly belongs to the subfamily Histerinae due to the combination of the following characters: asetose and impunctate labrum (Fig. 1D ), well-developed prosternal lobe, antennal cavity partially closed by prosternal alae (Fig. 1D, E ).

Platycretus muscularis was resolved as sister group to Haeteriinae in the phylogenetic analyses. They share several synapomorphies - very wide profemur and long, square shaped labrum which appears to be fused with clypeus (see Fig. 5C ). However, the species lacks other defining characters of the Haeteriinae - namely the robust triangular scape. Some characters important for tribal assignment (e.g., meso- and metaventrite) are obfuscated.

Etymology.

The Latin adjective muscularis (muscular) refers to the thickened meso- and metafemora most likely accommodating a large amount of muscle tissue.

Subfamily Dendrophilinae Reitter, 1909

Without tribal placement

Figures

Figure 1. Platycretus muscularis Simon Prazak & Lackner, gen. & sp. nov. A dorsal view; B amber specimen; C mouthparts; D frontal view; E foreleg; F middle leg; G hind leg; H cross section at the breakage point - posterior part.

Figure 5. Maximum parsimony analysis of the position of known fossil species of the Histeridae. A the complete tree showing the position of the fossils and the current subfamily classification of the lineages; B Dendrophilinae part of the tree with mapped characters; C Histerinae part of the tree with mapped characters. Bold taxa = fossils, red taxa = fossils described in this paper. For characters see Zhou et al. (2020).