Olexum complanatum 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

Olexum complanatum Simon Prazak & Lackner sp. nov.

Type material.

Holotype specimen (1408/E), female, Northern Myanmar, inclusion in burmite (ca. 99 Ma), amber piece clear, ca. 11 ×5× 1 mm, with unidentified insect fragments and pieces of debris.

Description.

Measurements. Head width: 0.2 mm, width between anterior pronotal angles: 0.23 mm, width between posterior pronotal angles: 0.4 mm, pronotal length: 0.2 mm, elytral length: 0.53 mm, elytral width (across widest point): 0.48 mm. - Body shape elongate oval, flattened, dorsally convex. Head separated from body, located next to the specimen. Cuticle chestnut brown to black. Pronotum with a fairly dense punctuation, punctures (diameter 5 microns) separated approximately by 2 times their diameters. Punctuation of elytra not visible due to white coating covering dorsal surface. Body venter without visible punctuation. Legs light, brown. Female genitalia exposed. - Head almost quadrate, clypeus massive, its length approximately 2/3 of frontal length. Clypeus with irregular sparse prominent setae. Setation of frons not observable. Supraorbital area with a row of regularly separated prominent setae (length 50 microns). Frontal and supraorbital striae indiscernible. Clypeolabral suture well visible. Gular sutures narrowly separated. Labrum short, rectangular, dorsally multisetose, with two pairs of long labral setae located at anterolateral corners, intermingled with shorter sparse setae. Right lateral side of labrum with clearly protruding labral fringe. Right mandible bidentate. Subapical tooth prominent and acute, subparallel with outer mandibular margin. Left mandible and rest of the mouthparts missing. Antennal scape short, thickened, apically truncate with several setae. Pedicel at least 3/4 of scape length, thickened, oval, with individual setae. Antennomeres 3-8 approximately of the same length as scape and pedicel combined. Antennomere 8 significantly wider than antennomeres 3-7. Antennal club slightly longer than antennomeres 3-8, strongly flattened with dense setation. Eyes small, almost completely flattened, almost invisible from dorsal view. - Thorax. Pronotum rectangular, ca. 1.5 × wider than long across midline. Lateral stria present, cariniform. Lateral pronotal margin slightly bisinuate, anterior angles rather broadly and obliquely truncate. Pronotal disc asymmetrical (possibly a teratological specimen?). Right lateral area with a single elongate furrow like depression subparallel to the lateral pronotal margin. Left lateral area with two furrow-like depressions, both parallel to the lateral pronotal margin. Single depression at the left anterior angle. Scutellar shield invisible. Elytra longer than wide. Elytral humeri not prominent. Outer lateral margin keel-like, next to it a longitudinal furrow present, reaching approximately 2/3 of elytral length apically. Next to the furrow two faint longitudinal tubercles present medio-apically. Rest of elytral disk without sculpture, elytral suture elevated in apical third. Propygidium almost entirely covered by elytra. Elytral epipleuron with keel-like structures and depressions. Prosternum with a short and broad prosternal lobe with antennal fissures laterally. Basal half of prosternal process between procoxae parallel sided, thence strongly diverging apically. Carinal prosternal stria absent on prosternal apophysis, parallel sided between procoxae, thence strongly diverging anteriorly, concurrent with the prosternal margin. Lateral prosternal stria absent. Antennal cavity present, open anteriorly. Mesoventrite very broad, subtrapezoidal, 4 × wider than long. Marginal mesoventral stria faint. Mesoventral disc glabrous. Meso-metaventral suture present. Metaventrite very large and broad, 1.5 × wider than long. Marginal metaventral stria absent, postmesocoxal stria faint. Lateral metaventral disc not clearly separated from metaventral disc. Metepisternum fully covered by elytral epipleuron. Lateral disc of metaventrite glabrous. - Abdomen. First visible abdominal ventrite rectangular, glabrous, twice as long as wide, without striae. Rest of the abdomen telescopically inflexed. Propygidium almost entirely covered with elytra. Pygidium short, triangular, bistriate. Female genitalia exposed, apex of valvifers cut off during polishing process. - Legs. Profemur flattened. Protibia strongly dilated, rounded, protibial spur massive; smaller apical spur present underneath it. Outer protibial margin slightly inwardly arcuate in the first anterior forth. Rest of the outer protibial margin round, with regularly separated short denticles, diminishing in size in basal direction, absent in the inwardly arcuate part. Inner protibial margin with a row of setae. Protarsal groove deep, well developed. Terminal protarsomere as long as protarsomeres 1-4 combined. Mesofemur flattened. Outer margin of mesotibia with a row of regularly separated denticles, diminishing in size in posterior direction. Mesotibia with tarsal groove. Mesotarsus with 5 tarsomeres. Mesotarsal claws well developed, longer than half of the terminal mesotarsomere. Metatibia slenderer than mesotibia, curved, with a row of spikes on outer margin sparser than in mesotibia. Metafemur flat. Metatibia with a tarsal groove. Metatarsus with five tarsomeres; tarsomere 5 2.5 × longer than 4; metatarsal claws well developed, longer than half of terminal metatarsomere.

Taxonomic assignment.

Olexum complanatum can be placed within the subfamily Dendrophilinae based on the following characters: prosternal lobe short, with incision for the passage of antenna (Fig. 2F ), antennal cavities widely open.

According to the phylogenetic analysis the species is related to the extant genus Dendrophilus Leach, 1817, as well as fossil Dendrophilinae genera Yethiha Caterino, 2021 and Druantia Caterino, 2021 (Fig. 5 ). Dendrophilinae subfamily is most likely polyphyletic ( Zhou et al., 2020) and comprehensive phylogenetic study is desirable. Hence, we refrain from placing O. complanatum into a tribe.

Etymology.

The specific name complanatum refers to the flattened body shape of this species.

Tribe Anapleini Olexa, 1982

Figures

Figure 2. Olexum complanatum Simon Prazak & Lackner, gen. & sp. nov. A dorsal view; B ventral view; C head, dorsal view (underlight); D ditto, ventral view (overlight); E ditto, ventral view (fluorescence); F prosternum, mesoventrite (fluorescence); G amber specimen; H protibia; I prosternal process, mesoventrite.

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).