Xyletinus besseli Alekseev & Bukejs 2019, sp. nov.

Xyletinus (s. str.) besseli sp. nov.

(Figs 1–3, 9)

urn:lsid:zoobank.org:act: 73241612-BCF2-4A4E-A868-7A4B5563E1AF

Material examined. Holotype: No. 1771-7 [CCHH]. Adult, sex unknown. The beetle inclusion is preserved in a polished piece of transparent amber with a yellowish shade. The small rounded amber slide is embedded in a block of polyester resin with dimensions 10×8× 4 mm. The complete beetle inclusion in good preservation state, but right elytron and right side of pronotum appear to be slightly roasted and deformed. The beetle is partially covered by sticked on surface and pubescence gas microvesicles, making conclusion on punctation and reticulation of the cuticular surface problematic. Syninclusions: one stellate fagacean trichome.

Type horizon. Baltic Amber, Upper or mid-Eocene.

Type locality. The Samland (Sambian) Peninsula, the Kaliningrad region, Russia.

Description. Total body length about 3.1 mm; elytra length 2.3 mm, combined width of elytra at humeri 1.7 mm; pronotum length 0.8 mm, pronotum maximum width 1.4 mm. Body elongate, subparallel, moderately convex; pubescence fine, moderately dense, semirecumbent; covered with very fine punctures and microreticulation (appearing partially granulate, shagreened or rugose due to small gas vesicles on body surface); unicolorous blackish brown (as preserved).

Head hypognathous, turned downwards and not visible in dorsal view; covered with fine, sparse punctation and transverse microreticulation; frons almost flat, without trace of carina or longitudinal impression, with shallow transverse impression anteriorly. Compound eyes relatively small, slightly convex, entire, oval (slightly vertical) in lateral view, with small distinct facets, without ommatidial setae; widely separated, frontal distance between compound eyes nearly equal to 2.5× transverse diameter of one eye. Antennal insertion limited by short carina posteriad. Antennae 11-segmented, serrate medially, short (reaching middle of pronotum), with two distal antennomeres forming club; antennomeres 1–3 elongate; scape longest; antennomeres 4–9 serrate, triangular; antennomeres 10–11 transverse, rounded; antennomere 10 the widest. Relative length ratios of antennomeres equal to 3.4–1.6–1.4– 1.0–1.0–1.0–1.0–0.8–1.0–1.2–1.0. Terminal labial palpomere triangular. Terminal maxillary palpomere triangular, slightly widened.

Pronotum convex, transverse, 1.8× as wide as long; margined; finely pubescent; sparsely punctured like head. Anterior pronotal angles subrectangular; posterior pronotal angles widely rounded and barely marked. Lateral margins visible in dorsal view.

Scutellar shield triangular with slightly rounded apex.

Elytra moderately short, 1.4× as long as wide, with distinct humeri, longitudinally striate (strial punctures indistinct). Each elytron with 12 shallowly impressed striae: striae 5–8 shortened and joined pairwise in apical two-thirds of elytron; stria 12 shortest, reaching abdominal ventrite 1. Elytral intervals slightly convex laterally, almost flat medially; with minute, moderately dense punctation (distance between punctures approximately 0.5–1.0× diameter of one puncture), interspaces between punctures partially forming minute wrinkles. Interval pubescence semierect, very short. Metathoracic wings not apparent. Metathorax with punctation dense, deep, rough, and partly forming wrinkles; distance between punctures about 0.3–1.0× diameter of one puncture; metaventrite without excavation for reception of mesothoracic legs.

Legs slender, comparatively short, pentamerous. Procoxae contiguous. Femora flattened internally for reception of tibia. Tibiae tetrahedral in cross-section; with outer margin bearing suberect setae; apex setose. Tarsus shorter than tibia; tarsomere 1 longest, as long as tarsomeres 2 and 3 combined; meso- and metarsomeres 4–5 widened. Tarsal claws simple, thickened basally, falcate, symmetrical.

Abdomen with five freely articulated, visible ventrites, without excavation for reception of metathoracic legs; finely and densely punctured and pubescent. Relative length ratios of ventrites 1–5 equal to 3–4–3–3–5.

Differential diagnosis. The new fossil species belongs to the genus Xyletinus based on following morphological characters: head deflexed; metathoracic ventrite and abdomen without excavation for reception of mesothoracic and metathoracic legs; antennae serrate; eyes widely separate; elytra striate; three terminal antennomeres no longer than preceding segments combined. The combination of the following features allows the assignment of Xyletinus besseli sp. nov. to the nominative subgenus: eyes oval, slightly convex, not strongly prominent; frons wide; terminal labial palpomere triangular; and tarsus shorter than tibia.

Xyletinus (s. str.) besseli sp. nov. resembling recent species of the genus and the subgenus in visible morphology, but can be easily distinguished by unique strongly transverse and widened antennomeres 10–11 (looking like 2-segmented antennal club), microreticulated dorsal surface, fine and sparse pronotal punctuation, and almost flat frons without carina or longitudinal impression.

Derivatio nominis. Patronymic, this new species is named after the director of the Königsberg observatory and professor of the Albertina (the Königsberg University) Friedrich Wilhelm Bessel (1784–1846), prominent Prussian astronomer, mathematician, physicist and geodesist.