Usechus andrushchenkoi Alekseev et Bukejs, 2024

ALEKSEEV, VITALII, MCKELLAR, RYAN C. & BUKEJS, ANDRIS, 2024, A revision and addition to Zopheridae (Coleoptera: Tenebrionoidea) in Baltic amber: possible connections between modern Holarctic distributions and Eocene ‘ amber forests’, Zootaxa 5536 (2), pp. 201-247 : 203-205

Usechus andrushchenkoi Alekseev et Bukejs sp. nov.

( Figs 1–4 )

Type material. Holotype: No. KAM 8820 [ KRAM]; “*Holotype / Usechus / andrushchenkoi sp. nov. / Alekseev et Bukejs des. 2024” [red handwritten label]; adult, sex unknown ; submentum and last abdominal ventrite not observable in the specimen. A complete beetle is included in a parallelepipedic, transparent, yellow amber piece with dimensions of 23× 15 mm and a maximum thickness of 6 mm, preserved without supplementary fixation. The amber piece with the studied specimen was accidentally cracked during mechanical processing and then glued together. As a result, the state of preservation and visibility of the inclusion lying at the junction of the amber layers deteriorated: the cuticle was destructed in some places (antennomeres), and air bubbles appeared in cracks and on the surface of the inclusion in some places (head). Ventral side of specimen is obscured by “milky” opacity. Syninclusions: a few small stellate Fagaceae trichomes.

Type stratum. Baltic amber from Eocene amber-bearing Blaue Erde deposits; estimated age: middle–late Eocene (Standke 1998).

Type locality. Yantarny settlement (formerly Palmnicken), Sambian (Samland) Peninsula, Kaliningrad Region, Russia.

Description. Measurements: total body length 4.1 mm, maximum body width (across elytra) 1.4 mm; pronotal length 0.9 mm, maximum pronotal width 1.3 mm, basal pronotal width 1.0 mm; elytral length (along the elytral suture, including scutellum) 2.9 mm; antennal length 0.6 mm.

Body subparallel-sided, elongate, 2.9× longer than wide; weakly convex dorsally and ventrally; unicolorous black (as preserved). Pubescence: pronotum covered with decumbent, narrow, elongate setae directed anteriad and laterad; elytra homogenously covered with shorter, decumbent, narrow, elongate setae, not forming patches.

Head partially retracted in pronotum, transverse, widest across eyes, not constricted behind eyes, and without temples. Compound eyes vertical, reniform (emarginate anteriorly at antennal base), flat, not visible in dorsal view, apparently without interfacetal setation. Antennal insertions concealed dorsally by lateral expansion of frons; antennal grooves ventral to eye absent. Antennae short, stout, extending to about middle of pronotum; 11-segmented with 3-segmented weakly flattened club; scape short, subquadrate; pedicel subcylindrical, longest; antennomeres 3–6 as long as wide, rounded; antennomeres 7–8 weakly transverse; antennomeres 9–10 widest, subtrapezoidal, transverse; antennomere 10 shorter than antennomere 9, about 2× as wide as long; antennomere 11 round and weakly flattened, as long as wide.

Pronotum nearly hexagonal, transverse, 1.4× as wide as long; deep anterolateral antennal cavities extend to middle of pronotal length and end in round fossa; widest point at middle, coinciding with endpoint of antennal cavities; distinctly narrowed anteriad and posteriad, with maximum pronotal width / basal pronotal width = 1.3; basally bordered. Pronotal disc convex medially, without transverse basal groove; lateral sides explanate, bordered. Pronotal surface coarsely granulate. Anterior and posterior pronotal angles obtuse. Prothoracic notosternal suture complete. Hypomeron densely granulose. Intercoxal prosternal process elonagte, distinctly widened apically and forming posterolateral projections extending to posterior margin of procoxae. Procoxal cavity closed posteriorly by projection of hypomeron and posterolateral projection of intercoxal prosternal process.

Scutellar shield small, cordate, transverse, about 2.0× as wide as long.

Elytra almost parallel-sided within anterior two-thirds of their length, tapered at apex, convex, 2.1× as long as wide combined, and 3.2× longer than pronotum. Humeral angles rounded. Elytra striate-punctate. Scutellary striole not apparent. Interstriae convex, without tubercles. Epipleura well-developed, widest in basal half, slightly narrowing posteriorly, reaching elytral apices. Metathoracic wings not apparent.

Legs short, rather robust. Procoxa nearly rounded, narrowly separated; metacoxae narrowly oval, transverse, separated by about one longitudinal diameter of metacoxa. Pro- and mesofemora nearly spindle-shaped, slightly widened; metafemora weakly curved; femora and tibiae subequal in length. Tibiae with fringe of setae apically. Tarsal formula 5-5-4; tarsomeres with fine setation ventrally; metatarsomere 4 longest, as long as metatarsomeres 2–3 combined. Claws simple, large, about 0.3× as long as tarsomere 4.

Abdomen with five visible ventrites, abdominal sutures distinct throughout length. Relative length ratios of ventrites 1–5 equal to 7:6:4:3:3. Setation and punctation of ventrites not apparent (possibly absent). Abdominal ventrite 1 with apically rounded intercoxal process in form of equilateral triangle, about as wide as metacoxal length. Ventrite 5 with arcuate posterior margin.

Remark. The ventral side of the specimen (mesoventrite, metaventite, abdominal ventrites) is obscured by ‘milky’ opacity making these areas indistinct or insufficiently visible. The closure of mesocoxal cavities (possibly closed) and the presence of impression on abdominal ventrite 5 (=preapical groove) remain unknown.

Differential diagnosis. Usechus andrushchenkoi sp. nov. differs from all extant congeners in the combination of the following characters: relatively long elytra, about 2.1× as long as wide; weakly developed elytral relief (alternate intervals not costate, tubercles absent); transverse pronotum, 1.4× as wide as long; distinct scutellar shield; and long pedicel, as long as antennomeres 3–5 combined.

The new species can be distinguished from all other described Baltic amber Zopheridae by the nearly hexagonal pronotum and the deep anterolateral pronotal antennal cavities.

Derivatio nominis. Patronymic, the name of new species is dedicated to Konstantin V. Andrushchenko (Kaliningrad), a great enthusiast in collecting Baltic amber insect inclusions, who provided the type specimen for description and donated the amber from his private collection to the museum.

Distribution of congeners. The genus Usechus has a disjunct Recent distribution ( Blaisdell 1929; Kulzer 1960; Saitô 1999; Ślipiński & Lawrence 1999; Kim & Ahn 2009; Jung et al. 2023). The group occurs in western North America (2 species: U. lacerta Motschulsky, 1845 —California; U. nucleatus Casey, 1889 —California, Oregon, Washington) and in East Asia (4 species: U. chujoi Kulzer, 1960 — Japan, Korea; U. tsushimensis Kamiya, 1963 — Japan, Korea; U. ohdaiensis Sasaji, 1987 — Japan; U. sasajii Saito, 1999 — Japan). The fossil species described in the current paper establishes that the tribe Usechini , which restricted to the present-day Holarctic realm, dates to at least the Eocene. It also confirms the presence of the tribe in the Western Palearctic in the Paleogene. The Eastern Palearctic, north-eastern Oriental, Western and Eastern Nearctic Regions are generally regarded as typical refuge areas for Eocene Laurasian relicts (e.g. Andrée 1951; Larsson 1978; Weitschat & Wichard 1998).

Ecology of congeners. Recent representatives of Usechus occur under the bark and in dead, rotting wood infested by fungi. They have been reported from deciduous ( Acer , Quercus ) and coniferous trees ( Blaisdell 1929; Doyen & Lawrence 1979; Saitô 1999). The Eocene Usechus representative, described herein, could also be associated with an analogous forest community of coniferous and fagacean wood and fungi.

Figures

FIGURES 1–2. Usechus andrushchenkoi sp. nov., holotype, No. KAM 8820 [KRAM]: 1—habitus, dorsal view; 2—habitus, ventral view. Scale bars = 1.0 mm.

FIGURES 3–4. Usechus andrushchenkoi sp. nov., holotype, No. KAM 8820 [KRAM]: 3—schematic drawing of antenna; 4—anterior part of body, dorsal view. Scale bar = 1.0 mm.