Coxelus carstengroehni Alekseev et Bukejs 2024, sp. nov.

Coxelus carstengroehni Alekseev et Bukejs sp. nov. (Figs 5–6) Type material. Holotype: GPIH no. 5208, CCGG no. 8705 (ex coll. Jonas Damzen JDC-12539); “Holotype / Coxelus carstengroehni sp. nov. / Alekseev et Bukejs des. 2024” [red printed label]; adult, sex unknown.A complete beetle with partially exposed metathoracic wings is included in a transparent, yellow amber piece with 11× 8 mm and a maximum thickness of 1 mm, preserved without supplementary fixation. Syninclusions: two specimens of male Chironomidae (Diptera: Nematocera), one of them is partially damaged, and several 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 (including visible part of head) 2.3 mm, maximum body width (across elytra) 0.8 mm; pronotal length 0.6 mm, maximum pronotal width 0.7 mm, basal pronotal width 0.6 mm; elytral length (along elytral suture, including scutellum) 1.5 mm. Body subparallel-sided, elongate, about 2.9× as long as wide, weakly convex dorsally and ventrally, unicolorous black (as preserved). Pubescence: head with short, erect setae; pronotum sparsely covered with stout, erect setae; elytra sparsely and uniformly covered with longer, erect, narrow, slightly curved setae arranged in rows, and not forming patches. Head prognathous, transverse; head densely granulose dorsally, granules rather large, about 2× as wide as diameter of one eye facet, and distance between granules distinctly smaller than diameter of one granule, with each granule bearing short, narrow seta; dense, coarse punctation present ventrally. Anterior margin of clypeus rounded. Compound eyes small, hemispherical, convex, without interfacetal setation. Antennal insertions concealed dorsally; antennal grooves ventrad eye present, short. Antennae short, extending to about middle of pronotal length; 11-segmented with distinct, compact, 2-segmented club; covered with sparse, short, semierect setae; scape wide, subcylindrical; pedicel cylindrical, 2× as long as wide, narrower and shorter than scape; antennomere 3 about 2.2× as long as wide, subcylindrical, slightly dilated apically, about 1.4× longer than pedicel; antennomeres 4–5 slightly elongate, subcylindrical; antennomeres 6–8 subquadrate, nearly as long as wide; antennomere 9 weakly transverse, 1.3× as wide as long, slightly dilated apically; antennomere 10 subtrapezoidal, dilated apically, widest, transverse, 2.2× wider than antennomere 9; antennomere 11 oval, transverse, 1.5× as wide as long, distinctly narrower than antennomere 10, finely setose. Terminal maxillary palpomere obliquely truncate, pointed apically, slightly longer than penultimate palpomere. Pronotum slightly transverse, 1.2× as wide as long, widest near midlength and slightly narrowed anteriad and posteriad, maximum pronotal width / basal pronotal width = 1.2; pronotal disc weakly convex, with large, oval, longitudinal impression medially; pronotal surface densely covered with granules, bearing erect, stout setae. Lateral margins weakly rounded, with fine crenulation; anterior margin arcuate; posterior margin rounded. Anterior angles prominent, acute, slightly rounded; posterior angles obtuse. Prothoracic notosternal suture complete. Hypomera and prosternum densely and coarsely punctate. Intercoxal prosternal process elongate, narrow, distinctly narrower than one procoxal diameter. Procoxal cavities open posteriorly. Scutellar shield minute, semioval, slightly transverse. Elytra almost parallel-sided in anterior two-thirds of length, tapered at apex, about 1.9× as longe as wide combined, non-carinate, convex, 2.5× longer than pronotum. Elytral base distinctly wider than pronotal posterior margin.Humeral angles rounded. Elytra striate-punctate. Scutellary striole not apparent.Interstriae flat, with elongate granules. Epipleura well-developed, widest in basal half, slightly narrowed posteriad, reaching elytral apices. Mesoand metaventrites densely and coarsely punctate. Mesocoxal cavities closed. Metanepisternum narrow, about 7.9× as long as wide, covered with dense and coarse punctures. Metaventrite with disc slightly convex, slightly shorter than abdominal ventrite 1; with discrimen distinct in posterior two-thirds of length. Relative length ratio of pro- to meso- to metaventrite to abdomen approximately equal to 10:7:8:22. Metathoracic wings developed. Legs rather short and robust, finely punctate. Procoxa nearly rounded, narrowly separated; mesocoxae relatively round, narrowly separated by about 0.5× diameter of mesocoxa; metacoxae oval, transverse, separated by about one longitudinal diameter of metacoxa. Femora nearly spindle-shaped, flattened, and widened medially; femora and tibiae subequal in length. Tibiae straight, slightly dilated apically, with fringe of short, fine setae apically. Tarsal formula 4-4-4; tarsomeres with sparse, fine setation ventrally; tarsomere 4 longest, longer than metatarsomeres 1–3 combined, and slightly curved. Claws simple, large, about 0.5× as long as tarsomere 4, and robust. Abdomen with five visible ventrites, abdominal sutures distinct throughout length; densely covered with punctures that grade from large to small (ventrites 1 and 2 with distinctly larger punctation than ventrires 3–5), distance between punctures 0.5–1.5× diameter of one puncture. Relative length ratios of ventrites 1–5 equal to 5:5:4:3:3. Abdominal ventrite 1 with small, apically acute intercoxal process in form of equilateral triangle that is about as wide as metacoxal lenght. Abdominal ventrite 5 subtrapezoidal, truncate apically, without marginal groove. Differential diagnosis. Coxelus carstengroehni sp. nov. differs from all extant congeners based on the combination of the following characters: eyes without interfacetal setae; metathoracic wings present and metathorax not shortened (in contrast to apterous Coxelus of Recent fauna); relatively long elytra, about 1.9× as long as wide; weakly transverse pronotum, 1.2× as wide as long; and elytral setation uniform and not forming patches. The new fossil species resembles the North American C. longus (Stephan, 1989) in its elongate body, but can be distinguished from it based on the smaller body length of 2.3 mm (2.9 mm in the holotype of C. longus according to the original description), larger eyes (eyes reduced, with 26–32 facets in C. longus), metathoracic wings present, and elytra not connate. Among the Synchitini described from Baltic amber, the new species may be confused only with the apparently similar and probably related Semicoxelus sontagae Alekseev et Pankowski, 2020. Semicoxelus sontagae (body length of the holotype is 1.85 mm) differs from Coxelus carstengroehni sp. nov. in its vertically elongate eyes (hemispherical in the new species), subantennal grooves absent (present in the new species), antennae stouter and antennal club is more weakly expressed (antennal club is well-defined, and antennomere 10 about 2.2× as wide as antennomere 9 in C. carstengroehni sp. nov.), and distinctly curved elytral setation (erect and only slightly curved in the new species). Derivatio nominis. The specific epithet is a patronym; the new species is named in honor of Mr. Carsten Gröhn (Glinde, Germany), who provided the amber piece with the holotype for our study. Distribution of congeners. The genus Coxelus occurs in the western Nearctic and Neotropics (2 species: C. longus (Stephan 1989) and C. serratus Horn 1885), and in temperate and southern areas of the western and eastern parts of Palearctic (6 species: C. alinae Dajoz, 1973; C. bituberculatus (Frivaldszky, 1893); C. humeridens Reitter, 1885; C. luteopilosus Pie, 1901; C. pictus (Sturm, 1807); and C. yeti Ślipinśki, 1985) (Ivie et al. 2016; Ślipiński & Schuh 2008). The fossil species described in the current paper confirms the presence of the genus in the Paleogene Western Palearctic, which happens to be the area where the genus is represented by the largest number of Recent species (4 spp.) as well. Ecology of congeners. Recent representatives of Coxelus are usually found in leaf litter and probably feed on decaying vegetation or fungi (Ślipiński & Lawrence 2010). They are considered saproxylophagous, being associated with dead wood and woody rotting material (as in C. pictus, according to Parisi et al. 2021). Coxelus pictus were numerous in samples taken from the decaying small and medium diameter wood debris in two Italian beech forests (Macagno et al. 2015), C. serratus has been reported from redwood duff and under bark of dead Douglas fir, whereas C. longus has been associated with dead ponderosa pines (Stephan 1989). The Eocene C. carstengroehni sp. nov. could be also associated with an analogous primeval forest community of dead coniferous and fagacean wood.

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