Hemicoelus favonii Bukejs, Alekseev & McKellar, 2017

Bukejs, Andris, Alekseev, Vitalii I., Cooper, David M. L., King, Gavin A. & Mckellar, Ryan C., 2017, Contributions to the palaeofauna of Ptinidae (Coleptera) known from Baltic amber, Zootaxa 4344 (1), pp. 181-188 : 182-185

Hemicoelus favonii Bukejs, Alekseev & McKellar sp. nov. ( Figs 1–2 )

Type material. Holotype: collection number P3300.84 [RSM], female. A rather complete beetle with partially exposed hind wings is included in small, transparent yellow amber piece embedded in block of Epotek 301 resin with overall dimensions 10 × 4 × 2 mm. Syninclusions: fungal hyphae, few small pieces of organic material, and small gas vesicles.

Type strata. Baltic amber, mid-Eocene to Upper Eocene.

Type locality. Baltic Sea coast, Yantarny settlement [formerly Palmnicken], Kaliningrad region, Russia.

Etymology. The species epithet, a noun in the genitive case, is derived from the Latin word “ favonius ” (west wind), referring to the most productive wind direction for amber collection in the Sambian peninsula.

Diagnosis. Hemicoelus favonii sp. nov. differs from extant species in combination of following characters: 11- segmented antennae; metathoracic ventrite with large impression in anterior portion; pronotum distinctly narrower than elytral base region; posterior suture of abdominal ventrite 1 weakly arcuate medially; sharp lateral pronotal margins incomplete, distinct in basal half only; elytral striae on disc not grouped in pairs; posterior pronotal angles rounded; elytral interstriae 3, 5, 7 and 9 slightly convex; and smaller body size.

Three fossil species of the tribe Anobiini have previously been described from Baltic amber ( Kuśka 1992; Hawkeswood et al. 2009; Bukejs & Alekseev 2015): Anobium jacquelinae Hawkeswood, Makhan & Turner, 2009 ; Microbregma sucinoemarginatum ( Kuśka, 1992) ; and Microbregma waldwico Bukejs & Alekseev, 2015 . Apart from genus-level differences (such as variation in abdominal ventrite lengths, and relief of pronotum dorsally), differences between known fossil species and Hemicoelus favonii sp. nov. include: (1) comparative widths of pronotum and elytral base region: in Microbregma waldwico , M. sucinoemarginatum and Anobium jacquelinae , pronotum nearly as wide as elytral base, but in new species pronotum distinctly narrower than elytral base; (2) body length: 3.8–4.6 mm in M. waldwico , 3.5 mm in M. sucinoemarginatum , 3.0 mm in A. jacquelinae , and 1.8 mm in Hemicoelus favonii sp. nov.

Description. Body length 1.8 mm, maximum width 0.8 mm; body shape elongate, cylindrical; body color (as preserved) brown, with legs and antennae paler, rufous; head and pronotum with fine recumbent pubescence, elytra apparently glabrous.

Head hypognathous, evenly and weakly convex dorsally, finely and densely granulated; frons weakly convex, without tubercles or carinae. Compound eyes small, oval, convex, entire, with distinct facets, without ommatidial setae; distance between compound eyes nearly equal to 1.5× vertical diameter of one eye. Antennae short, reaches elytral base; 11-segmented, with 3-segmented club; scape subcylindrical, weakly thickened; pedicel globose, 0.5× length of scape and 0.35× length of antennomere 11; antennomere 3 elongate, as long as pedicel, or about 0.3× length of antennomere 11; antennomeres 4–8 small, nearly as long as wide; antennomeres 9–11 elongate, about 0.8× as long as antennomeres 1–8 combined; antennomeres 9–10 equal in length and similar in shape, distinctly dilated apically, antennomere 11 spindleshaped, 1.4× as long as antennomere 10. Relative length ratios of antennomeres 1–11 equal to 15-7-7-4-5 - 4-5-4-13 -13- 19. Antennal insertions widely separated, distance equal to 0.75× width of frons.

Pronotum weakly transverse, about 1.1× as wide as long, length 0.45 mm, maximum width 0.5 mm, distinctly narrower than elytral bases; covered with fine, dense granulation. Anterior margin arcuate, posterior margin weakly convex; lateral margins crenulated, weakly convex in dorsal view, lateral edge sharp but incomplete, distinct in basal half only. Posterior pronotal angles rounded, anterior angles apparently rounded. Pronotum with two oblique, oval, laterobasal impressions on dorsal surface; narrow transverse impressions near posterior margin; and medial longitudinal impression in anterior half; with V-shaped ridge in basal half (almost divided by longitudinal medial impression), and two latero-basal gibbosities.

Scutellar shield large, transverse. Elytra subparallel, elongate, length 1.25 mm, maximum width 0.8 mm, about 1.6× as long as combined width, 2.8× as long as pronotum; humeral calli developed, basal margin weakly concave. Elytral apices indistinctly truncated. Elytral punctures small, dense, arranged in regular striae, each elytron with 10 striae plus shortened scutellar stria, striae distinct throughout entire length of elytron, distance between strial punctures equal to 0.3–1.2× diameter of one puncture, elytral striae on disc not grouped in pairs; interstrial intervals shagreened, interstriae 3, 5, 7 and 9 more or less convex, distance between striae about 1.0–2.0× diameter of one strial puncture. Pygidium completely covered by elytra. Hind wings apparently well developed, partially exposed.

Hypomera impunctate, distinctly concave, more or less enclosing head. Prosternal intercoxal process wide, about 0.7× as wide as diameter of procoxa, concave, with lateral margins parallel, and anterior margin widely emarginate. Mesothoracic ventrite almost impunctate, shagreened; with deep longitudinal medial drop-like impression, and with two subtriangular impressions at anterior margin (anterior to mesocoxae); impressions delimited by sharp carinae. Metathoracic ventrite convex, with large and dense punctures, distance between punctures less than puncture diameter; with large, oval transverse impression in anterior portion (joined to impression of mesoventrite), about 0.4× of metaventrite length (medially), and weakly concave, with impunctate medial area posteriorly. Metepisternum about 5× as long as wide, with large punctures, distinctly widened anteriorly and gradually narrowed posteriorly, anterior margin oblique, lateral margins nearly straight.

Legs moderately short. Procoxae oval, distinctly separated by about 0.7 times procoxal diameter; mesocoxae globose, separated by almost one mesocoxal diameter; metacoxae narrow, transverse, with groove for reception of metafemora. Trochanters subglobose; femora almost straight, short, barely projecting beyond lateral side of elytra, weakly dilated apicaly, clavate, ventrally with longitudinal groove for reception of tibiae; tibiae straight, nearly as long as femora; tarsi slender, about 0.85× length of tibia, tarsal formula 5-5-5. Relative length ratios of protarsomeres 1–5 equal to 11-9-7-5-9; metatarsomere 4 slightly emarginated. Claws free, falcate, thickened basally.

Abdomen with five visible ventrites; ventrite 1 covered with fine punctation, shagreened, with subsequent ventrites smoother; abdominal sutures distinct throughout length; posterior suture of ventrite 1 slightly arcuate medially, other sutures nearly straight. Ventrites 3 and 4 subequal in length; relative length (medially) ratios of ventrites 1–5 equal to 6- 5-4-3-4. Intercoxal process of abdominal ventrite 1 trapezoidal, with rounded anterior angles.

Note. Probable sex of fossil specimen was determined using synchrotron X-ray micro-CT observations. Sexual dimorphism is known for extant species of Hemicoelus : males and females may be separated based on their sexually dimorphic antennae in which the antennal club of males is more elongate and about one-half longer than that of females ( Arango & Young 2012). Females also tend to be larger than males, although this character is unreliable.

Figures

FIGURE 1. Hemicoelus favonii sp. nov., holotype RSM P 3000.84. A, B—dorsal habitus photomicrograph, and corresponding SR xray µCT rendering; C, D—ventral habitus photomicrograph, and corresponding SR x-ray µCT rendering. Scale bars represent 0.5 mm.

FIGURE 2. Hemicoelus favonii sp. nov., holotype RSM P 3000.84. A, B—lateral habitus photomicrograph, and corresponding SR xray µCT rendering; C, D—anterior habitus photomicrograph, and corresponding SR x-ray µCT rendering; E—detail of right antenna in dorsal view; F—detail of right mesotarsus in dorsal view; G—detail of elytral punctation and interstriae. Scale bars represent 0.5 mm in A, D; 0.25 mm in E; 0.1 mm in F, G.