Eustrophus praecursor, Alekseev & Bukejs, 2022

Alekseev, Vitalii I. & Bukejs, Andris, 2022, Eustrophus praecursor sp. nov.: the first Eocene species of Eustrophinae (Coleoptera: Tetratomidae) from Baltic amber, Zootaxa 5165 (3), pp. 435-442 : 436-440

Eustrophus praecursor sp. nov.

( Figs 1–5 )

Type material. Holotype: collection number No 6819 [ MAIG] (ex coll. Jonas Damzen 9230); adult, sex unknown. A complete beetle with a deformed right elytron is included in a transparent, yellow amber piece with dimensions of 6× 8 mm and a maximum thickness of 7 mm; preserved without supplementary fixation. Syninclusions: few stellate trichomes of Fagaceae and detrital particles.

Type stratum. Baltic amber from amber-bearing blue Earth layers; Middle-Late Eocene, 48–34 Ma ( Sadowski et al. 2017; Seyfullah et al. 2018; Bukejs et al. 2019; Kasiński et al. 2020).

Type locality. Yantarny village (formerly Palmnicken), the Kaliningrad Region, Russia .

Description. Measurements: body length (from anterior margin of pronotum to elytral apex along the midline) about 5.1 mm; maximum width across both elytra 2.2 mm (measured in basal one-third); pronotum length 1.3 mm, pronotum maximum width 2.1 mm; elytra length 3.8 mm, elytron maximum width 1.1 mm. Body oblong-oval, moderately convex dorsally; integument uniformly dark brown (as preserved); densely covered with fine, short, semierect setation.

Head deflexed, uniformly and coarsely punctate. Compound eyes slightly flattened, large, with vertical diameter about 1.8× transverse diameter, deeply and narrowly emarginated around antennal insertions, without interfacetal setae; widely separated dorsally, distance between eyes about 1.5× as wide as transverse eye diameter. Antennae 11-segmented, not clubbed, with gradually widened distal antennomeres, moderately long, extending about to the elytral base. Scape subcylindrical, about 2.0× as long as pedicel; pedicel cylindrical, as long as antennomere 4; antennomeres 3–4 conical, antennomere 4 about 0.7× as wide as antennomere 5; antennomeres 5–10 trapezoidal, distinctly widened apically, as wide as long, flattened; antennomere 11 longer than antennomere 10, ovoid, pointed apically. Relative length ratios of antennomeres 1–11 equal to 10:5:6:5:7:7:7:7:7:7:9.

Pronotum semicircular, transverse, about 1.6× as wide as long, with maximum width in posterior one-fourth, as wide as elytra at base; pronotal disc slightly convex, with two paired very shallow, rounded subbasal impression. Lateral pronotal sides smooth, rounded. Pronotum weakly bisinuate at the base, with weak middle lobe, not margined posteriorly; evenly arcuate anteriorly. Anterior angles are weakly developed, obtusely rounded; posterior angles distinct, orthogonal. Pronotal surface uniformly and shallowly punctate; shortly pubescent. Prothoracic episternal suture absent. Prosternal punctation coarse and dense, the distance between punctures smaller than the diameter of one puncture, and interspaces convex. Hypomera rugosely punctate, without transverse suture. Procoxae are separated by the triangular prostenal process. Prosternal process elongate, with rounded apex, not extending to the posterior margin of procoxae.

Elytra elongate, slightly convex, striate-punctate. Punctation small, deep and dense, arranged in longitudinal striae, each elytron with 10 complete striae. Interstriae are densely covered with finer punctures bearing short, semierect setae. Epipleura narrowed posteriorly, extending to elytral apex. Metanepistenum densely and finely punctate; distinctly subdivided in two parts: anterior triangular area and posterior longitudinal, rectangle area. Metaventrite with dense, fine punctuation; disc convex, with sharp discrimen.

Legs slender, long, relatively similar in shape, finely and densely punctate. All coxae separated; procoxae large, nearly hemispherical, strongly convex; metacoxae simple. Femora slender, suboval, flattened; metafemora widest, about 3× as long as wide. Femora and tibiae are subequal in length. Tibiae almost straight; with two rather short (about 0.8× as long as metatibia width at apex), simple, paired apical spurs of equal length; outer surfaces of meso- and metatibiae with numerous oblique, comb-like ridges (with about 15 ridges on metatibia). Tarsal formula 5-5-4. All tarsomeres narrow, simple (non-lobed). Relative length ratios of metatarsomeres 1–4 equal to 14:7:5:5. Tarsal claws slender, without basal tooth or expansion.

Abdomen with five visible, freely articulated ventrites; densely covered with uniform, fine punctation and decumbent setation. Relative length ratios of ventrites 1–5 equal to 40:35:32:25:23 (measured medially).

Note. The presence or absence of ovate, setiferous pit on the ventral surface of profemur is unclear due to legs position in the studied specimen. The genitalia are not exposed, making definite sexual determination impossible. The shape of scutellar shield is unclear (probably trapezoidal and slightly transverse) because it is covered by air bubble.

Etymology. The specific epithet “ praecursor ” is used as noun in apposition and stems from the Latin substantive, meaning “forerunner, vanguard, predecessor”. The name refers to the pioneer status of this species, pre-dating the other species of the genus.

Differential diagnosis. Eustrophus praecursor sp. nov. can be distinguished from extant congeners by following characters: basal pronotal margination absent (in contrast to finely margined pronotum in E. tomentosus , E. dermestoides , and E. niponicus ), and compound eyes rather widely separated by a distance about 1.5× as transverse eye diameter (in contrast to the narrowly separated eyes with the distance between them about 0.4× as transverse eye diameter in E. yunnanensis ). The new extinct species from Baltic amber can be easily distinguished from extinct Eustrophinae from Cretaceous ambers by the combination of tribal and generic characters, most obviously in the presence of oblique, comb-like ridges on outer surfaces of meso- and metatibiae in contrast to the simple tibiae in all other described fossil Eustrophinae .

Figures

FIGURES 1–2. Eustrophus praecursor sp. nov., holotype, No 6819 [MAIG]: 1—habitus, dorsal view; 2—habitus, ventral view. Scale bar = 1.0 mm.

FIGURES 3–5. Eustrophus praecursor sp. nov., holotype, No 6819 [MAIG]: 3—details of head and prothorax, ventral view; 4—habitus, right lateral view; 5—right metatibia and metatarsus. Not to scale.