Atomaria (Anchicera) propinqua, Lyubarsky & Alekseev & Bukejs, 2023

Lyubarsky, Georgy Yu., Alekseev, Vitalii & Bukejs, Andris, 2023, A new fossil species and new record of Atomaria Stephens (Coleoptera: Cryptophagidae) from Eocene Baltic amber, Zootaxa 5375 (2), pp. 241-248 : 242-246

Atomaria (Anchicera) propinqua sp. nov.

( Figs 1–5 )

Type material. Holotype:No 062 [ ACAB]; adult, sex unknown.A complete beetle is included in a transparent, yellow amber piece with dimensions of 15× 14 mm and a maximum thickness of 2 mm; preserved without supplementary fixation. Syninclusions: absent.

Type stratum. Baltic amber; Middle–Late Eocene ( Sadowski et al. 2017, 2020; 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 head to elytral apex along midline) about 1.46 mm, body maximum width across both elytra 0.62 mm; head length 0.15 mm, head maximum width across eyes 0.33 mm; pronotum length 0.35 mm, pronotum maximum width 0.46 mm; elytra length 0.96 mm, elytra maximum width 0.62 mm. Body elongate, moderately convex; integument rufous (as preserved); pubescence simple, inconspicuous, semierect.

Head partially retracted into thorax, transverse, about 2.1× wider (including eyes) than long, not constricted behind eyes; coarsely punctate, distance between punctures 0.3–1.2× diameter of one puncture, punctation distinctly denser posterolaterally. Frons convex, without tubercles. Forehead flat, slightly convex laterally. Compound eyes hemispherical, convex, rather large, with 10–11 large facets at outer margin (in dorsal view). Antennae inserted close to eyes, under lateral margin of forehead; antennal insertions widely separated, distance between antennal insertions larger than distance between antennae and аnterior margin of eye; antennal grooves absent. Antennae 11- segmented with loose, 3-segmented club, rather long, reaching slightly beyond pronotal base; antennal flagellum (antennomeres 3–8) stout, about 0.33× as wide as eye length; antennomere 1 subcylindrical, elongate, 1.5× longer than wide; antennomere 2 subconical, slightly dilated apically, elongate, 1.6× longer then wide; antennomere 3 conical, elongate, about 1.3× longer than wide, slightly narrower (about 0.9× as wide as antennomere 2) and slightly shorter than antennomere 2; antennomere 4 elongate, 1.25× longer than wide; antennomere 5 subtrapezoidal, elongate, 1.3× longer than wide, nearly as long as antennomere 3; antennomeres 6–8 trapezoidal, dilated apically, equal in length, as long as wide; antennal club slender, rather narrow, slightly wider than flagellum; antennomeres 9–10 trapezoidal, nearly as wide as long, antennomere 10 about 1.3× as wide as antennomere 8; antennomere 11 oval, with narrowly rounded apex, 1.7× longer than wide; relative length ratios of antennomeres 1–11 equal to 9:8:6:5:6:5:5:5:6:7:12. Terminal maxillary palpomere subconical.

Pronotum weakly transverse, 1.3× wider than long, narrowed posteriad and anteriad, with maximum width slightly beyond middle; with convex disc; with finely margined posterior and lateral sides; lateral side borders visible from above only in basal half. Pronotal punctation coarse and rather dense, distance between punctures equal to 1.0–1.5× diameter of one puncture; pronotal punctation about as large as punctation of head. Posterior angles obtuse, pointed; anterior angles obtuse. Anterior edge weakly rounded, without excision; posterior edge bisinuate, lobed, with shallow depression and two basal pits; lateral edges widely rounded, without callosity and teeth, with fine crenulation. Prohypomera slightly impressed; with rather dense, coarse punctation. Prosternum with disc convex; sparsely covered with coarse punctation. Prostenal process convex, wide, about 0.7× as wide as procoxal width. Procoxal cavities closed externally.

Scutellar shield small, oval, strongly transverse, 2.3× wider than long, covered with fine punctation. Elytra elongate-oval, moderately convex, with maximum width in middle, 1.55× longer than wide, 2.7× as long as pronotum length; completely covering abdomen; lateral sides slightly rounded; humeri rounded, slightly prominent. Elytral punctation irregular, coarse and rather dense; punctures in basal part about as large as punctures on pronotal disc; distance between punctures equal to 1.0–1.5× diameter of one puncture. Epipleura narrowed posteriorly, incomplete, extending about to abdominal ventrite 4. Metaventrite with disc convex; covered with coarse, rather dense punctation.

Legs slender, long, relatively similar in shape, finely punctate. Procoxae large, widely suboval, slightly transverse; mesocoxae round; metacoxae narrowly oval, strongly transverse. Femora elongate-oval, flattened. Tibiae thin, almost straight, nearly as long as femora, apparently with apical spines. Tarsi long and thin; tarsomeres simple, not lobed; tarsal formula 5-5-5. Pretarsal claws simple.

Abdomen with five visible, freely articulated ventrites; intercoxal process rather wide; ventrite 1 longest, covered with rather coarse punctation, ventrites 2–5 with fine and denser punctures; ventrite 5 truncate apically. Relative length ratios of ventrites 1–5 equal to?29:11:9:8:9 (measured medially).

Diagnosis. Atomaria propinqua sp. nov. can be distinguished from related species by a combination of characters: antennal flagellum (antennomeres 3–8) stout, about 0.33× as wide as eye length; antennomeres 1 and 2 slightly wider than flagellum; antennomere 3 elongate, about 1.3× as long as wide, only slightly narrower and slightly shorter than antennomere 2; antennomere 5 about as long as antennomere 3; antennomeres 6–8 equal in length; antennomeres 9–10 trapezoidal, nearly as wide as long; antennal club slender, rather narrow, slightly wider than flagellum; pronotum and elytra with simple, semierect, inconspicuous pubescence; pronotum weakly transverse, narrowed toward base, with convex disc; lateral side borders of pronotum visible from above only in basal half of its length; pronotum maximum width nearly in middle of its length.

The new species resembles the fossil species A. alekseevi Lyubarsky et Bukejs, 2022 (Baltic amber) but differs in stout antennal flagellum, about 0.33× as wide as eye length (about 0.2× as wide as eye length in A. alekseevi ), comparatively shorter and wider antennomere 3 (about 1.3× longer than wide in the new species and about 2× longer than wide in A. alekseevi ; and 0.9× as wide as antennomere 2 width in the new species and 0.6× as wide as antennomere 2 width in A. alekseevi ); nearly as wide as long antennomeres 9–10 (strongly transverse in A. alekseevi ); more elongate antennomere 11 (1.7× longer than wide in the new species and 1.2× longer than wide in A. alekseevi ); and more narrowed posteriad and less transverse pronotum, 1.3× wider than long (pronotum less narrowed posteriad and more transverse, 1.5× wider than long and less in A. alekseevi ).

In the recently published paper ( Lyubarsky et al. 2023), a key to Eocene Atomaria species is provided, including A. propinqua sp. nov. as “ Atomaria sp. Lyubarsky, Alekseev et Bukeis in prep. 2023”.

Etymology. The specific epithet of this new species stems from the Latin adjective propinquus, meaning “related, similar, resembling, near”.

Ecological note. The present-day cosmopolitan distribution, morphological polymorphism and ecological plasticity of Atomaria ( Leschen 1996; Lyubarsky 1998) do not allow us to speculate about the detailed paleohabitat peculiarities of the species in the Eocene amber forest. The fourth species of Atomaria described from Baltic amber testifies to the species richness of the genus in the European Eocene and rather wide spectrum of ecological niches for mycophagous beetles, connected with rotting vegetation of primeval forest.

Figures

FIGURES 1–3. Atomaria (Anchicera) propinqua sp. nov., holotype, No 062 [ACAB]: 1—habitus, dorsal view; 2—habitus, ventral view; 3—right antenna, dorsal view. Scale bars = 0.2 mm for Figs 1–2; 0.1 mm for Fig. 3.

FIGURES 4–5. Atomaria (Anchicera) propinqua sp. nov., holotype, No 062 [ACAB]: 4—head and pronotum, dorsal view; 5—details of head, ventral view. Abbreviation: tmp—terminal maxillary palpomere. Not to scale.