Acritus (Acritus) sutirca, Alekseev & Bukejs, 2021

Alekseev, Vitalii I. & Bukejs, Andris, 2021, The first extinct species of Acritus LeConte, 1853 (Histeridae: Abraeinae) from Eocene Baltic amber: a microscopic beetle inclusion studied with X-ray micro-computed tomography, Fossil Record 24 (2), pp. 223-231 : 224-229

Acritus (Acritus) sutirca sp. nov.

urn:lsid:zoobank.org:act:81D788F9-A159-45AE-B9CC-CDBCCDEC41A4

Figs. 1–4

Type material

Holotype: no. 5541 ( MAIG), “ HOLOTYPE / Acritus sutirca sp. nov. /des. Alekseev V.I. & Bukejs A. 2021”; adult, male (determined based on micro-CT results). A complete beetle included in a very small, transparent, yellow amber piece with dimensions of 3 × 2 mm, maximum thickness of 1 mm; and preserved without supplementary fixation but subsequent to study it will be embedded in synthetic resin. The original amber piece (before cutting) included many syninclusions: three specimens of Dolichopodidae ( Diptera ), two specimens of Sciaridae ( Diptera ), one female specimen of Chironomidae ( Diptera ), one male specimen of cf. Ceratopogon sp. ( Ceratopogonidae : Diptera , det. R. Szadziewski), one female specimen of cf. Neurohelea cothurnata ( Meunier, 1904) ( Ceratopogonidae : Diptera , det. R. Szadziewski); and one specimen of Thysanoptera.

Type stratum

Middle–Late Eocene, 48–34 Ma ( Seyfullah et al., 2018; Bukejs et al., 2019; Kasiński et al., 2020; Sadowski et al., 2017, 2020).

V. I. Alekseev and A. Bukejs: The first extinct species of Acritus LeConte, 1853 ( Histeridae : Abraeinae ) 225

Type locality

Russia, Kaliningrad region, the Sambia Peninsula (Samland).

Description

Measurements: body length 0.75 mm, body maximum width 0.56 mm; pronotum length 0.28 mm, pronotum maximum width 0.51 mm; elytra length 0.49 mm, elytra maximum combined width 0.56 mm.

Body widely oval, convex; integument unicolorous black (as preserved).

Head large and wide, partially retracted into prothorax, apparently covered with fine punctation. Fronto-clypeal suture absent. Compound eyes moderately large, rounded, slightly convex. Antennae clavate, apparently 9-segmented, short, slightly longer than protibia. Scape large, elongate, clavate; pedicel cylindrical; antennomeres 3–8 transverse; antennal club (formed from fused antennomeres 9–11) without sutures, widely oval, with rounded apex, about as long as antennomeres 3–8 combined.

Pronotum transverse, 1.82× as wide as long, widest at base. Punctation distinct, small, sparse; punctures round and small; distance between punctures equal to 1.0–3.5× diameter of one puncture; interspaces smooth. Lateral mar-

226 V. I. Alekseev and A. Bukejs: The first extinct species of Acritus LeConte, 1853 ( Histeridae : Abraeinae )

gins widely rounded; anterior and posterior margins convex. Anterior angles orthogonal; posterior angles nearly orthogonal. Marginal stria complete. Antescutellar stria absent. Basal pronotal margin with sparse punctures. Prosternum non-lobed anteriorly, transversely trapezoidal in form, with slightly arcuate anterior and posterior margins, moderately convex, without distinct punctation.

Scutellar shield very small, triangular.

Elytra nearly as wide as long, widest anterior to midlength, truncated, not completely covering abdomen; basal

228 V. I. Alekseev and A. Bukejs: The first extinct species of Acritus LeConte, 1853 ( Histeridae : Abraeinae )

margin of elytra almost straight, as wide as pronotal posterior margin; elytral punctation distinct: disc evenly, finely and sparsely punctate; dorsal elytral striae or any impressions absent; marginal stria complete. Mesoventrite almost flat, narrow, about 3.3× as wide as long, without distinct punctures. Meso-metaventral suture obtusely angulate medially, without punctures. Metaventrite with flat, finely punctate disc; lateral sides with larger punctation, distance between punctures about 2.5–5.0× diameter of one puncture; posterior margin straight. Lateral metaventral stria bent and curved backwards reaching mesepimeron.

Legs rather long. All coxae widely separated, ratio of relative distances between pro-, meso- and metacoxae equal to 4: 7.5: 8.5. Femora flattened, slightly widened mesally. Tibiae nearly as long as femora; protibia setose, without teeth, slightly dilated in apical portion; meso- and metatibiae narrower, slightly dilated apically, with two thin apical spurs. Tarsi long, slender; pro- and mesotarsi pentamerous, metatarsi tetramerous; ratio of relative lengths of metatarsomeres 1–4 equal to 17: 8: 8: 20. Tarsal claws thin, free, simple.

Abdominal ventrite 1 largest, distinctly longer than ventrites 2–5 combined, nearly impunctate (with few punctures laterally), with straight femoral lines across ventrite 1; sutures between ventrites 1–4 arcuate. Pygidium and propygidium visible in ventral and caudal views, glabrous, apparently impunctate.

Aedeagus with almost parallel lateral sides; curved in lateral view; truncate apically ( Fig. 4a–d ).

Differential diagnosis

Acritus (s. str.) sutirca sp. nov. differs from the widely distributed extant Palaearctic A. nigricornis , as well as from A. komai Lewis, 1879 , A. italicus Reitter, 1904 , and many other representatives of the genus in the absence of distinct arcuate transverse antescutellar striae on its pronotal base. The shape of the lateral metaventral stria within the new species is bent and curved backwards, reaching the mesepimeron (a characteristic which is similar to A. nigricornis and different from A. komai ). The combination of other characters mentioned in the species description (e.g. body part ratios), as well as details of punctation, should distinguish the new fossil species from all Recent congeners.

Acritus (s. str.) sutirca sp. nov. can easily be distinguished from two dendrophiline histerids that have been described from Baltic amber, Carcinops donelaitisi Alekseev, 2016 , and Xestipyge ikanti Alekseev, 2016 , in that the new species is distinctly smaller in body size (both mentioned species are about 1.5 mm long) and based on its lack of dorsal elytral striation. The new species differs from the similar-looking dendrophiline microhisterids that have recently been described from Rovno and Baltic amber ( Bacanius kirejtshuki Sokolov and Perkovsky, 2020 and B. gorskii Alekseev and Bukejs, 2021 ), based on subfamilial and generic characters. These characters include the following: the anteriorly non-lobed transverse prosternum (prosternal lobe present in Bacanius , so the prosternum is longer than it is wide); 4-segmented metatarsi (in contrast to 5-segmented in Bacanius ); truncated elytra and partially exposed pygidium (in contrast to the nontruncated elytra completely covering pygidium of Bacanius ); and small, triangular scutellar shield (not distinct in Bacanius ).

Derivatio nominis

The species name “ sutirca ” is based on the generic name Acritus read backwards, and it is used as a noun in apposition.

Figures

Figure 1. Photomicrographs of Acritus sutirca sp. nov., holotype, no. 5541 (MAIG), habitus: (a) ventral view; (b) dorsal view; (c) left lateral view; (d) frontal view. Scale bar represents 0.2 mm.

Figure 2. X-ray micro-CT renderings of Acritus sutirca sp. nov., holotype, no. 5541 (MAIG), habitus: (a) dorsal view; (b) left lateral view; (c) ventral view; (d) right lateral view. Scale bar represents 0.2mm.

Figure 3. X-ray micro-CT renderings of Acritus sutirca sp. nov., holotype, no. 5541 (MAIG), habitus: (a) frontal view; (b) caudal view. Scale bar represents 0.2 mm. Abbreviations: a1 – antennomere 1 (scape); ey – compound eye; py – pygidium; pp – propygidium.

Figure 4. X-ray micro-CT renderings of Acritus sutirca sp. nov., holotype, no. 5541 (MAIG): (a–d) aedeagus in dorsal, ventral view and lateral views; (e) antennae. Scale bar represents 0.1 mm.