Foss. Rec., 24, 223–231, 2021 https://doi.org/10.5194/fr-24-223-2021 © Author(s) 2021. This work is distributed under the Creative Commons Attribution 4.0 License. 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 Vitalii I. Alekseev1,2,3 and Andris Bukejs4 1Shirshov Institute of Oceanology, Russian Academy of Sciences, Nahimovskiy prospekt 36, Moscow 117997, Russia 2Institute of Life Sciences, Immanuel Kant Baltic Federal University, Nevskogo street 14, Kaliningrad 236016, Russia 3Kaliningrad Regional Amber Museum, Marshal Vasilevskii square 1, Kaliningrad 236016, Russia 4Institute of Life Sciences and Technologies, Daugavpils University, Vien¯ıbas 13, Daugavpils 5401, Latvia Correspondence: Andris Bukejs ([email protected]) Received: 26 March 2021 – Revised: 11 June 2021 – Accepted: 17 June 2021 – Published: 8 July 2021 Abstract. Acritus sutirca sp. nov. is described and il- small mammals (Kryzhanovskij and Reichardt, 1976). The lustrated from Eocene Baltic amber on the basis of one present-day distributional range of several species (e.g. Acri- adult male specimen. As the first extinct member of the tus komai Lewis, 1879) which also occur in anthropogenic subfamily Abraeinae (Histeridae) and the smallest known habitats may be secondarily widened by accidental human fossil histerid specimen, this material was examined us- introduction (Lackner, 2013). ing a combination of light microscopy and X-ray micro- Histerid inclusions are scarce in Baltic amber collections computed tomography (µCT). Internal features of the ab- (e.g. Klebs, 1910; Hieke and Pietrzeniuk, 1984; Kulicka domen are well preserved, allowing us to study sclero- and Slipi´ nski,´ 1996) and are represented by small and eas- tized parts of the aedeagus and illustrate these structures ily overlooked specimens. In general, the clown beetles in detail. The current finding expands the range of known of Eocene Baltic amber are insufficiently studied (Alek- Histeridae diversity from the Eocene (48–34 Ma) “amber seev, 2013, 2017a). The oldest reports of the family from forests” of Europe. The absolute size limits and the proba- this lagerstätte are not precise and include the notes “His- ble average size for Baltic amber coleopteran inclusions are ter” within the “Clavicornia” group (Berendt, 1845; Menge, also briefly discussed (urn:lsid:zoobank.org:pub:A2B17479- 1856), as well as “Histeridae” (Helm, 1896). The generic 8628-47C7-A8D4-3E0789BF088B). data on clown beetles in Baltic amber were first provided by Klebs (1910), who listed five different specimens from the famous Königsberg amber collection: two specimens belong- ing to Abraeinae (“Abraeus” and “Acritus”), two belonging 1 Introduction to Dendrophilinae (“Bacanius?” and “Carcinops”) and one specimen belonging to Histerinae (“Platysoma”). This more The genus Acritus LeConte, 1853 is nearly cosmopolitan or less credible and brief information was repeated later with- in distribution and comprises more than 120 described ex- out any significant additions (Spahr, 1981), and the data have tant species worldwide (Kryzhanovskij and Reichardt, 1976; been partially supported or supplemented only within the Mazur, 2011), while 16 species are registered in the Recent last decade (Alekseev, 2016; Sokolov and Perkovsky, 2020; Palaearctic (Lackner et al., 2015). Representatives of this mi- Alekseev and Bukejs, 2021). crohisterid genus occur usually under the bark of deciduous The size of insect inclusions in ambers is often used for trees and in putrescent wood, as well as in decaying fungi, morphologic comparison with extant relatives. Sometimes, under stones, and in forest litter. Sometimes, the beetles can the size difference between extant and amber forms is in- be found cohabitating with ants or termites, and they have terpreted in the context of climate changes (Nadein and been sampled in boreholes of bark beetles or in burrows of Published by Copernicus Publications on behalf of the Museum für Naturkunde Berlin. 224 V. I. Alekseev and A. Bukejs: The first extinct species of Acritus LeConte, 1853 (Histeridae: Abraeinae) Perkovsky, 2019) or as an evolutionary trend within the Genus Acritus LeConte, 1853 group (Perkovsky and Nel, 2021). Without doubt, the ratio- nal and adequate use of data available from fossils should Subgenus Acritus LeConte, 1853 include the size information and is promising. However, it needs to be used for general conclusions with caution. In the Remarks current paper, a new microhisterid species of the genus Acri- tus LeConte, 1853 is described and illustrated from Eocene The specimen under study is assigned to the tribe Acri- Baltic amber using X-ray microtomography. The male geni- tini within the subfamily Abraeinae based on the following: talia are examined and illustrated. It is the first fossil member (1) the prosternum without anterior lobe; (2) body size less of this genus and one of the smallest described coleopteran than 1.0 mm; (3) metatarsi with four tarsomeres (i.e. tarsal species from this lagerstätte and from Eocene European am- formula 5–5–4); (4) body form oval, not subcylindrical; and bers in general. The problematic of the size range in amber (5) pronotum evenly convex, without deep transverse furrow inclusions is briefly discussed as well. medially. The generic placement and attribution to Acritus is based on a combination of the following characters: (1) eyes present; (2) protibiae setose, without spinules or denticles; 2 Material and methods (3) scutellar shield small but visible; (4) pygidium without marginal stria; and (5) pronotum with a transverse line of The material examined is deposited in the collection of large punctures along the basal margin. the Museum of Amber Inclusions, University of Gdansk´ The extant genus Acritus consists of two subgenera: Acri- (Poland) (MAIG). tus s. str. with 118 described species and Pycnacritus Casey, The X-ray micro-CT (µCT) observations of specimen 1916 with 9 species (Mazur, 2011). The studied specimen were conducted at Daugavpils University, Daugavpils, does not have any oblique dorsal striae or vague impressions Latvia, using a Zeiss Xradia 510 Versa system. Scans were on the elytra, and it lacks any microsculpture between its performed with a polychromatic X-ray beam at an energy of fine elytral punctation. Therefore, a newly described fossil 30 kV and power of 2 W. Sample to detector distance was species was placed in the nominative subgenus. set to 12.8 mm, and source to sample distance was set to 23.4 mm. Tomographic slices were generated from 3001 ro- Acritus (Acritus) sutirca sp. nov. ◦ × tation steps through a 360 rotation, using a 20 objective, urn:lsid:zoobank.org:act:81D788F9-A159-45AE- and exposure time during each projection was set to 17 s. Ac- B9CC-CDBCCDEC41A4 quired images were binned (4 × 4 × 4) giving a voxel size of 1.75 µm. Images were imported into the Dragonfly PRO (ver- Figs. 1–4 sion 2020.2) software platform for interactive segmentation and 3D visualization. Prior to the full scan, a 23 min warmup scan was conducted with identical stitch parameters, but with Type material reduced rotational steps (201) and exposure time set to 3 s. Holotype: no. 5541 (MAIG), “HOLOTYPE/Acritus sutirca The photographs of the specimen were taken using a Leica sp. nov./des. Alekseev V.I. & Bukejs A. 2021”; adult, male M205 (stereomicroscope with a camera). Extended depth of (determined based on micro-CT results). A complete bee- field at high magnification was achieved by combining mul- tle included in a very small, transparent, yellow amber piece tiple images from a range of focal planes using Leica Appli- with dimensions of 3 × 2 mm, maximum thickness of 1 mm; cation Suite software, and the resulting images were edited and preserved without supplementary fixation but subse- to create figures using Adobe Photoshop 7.0 software. quent to study it will be embedded in synthetic resin. The The following references were used for the generic place- original amber piece (before cutting) included many synin- ment of the examined specimen and its comparison with clusions: three specimens of Dolichopodidae (Diptera), two extant and extinct taxa: Wenzel (1955), Kryzhanovskij and specimens of Sciaridae (Diptera), one female specimen of Reichardt (1976), Gomy (1994), Lackner (2013), Alek- Chironomidae (Diptera), one male specimen of cf. Cerato- seev (2016), Sokolov and Perkovsky (2020), and Alekseev pogon sp. (Ceratopogonidae: Diptera, det. R. Szadziewski), and Bukejs (2021). one female specimen of cf. Neurohelea cothurnata (Meunier, 1904) (Ceratopogonidae: Diptera, det. R. Szadziewski); and 3 Systematic palaeontology one specimen of Thysanoptera. Family Histeridae Gyllenhal, 1808 Type stratum Subfamily Abraeinae MacLeay, 1819 Middle–Late Eocene, 48–34 Ma (Seyfullah et al., 2018; Bukejs et al., 2019; Kasinski´ et al., 2020; Sadowski et al., Tribe Acritini Wenzel, 1944 2017, 2020). Foss. Rec., 24, 223–231, 2021 https://doi.org/10.5194/fr-24-223-2021 V. I. Alekseev and A. Bukejs: The first extinct species of Acritus LeConte, 1853 (Histeridae: Abraeinae) 225 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. Type locality Head large and wide, partially retracted into prothorax, ap- parently covered with fine punctation. Fronto-clypeal suture Russia, Kaliningrad region, the Sambia Peninsula (Samland). absent. Compound eyes moderately large, rounded, slightly convex. Antennae clavate, apparently 9-segmented, short, Description slightly longer than protibia. Scape large, elongate, clavate; pedicel cylindrical; antennomeres 3–8 transverse; antennal Measurements: body length 0.75 mm, body maximum width club (formed from fused antennomeres 9–11) without su- 0.56 mm; pronotum length 0.28 mm, pronotum maximum tures, widely oval, with rounded apex, about as long as an- width 0.51 mm; elytra length 0.49 mm, elytra maximum tennomeres 3–8 combined. combined width 0.56 mm. Pronotum transverse, 1:82× as wide as long, widest at Body widely oval, convex; integument unicolorous black base.