Research Article |
Corresponding author: Alexey V. Shavrin ( ashavrin@hotmail.com ) Academic editor: Jan Klimaszewski
© 2020 Alexey V. Shavrin, Shûhei Yamamoto.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Shavrin AV, Yamamoto S (2020) A remarkable new species of the rove beetle genus Anthobium Leach, 1819 from Eocene Baltic amber (Coleoptera, Staphylinidae, Omaliinae). ZooKeys 973: 89-101. https://doi.org/10.3897/zookeys.973.53940
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An unusual new omaliine species, Anthobium alekseevi sp. nov., is described and illustrated from Eocene Baltic amber, tentatively placed in the megadiverse genus Anthobium Leach, 1819. A new monotypic species-group is established. The new species can easily be distinguished from other species of the genus by the larger body, shape of the subrectangular pronotum, and the presence of a median carina on the prosternum and large, subtriangular tooth on the inner side of each mesotibia, likely exhibiting a peculiar sexual dimorphism in the male. Based on the study of the specimen with support of microtomographic images, a brief comparative analysis of a new species with described extant species of Anthobium is provided.
Anthobium, Anthophagini, fossil, Omaliini, palaeontology, sexual dimorphism, x-ray micro-CT
The rove beetles of the subfamily Omaliinae, with about 1700 species in 118 extant and 14 extinct genera, are distributed in northern temperate areas, with greatest diversity in the Holarctic region. Omaliines are common in various types of biotopes. An overwhelming number of species are hygrophiles, and they can be found near swamps, banks of rivers, mountain streams at high elevations, etc. Most species are predators of small invertebrates or are sapro- and mycophagous, and some species are even pollen-feeders (e.g. Amphichroum Kraatz, 1858).
The fossil history of Omaliinae was briefly discussed by
Due to the poor visibility of several main details of the body, the single specimen of a possible omaliine, as an inclusion within a piece of the Baltic amber from the collection of V. Alekseev (Kaliningrad), was difficult to attribute to any taxon, and therefore it was not included in our last study on fossil omaliines (
The studied material is housed in the private collection of Vitalii I. Alekseev (Kaliningrad, Russia) and eventually will be deposited in Borissiak Paleontological Institute of the Russian Academy of Sciences, Moscow, Russia (
All measurements are given in millimeters and were made with a stereoscopic microscope equipped with an ocular micrometer. Measurements were made from the dorsal side of the specimen except for ocular length and width of the abdomen which were made from the ventral side. Measurement of the total length of the body was difficult to do because of the specimen’s orientation within the amber piece; the resulting approximate values are marked with “~”. The type labels are cited in inverted commas and separated from each other by a comma, different lines in labels are separated with ‘|’; explanations of the type labels are given in square brackets, necessary notes within the label are given in angle brackets.
The specimen was examined using a Nikon SMZ 745T stereomicroscope. A Sony Alpha DSLR-A300digital camera was used for photographs of amber, habitus, and its details. Micro-CT observations of the specimens were conducted at the Daugavpils University (Daugavpils, Latvia) using Zeiss Xradia 510 Versa system. Scans were performed with a polychromatic x-ray beam at an energy of 40 kV and power of 3 W. Sample-detector distance was set to 17.6 mm and source to sample distance 32.6 mm. Tomographic slices were generated from 3001 rotation steps through a 360-degree rotation, using a 4× objective, and exposure time during each projection was set to 3 s. Acquired images were binned (2 × 2 × 2) giving a voxel size of 4.3 μm. Images were imported into Dragonfly PRO (ver. 4.1) software platform for interactive segmentation and 3D visualization. Prior to the full scan a 23-minute warmup scan was conducted with the same scan parameters except rotation steps which had been reduced to 201 and exposure time which was reduced to 1 s.
Subfamily Omaliinae MacLeay, 1825
Tribe Anthophagini Thomson, 1859
Omalium atrocephalum Gyllenhal (= Silpha melanocephalum sensu Marsham), for details see
Body medium-sized; forebody convex, shiny; apical segment of maxillary palp twice as long as preceding segment; anterior angles of subrectangular pronotum slightly protruded anteriad, mediobasal third of pronotum with oval impression; lateral edges of pronotum without crenulation; prosternum with distinct median carina; surface of elytra without elevations; inner side of each mesotibia with large tooth in middle.
†Anthobium alekseevi sp. nov.
Anthobium alekseevi sp. nov. differs from the remaining species of the genus by the larger body, the shape of the subrectangular pronotum, the presence of distinct median carina on the prosternum, and the presence of a large median tooth on the inner side of the mesotibia.
Holotype: male, complete specimen as inclusion in a piece of small yellow Baltic amber, 11.0 mm × 0.7 mm × 0.5 mm in size (Figs
The specimen is poorly visible because it is partially covered with white microbubbles, and some details of the structure of the body are not visible: head, median portion of pronotum and scutellum, ventral side of the body and abdomen (Figs
Baltic amber from Kaliningrad Area, westernmost Russia; mid-Eocene (ca 44 Ma;
Measurements: maximum width of head including eyes: 1.30; length of head (from base of labrum to neck constriction along head midline in dorsal view): 0.75; ocular length: 0.40; length × width of segments III and IV of maxillary palpi: III: 0.15 × 0.10, IV: 0.30 × 0.10; length of antenna: 2.70; length of pronotum: 1.35; maximum width of pronotum: 1.75; sutural length of elytra from apex of scutellum to posterior margin of sutural angle: 2.60; length of elytron from basal to apical margin: 2.95; maximum width of elytra: 2.30; length of metatibia: 1.60; length of metatarsus: 0.80; maximum width of abdomen (at segment IV): 2.10; total length (from anterior margin of clypeus to apex of abdomen): ~5.40.
Body oblong, moderately wide, shiny (Fig.
Head transverse (Figs
Pronotum subrectangular, 1.2 times as wide as long, 1.3 times as wide as head, widest in middle, evenly rounded both anteriad and posteriad (Fig.
Elytra convex, slightly longer than wide, about twice as long as pronotum, indistinctly widened in middle, reaching basal margin of abdominal tergite VI, with widely rounded apical margins (Fig.
Legs long (Fig.
Abdomen distinctly narrower than elytra (Fig.
Male. Protarsomeres 1–4 distinctly wide, with very long lateral setae (Fig.
Female unknown.
In general, A. alekseevi sp. nov. can be characterized by the similar coloration and shape of the convex, shiny, and glabrous body, as in many species of Anthobium. The punctation of the forebody is poorly visible except in the lateral and basal portions of the pronotum and elytra; the median portion of the elytra bears longitudinal rows of punctures. The pronotum is shiny, and the microsculpture between punctures is missing. Similar punctation on the shiny forebody is specific for some East Asiatic species groups (e.g. gracilipalpe and nigrum). Similar to many species of the genus, the head of A. alekseevi sp. nov. has distinct median elevation and elongate grooves in front of the ocelli. Unfortunately, each latero-apical portion of the head is hidden by adjoining basal anttennomeres, so the presence and shape of an antocular identation are invisible. Usually, the antocular identation of Anthobium is variable in its shape: some species have distinct and semicircular notch as figured by
Anthobium alekseevi sp. nov. has several significant morphological features that distinguish it from other known species of the genus, which led us to propose a new, separate species group for it. The first peculiar feature is the large body of the new species, which is roughly 5.4 mm long. So far, the largest specimens of Anthobium have been 4.75 mm long: Himalayan A. nigrum (Cameron, 1924) and Chinese A. puetzi Shavrin & Smetana, 2017. On average, the body length of known species of the genus varies from 3.0 to 4.0 mm, and the smallest species is the Chinese A. liliputense Shavrin & Smetana, 2018, with specimens as small as 1.8 mm in body length. The second peculiar feature is the shape of subrectangular pronotum (Fig.
Anthobium alekseevi sp. nov. 14 forebody, dorsal view 15 head, apical portion of pronotum and legs, frontal view 16 head, pronotum and antenna, dorsal view 17 elytra and abdomen, dorsal view 18 head and thorax, lateroventral view 19 apical portion of elytra, abdomen and legs, posterodorsal view. Scale bar: 1.0 mm.
Species of Anthobium are strongly dependent on a temperate climate, living in forest litter and wet moss, and most commonly inhabit wet habitats near swamps and along banks of streams and rivers. Hypothetically, A. alekseevi sp. nov. may have lived in wet biotopes near rivers or swamps. A similar temperate-loving, extinct, and potentially rheophilous species, Geodromicus balticus Shavrin & Yamamoto, 2019, was also described from Eocene Baltic amber.
We are grateful to Vitalii I. Alekseev (Kaliningrad, Russia) for the material, Kristaps Kairišs (Daugavpils, Latvia) for the providing of microtomographic images, and Giulio Cuccodoro (Genève, Switzerland) for helpful comments and suggestions. This study was partly supported by Grant-in-Aid for JSPS Fellows (JSPS KAKENHI Grant Number 20J00159) to SY from the Japan Society for the Promotion of Science (JSPS), Tokyo, Japan.