Geodromicus balticus Shavrin & Yamamoto

Shavrin, Alexey V. & Yamamoto, Shuhei, 2019, Unexpected palaeodiversity of omaliine rove beetles in Eocene Baltic amber (Coleoptera, Staphylinidae, Omaliinae), ZooKeys 863, pp. 35-83 : 35

† Geodromicus balticus Shavrin & Yamamoto sp. nov. Figures 1, 2, 17-18, 19-20

Type materials examined.

Holotype: female, FMNHINS-3965993, complete specimen as inclusion in a piece of light yellow Baltic amber, 3.4 cm × 2.4 cm × 0.5 cm in size (Figs 1, 2), with glued small paper on plastic envelope labeled “6083”, with three colour photographs of habitus of the beetle (two of dorsal and one of ventral view) with rectangular stamp on the back of each labeled "Certificate 6083 [handwritten in blue] | Natural Baltic Amber with Inclusions | expert Jonas Damzen | International Amber Association | Names of Inclusions: | Staphylinidae [handwritten in blue] | Rove beetle [handwritten in blue]" <with additional round stamp on the left side: "+SOCIETAS SVCCINORVM+INTERNATIONALIS"]>, with the following labels: "Baltic amber | Yantarny, Kaliningrad | Russia | (S. Yamamoto Coll.) |?Geodromicus | Omaliinae, Anthophagini | Protobiae with minute | hairs | tarsi with long hairs" <rectangular label; handwritten on both sides of the label>, "HOLOTYPE | Geodromicus | balticus sp. nov. | Shavrin A. & Yamamoto S. des. 2019" <red rectangular label, printed> (FMNH).

Preservation.

The specimen is poorly visible because it is partially covered with white cloud of microbubbles created by decay products interacting with resin, a characteristic of authentic Baltic amber ( Cai and Huang 2013). This is especially noticeable on the anterior half of the body, under the apical and basal portions of the head, including the usual location of ocelli, and most of the pronotum. The abdominal tergites are not visible dorsally, as they are covered by the hind wings. The ventral side of the specimen is visible in detail except for the basal portion of the thoracic sclerites.

Locality and horizon.

Baltic amber from Yantarny, Kaliningrad, westernmost Russia; mid-Eocene (ca 44 Ma; Wappler 2005).

Description.

Measurements: HW (ventral): 0.76; HL (ventral): ~0.40; OL (ventral): 0.25; PLL×PLW (II, III): II: 0.05 × 0.03, III: 0.08 × 0.02; PML × PMW (III, IV): III: 0.10 × 0.06, IV: 0.16 × 0.05; PL (ventral): ~0.47; PW (ventral): ~0.87; ESL: 1.40; EW: 1.51; MTbL: 1.00; MTrL: 0.36 ( I–IV: 0.20; V: 0.16); AW (IV): 1.41; TL: 3.80 (head of specimen slightly out of pronotum, thus the total length likely to be slightly shorter). Antennomeres with lengths × widths: 1:? × 0.07; 2: 0.16 × 0.06; 3: 0.11 × 0.06; 4-5: 0.15 × 0.05; 6-7: 0.15 × 0.07; 8: 0.14 × 0.07; 9-10: 0.12 × 0.07; 11: 0.25 × 0.07.

Body elongate; forebody convex. Specimen dark-brown and glossy, with antennomeres brown, mouthparts reddish-brown, legs yellow-brown with a somewhat darkened tibia. Habitus as in Figures 17-20.

Head transverse, slightly elevated in middle, about twice as wide as long, with short temples, moderately strongly narrowing toward neck, with diagonal moderately deep grooves (visible only apical part of left groove), reaching level of apical third of eye; gular sutures slightly separated at narrowest point on level of basal third of length of eyes (Fig. 20). Eyes large and widely convex, with medium-sized facets. Medioapical portion with irregular, dense and moderately deep, small punctation, without microsculpture, basal portion of head between eyes and gular sutures with dense diagonal mesh. Middle portion of swollen neck (ventral view) with dense isodiametric microsculpture (Fig. 18). Labrum transverse, with moderately widely rounded apical margin and elongate apical setae, without visible median emargination. Mandibles with strongly curved acute apex; left mandible with two elongate subapical teeth on inner side of cutting edge; distal third of cutting edge of right mandible not clearly visible, with elongate tooth somewhat shorter than that in left mandible. Maxillary palpi moderately long, with several long setae on apical margins of palpomeres 2 and 3; palpomere 2 narrower basally, gradually and slightly widened apicad; palpomere 3 distinctly longer than broad, narrow basally, markedly widened apicad; apical palpomere elongate, 1.8 times as long as penultimate segment one and visibly narrower at base than apex of penultimate one, somewhat parallel-sided in middle, slightly narrowed toward moderately acute apex (Figs 17-20). Labial palpomeres 2 and 3 distinctly longer than their width, apical palpomere 1.6 times as long as preceding segment, gradually narrowing toward apex from middle. Antenna reaching basal third length of elytra, with moderately wide antennomeres, with short dense setation; antennomere 2 slightly narrower than 1; antennomere 3 slightly shorter than antennomere 2, narrow basally and slightly widened apicad; antennomeres 4 and 5 three times as long as broad; antennomeres 6 and 7 slightly longer and distinctly wider than antennomere 5; antennomere 8 twice as wide as long; antennomeres 9 and 10 slightly shorter than antennomere 8; apical antennomere elongate, twice as long as penultimate segment and more than three times as long as broad (Figs 17, 19).

Pronotum transverse, about 1.3 times as wide as long, slightly wider than head, widest slightly in front of middle, markedly more narrowed posterad than anterad, indistinctly emarginate laterally; anterior angles widely rounded, posterior angles obtuse. Lateral portions of pronotum with small irregular punctation, without microsculpture. Pubescence appears regular, accumbent. Pronotal hypomeron and postcoxal process well developed; intercoxal process almost reaching basal third of length of procoxae, with acute sharp apex; pronotosternal suture distinct; mesoventrite with acute intercoxal process, reaching basal third of mesocoxae; metaventrite broad, with moderately acute apex of intercoxal process, not reaching mesosternal process (Figs 18, 20). Median part of prosternum with very sparse, irregular, small punctation; metaventrite with dense small punctation (Fig. 18); prosternal process with dense isodiametric microsculpture.

Elytra slightly broader than long, reaching apical margin of abdominal tergite III, markedly more than twice as long as pronotum, gradually widened apicad, with straight hind margin (Figs 17, 19). Punctation dense, small and deep, markedly smaller in basal portion, near scutellum and along suture. Pubescence regular, accumbent. Hind wings fully developed (Figs 17, 18).

Legs of moderately similar length, slender and moderately long; procoxae wide, protruding ventrad; mesocoxae large and oval, contiguous; metacoxae strongly transverse; protrochanter narrow, elongate; mesotrochanter relatively small, semioval; metatrochanter elongate; femora widest about middle; pro- and mesotibiae about as long as femora, slightly widened from narrowest basal portions toward middle, covered with regular moderately short pubescence and elongated setae on lateral margins (more visible in protibiae); metatibia markedly longer than metafemora and more than twice as long as metatarsus; apical metatarsomere slightly shorter than preceding tarsomeres together; tarsal claws simple and moderately long, without modifications (Figs 18, 20).

Abdomen slightly narrower than elytra (Figs 17-20). Abdominal tergite III to IV similar in width, beginning from segment V gradually narrowed apicad; tergite VII strongly narrowed to truncate apex (Figs 18, 20).

Male unknown.

Female. Apical margin of abdominal sternite VIII straight (Figs 18, 20). Genital segment with elongate gonocoxites, and moderately small, narrow styli (Fig. 20).

Etymology.

The specific epithet is the Latinized adjective derived from the name of the Baltic Sea.

Remarks.

Based on the shape of elongate antennomeres 8-10, the general shape of the apical maxillary palpus with elongate apical palpomere not dramatically narrower than the penultimate one, and on the length of tarsomeres 1-4 together distinctly longer than apical tarsomere, the studied specimen undoubtedly belongs to the tribe Anthophagini ( Moore and Legner 1979; Newton and Thayer 1995; Newton et al. 2000). Judging from the combination of visible morphological details of the fossil, such as shapes of the forebody, maxillary palpomeres, gonocoxites, and mandibles with developed large teeth on inner side of the cutting edge, as well as by presence of distinct grooves in front of ocelli, the species belongs to the Hygrogeus complex of genera ( Zerche 1992, 2003). The representatives of these taxa are widespread in the Holarctic Region and contain several genera reaching their greatest diversity in Central and East Asia ( Shavrin 2017a). Unfortunately, the condition of the specimen described here does not allow the observation of the presence of neither the anteocellar impressions nor the ocelli. The presence of impression between ocelli depends on the degree of convexity of head and can be significantly variable among genera and even among species of a species group, from indistinct to very deep. Regarding ocelli, these structures can be large and very convex, small, flattened, and indistinct, or reduced (sometimes in one genus) as in the tribe Anthophagini , but also in Coryphiini , Eusphalerini , and Omaliini ( Zerche 1990, 1992; Shavrin 2016). Besides proportions of the forebody, internal and external structure of the aedeagus, genera of the Hygrogeus complex can vary by the shape of the apical two maxillary palpomeres ( Coiffait 1981; Zerche 1992). The fossil cannot be member of Altaiodromicus Zerche, 1992, Hygrodromicus Tronquet, 1981, Liophilydrodes Nakane, 1983, Microedus LeConte, 1874, or Philydrodes Bernhauer, 1929, because members of these taxa share a short and very narrow apical maxillary palpomere. The new species cannot be placed in Trichodromeus Luze, 1903, species of which share a moderately small apical conical-shaped maxillary palpomere, shorter than an indistinctly widened apicad penultimate segment, or Paratrichodromeus Zerche, 1992, species of which have an apical maxillary palpomere distinctly narrower and shorter than the preceding segment. Additionally, from all these taxa the new species differs by the following: from Altaiodromicus , Hygrodromicus , and Trichodromeus by the larger eyes, less transverse head and pronotum, and elongate elytra gradually widened apicad; from Liophilydrodes and Microedus by longer elytra and absence of microsculpture between punctures on the head; from Philydrodes by the smaller head, larger pronotum, wider and longer elytra; from Paratrichodromeus by the wider pronotum and elytra and shorter antennae. The elongate apical and penultimate segment of the maxillary palpomere in this fossil are somewhat like that of some Asian species of the genus Hygrogeus Mulsant & Rey, 1880, especially the narrowed apical portion of the apical segment as in European H. aemulus (Rosenhauer, 1847). In general, the fossil differs from Hygrogeus by the slightly convex body, distinctly larger eyes and shorter temples, significantly shorter labial palpomeres and markedly wider pronotum. The relatively small body and its coloration, slightly convex dorsal side of the head, similar location of grooves and shape of mandibles are like some species of the genus Anthophagus Gravenhorst, 1802. However, it differs from Anthophagus by the shape of the apical maxillary palpomere (in Anthophagus significantly narrowed apicad and shorter than preceding palpomere), wider pronotum with markedly transverse prosternum and distinctly elongate mesosternal process ( Anthophagus with a small mesosternal process extending short distance between coxae; Moore and Legner 1979), and, more importantly, different shape and structure of metatarsi: the first metatarsomere very short as opposed to markedly elongate as in Anthophagus (sometimes slightly shorter than apical tarsomere) and absence of modifications at base of tarsal claws ( Anthophagus with two membranous patches at the base).

Based on the general shapes of the forebody, eyes, gular sutures, preapical and apical maxillary palpomeres, and antennomeres, as well as characters of the punctation and microsculpture of the body, shapes of thoracic sclerites, and length of basal metatarsomere, the new species can be placed as a putative Geodromicus . The extant representatives of the genus are widely distributed in the Holarctic Region, reaching their greatest diversity in Asia. The genus includes more than 120 species, the majority of which are distributed in the eastern Palaearctic Region and strongly associated with mountain regions ( Herman 2001; Schülke and Smetana 2015; Shavrin 2018). According to the observed morphological data, it is rather difficult to place the new species into one of the subgenera ( Geodromicus sensu stricto or Brachydromicus A. Bordoni, 1993) or any species group because the subgeneric subdivision seems to be artificial and species group placements were provided only for some taxa of the western ( Bordoni 1984) and eastern ( Shavrin 2018) Palaearctic and based on external and internal morphology of the aedeagus. The species can be tentatively compared with the smallest specimens (about 4.00 mm length, known to the first author) of the Palaearctic species G. plagiatus (Fabricius, 1798).

From all species of the genus, G. balticus sp. nov. differs by the markedly elongate apical segment of maxillary palpi. It highlights the need to revise the supraspecific taxonomy of the Hygrogeus complex, some of which have unclear status.