† Coptotomus balticus sp. n.

(Figs 1, 2)

Type locality. Russia, Kaliningrad Region, Yantarny mine.

Age. According to Wolfe et al. (2016) Baltic amber is thought to be of middle Eocene age (Lutetian: 41.3–47.8 million years ago).

Holotype (male). In a plastic box in the main collection of the Zoologische Staatssammlung München (ZSM) with a printed label: “ Russia, Kaliningrad Region, Yantarny mine. HOLOTYPE † Coptotomus balticus sp. n. Hendrich & Balke det. 2020”.

Two labelled pictures were provided with the specimen: “Certificate 4774 Natural Baltic Amber with Inclusions expert Jonas Damzen International Amber Association Names of Inclusions: Coleoptera, Dytiscidae ”.

Paratype (female). In a plastic box in the Geologisch Palaeontologogisches Institut, University Hamburg, now: CeNak (Centrum of Natural History), with a printed label: “ Russia, Kaliningrad Region, Yantarny mine. GPIH no. 5040, coll. Gr ̂hn no. 8188. PARATYPE † Coptotomus balticus sp. n. Hendrich & Balke det. 2020”.

Description. Beetle elongate-oval, somewhat pointed posteriorly (Fig. 1A); length of beetle with head about 5.2 mm, without head about 4.8 mm; greatest width about 2.8 mm.

Surface sculpture. Head with fairly small punctation. Pronotum, elytron and ventral side with dense and larger punctation. Profemur and tibia dorsally and ventrally with coarse punctation. Mesofemur and tibia dorsally and ventrally with coarse punctation.

Structures. Head with anterolateral foveae elongate but widely separated. Antennae filiform and slender. Apical palpomere of both the maxillary and labial palps distinctly bifid (Fig. 1C). Pronotum with lateral rim. Scutellum visible. Metacoxal lobes large and rounded with metacoxal lines not closely approximated; prosternum and prosternal process in same plane and pro- and mesotarsi distinctly pentamerous. Prosternal process elongate and acutely pointed apically, received into V-shaped impression on metasternum. Metaventrite comparably broad laterally, width at narrowest point adjacent to mesocoxa 0.35 mm (Fig. 1B). Metatibia and metatarsomeres with natatorial setae. Protarsomeres 1 to 3 narrowly dilated, together bearing four transverse rows of narrow scales on ventral surface. Metatarsomeres 1 to 4 with externoapical angles lobed; metatarsal claws subequal (Fig. 1D).

Female. Length of beetle with head about 5.9 mm, without head about 5.6 mm; greatest width about 2.7 mm. Pronotum and elytron with longer longitudinal striae (Fig. 2A, C).

Etymology. Latin adjective for being from the Baltic Sea (or Baltic amber). The name is an adjective in the nominative singular.

Distribution. Only known from the type locality.

Differential diagnosis. As the shape of the aedeagus and the dorsal colouration could not be examined, the present differential diagnosis for the holotype is based on the relation of the width of the metacoxal wing to the metaventral width (MC / MV) (Larson 1975, p. 445), and the total length of body.

Based on the relation of the width of metaventral wing to metacocal width (male: MC/MV = 4.2, female: 4.6) (Fig. 1B), † C. balticus sp. n. is morphologically similar to the extant C. longulus longulus LeConte, 1852 (Fig. 3B), but † C. balticus sp. n. is much smaller (TL = 5.2–5.9 mm versus 6.8–8.2 mm) (Larson et al. 2000). † C. balticus sp. n. is the smallest species of the genus (general size range of extant Coptotomus species TL = 5.7–8.6 mm, see Larson et al. 2000).

The female of † C. balticus sp. n. possesses longer and more conspicuous longitudinal striae on pronotum and elytron than the extant species (Fig. 2A, C), which only possess fine strioles.

Habitat. Unknown. All extant species inhabit ponds or slowly flowing streams with dense vegetation (Bergsten & Miller 2016). The larvae of C. longulus lenticus Hilsenhoff, 1980 are benthic inhabitants of ponds (Bacon et al. 2000). All species are capable to flight and have been collected at light.