Rhantus villumi, Prokin & Hájek & Vasilenko & Perkovsky, 2024

Prokin, Alexander A., Hájek, Jiří, Vasilenko, Dmitry V. & Perkovsky, Evgeny E., 2024, The oldest Rhantus (Coleoptera, Dytiscidae) from the earliest Eocene Fur Formation, Denmark, Zootaxa 5458 (2), pp. 263-274 : 264-268

† Rhantus villumi sp. nov.

( Figs. 1–7 )

Type material. Holotype: FUM-N 11573, complete (direct and counter impressions) female imprint. Oval beetle, with genitalia, partly preserved antennae, maxillary palpi, hind legs, and distal portion of protarsus.

Description. A large Rhantus species, total length 21.4 mm, length without head 18.6 mm, length of elytra 15.2 mm, maximum width 10.9 mm. Head 1.85 times as wide as long, pale with darkened clypeus and subtrapezoidal transverse dark spot medially ( Figs. 1 , 3 ). Antenna filiform with antennomeres I–IV longer than others, ultimate maxillary palpomere longer than penultimate ( Fig. 6 ). Eye 0.24 as wide as head. Pronotum 3.4 times as wide as long, 1.5 times as wide basally as at its anterior margin; pale medially, with broadly darkened anterior part, lateral parts, and narrowly darkened posterior part projecting anteridad submedially as two spots; lateral bead not evident ( Figs. 1 , 3 ). Scutellum broadly triangular ( Figs. 2 , 5 ). Elytron translucent with black irrorations around small spots, integrated in rows better visible near lateral margin; epipleuron broad basaly, continuously decreasing to level of posterior margin of abdominal ventrite IV ( Figs. 1–4 ). Metacoxal plate about twice as long as lateral lobe of metaventrite (also termed “metasternal wing”) ( Figs. 2 , 4 ). Abdominal ventrites II and III nearly fused, ventrites I–V subequal in length; ventrite VI about 1.5 times as long as previous ventrites, evenly rounded ( Figs. 2 , 4 ). Ultimate protarsomere (protarsomere 5) 3 times as long as penultimate protarsomere (protarsomere 4); protarsal claws subequal in size and shape ( Fig. 6 ). Metafemur 1.25 times as long as metatibia, with large cavity in apical third for receiving tibia ( Fig. 7 ). Metatibia slightly widened towards apex, with line of 8 setigerous punctures on anterior surface, strong sharp setae on dorsal and apical margins, and with long and slender setae on ventral margin ( Fig. 7 ). Metatarsomere I twice as long as metatarsomere II, III nearly 3 times as long as IV and V; all tarsomeres with long natatorial setae on dorsal margin, and strong sharp setae on ventral margin; claws unequal ( Fig. 8 ).

Comments on classification. Within the extant subfamilies of Dytiscidae , we place the fossil in the subfamily Colymbetinae , based on the following combination of characters: 1) eyes anterolaterally emarginate; 2) apical maxillary palpomere simple, not biramous; 3) mesoscutellum (scutellar shield) clearly visible with elytra closed; 4) prosternal process without longitudinal groove, obtusely lanceolate; 5) apex of elytron evenly rounded; 6) lateral parts of metaventrite broadly triangular; 7) metacoxal lines present, broadly separated; 8) metafemur without linear series of setae near anteroapical angle; 9) metatarsal claws unequal (see Miller & Bergsten 2016). It differs from the only extinct subfamily †Liadytiscinae in the maximum length of the metacoxal plate greater than the maximum length of the lateral part of the metaventrite, metatarsomeres 1–4 with lobed apical margins, metatibia shorter than metafemur, unequal metatarsal claws (see Prokin et al. 2013).

Within the subfamily Colymbetinae , the fossil is tentatively assigned to the cosmopolitan genus Rhantus Dejean, 1833 , mainly based on the following characters (see Balke et al. 2017): 1) body elongated oval, not slender or torpedo-shaped; 2) sides of pronotum with indistinct\narrow lateral bead; 3) posterior pronotal angles weakly obtuse; 4) anterior margin of metaventrite deeply incised for receiving prosternal process; 5) anterior surface of metatibia with several setigerous punctures arranged in linear row; 6) metatarsomeres weakly lobed on posteroapical margin. The genus Rhantus , in the current sense, is only molecularly defined, without any known morphological synapomorphy and without clear known differences, especially from the genus Carabdytes Balke, Hendrich & Wewalka, 1992 (see Balke et al. 2017). However, we consider the presence of the latter genus, which is currently distributed exclusively on Pacific Islands, in Eocene of Europe to be rather unlikely, and therefore prefer to classify the fossil as Rhantus , of which 11 species are currently known to occur in Europe (see Nilsson & HÁjek 2024).

The female holotype imprint lacks the usual characters used to identify recent Rhantus species (elytral sculpture, shape of male claws, shape of male genitalia; see, e.g. Nilsson & Holmen 1995). Therefore, the new species is mainly distinguished on the basis of its large body size, which is significantly larger than in all known Rhantus species (body size 5.7–17.8 mm, according to Miller & Bergsten 2016). In addition, the colouration of the head and pronotum may also be of limited use for alpha taxonomy, although it varies in many extant species (see, e.g. Nilsson & Holmen 1995: figs. 507–516; HÁjek 2009: figs. 138–146).

Etymology. This species is dedicated to the Villum Foundation, named after Villum Kann Rasmussen (1909– 1993), whose financial support enables the EEP to continue the study of the Eocene insect fauna.

Figures

FIGURES 1–2. †Rhantus villumi sp. nov., photographs of holotype: 1. direct impression; 2. counter impression. Scale bars, 4 mm.

FIGURES 3–4. †Rhantus villumi sp. nov., line drawings of holotype: 3. dorsal view; 4. ventral view.

FIGURES 5–6. †Rhantus villumi sp. nov., details of counter impression of holotype: 5. part of head, prosternum and mesoventrite, scale bar 1 mm; 6. anterior margin of head with maxillary palpi, base of antenna and distal portion of protarsus, scale bar 0.5 mm.

FIGURES 7–8. †Rhantus villumi sp. nov., details of holotype: 7. left hind leg in direct impression, dorsal view; 8. right hind leg in counter impression, ventral view. Scale bars 1mm.