Raractocetus sverlilo Nazarenko, Perkovsky & Yamamoto, 2022

Yamamoto, Shuhei, Nazarenko, Vitaly Yu., Vasilenko, Dmitry V. & Perkovsky, Evgeny E., 2022, First fossil species of ship-timber beetles (Coleoptera, Lymexylidae) from Eocene Rovno amber (Ukraine), Fossil Record 25 (1), pp. 65-74 : 65

† Raractocetus sverlilo Nazarenko, Perkovsky & Yamamoto sp. nov.

Figs 1 , 2 , 3 , 4A, C, D

Holotype.

SIZK L-814, female. The beetle inclusion can be observed from multiple angles, except obliquely. Near the inclusion there are cracks that partially cover the lower part of the body. Syninclusions: Scraptiidae ( Coleoptera ), Mymaridae ( Hymenoptera ), 3 Dolichopodidae ( Diptera ), larva of Syrphidae ( Diptera ), Lepidoptera , and Acari.

Etymology.

The name of the species is derived from the Russian name, сверлило ( Raractocetus sverlilo ), for Lymexylidae .

Type strata.

Rovno amber, Upper Eocene.

Type locality.

Voronki locality (Varash district), Rovno Region, Ukraine.

Description.

Measurements: body length ca. 7.1 mm, head length 0.60 mm, head width 0.70 mm, pronotum length 0.60 mm; elytra length 1.30 mm, elytra width 0.41 mm, hindwing length ca. 4.20 mm, hindwing width 1.10 mm; length of protibia 0.37 mm, mesotibia 0.67 mm, metatibia 0.78 mm; length of protarsus ca. 0.44 mm, mesotarsus 0.71 mm, metatarsus 1.10 mm; lengths of 4-7th sternites, respectively 0.77 mm, 0.86 mm, 0.80 mm and 0.86 mm.

Female. Body small, narrowly elongate, abdomen arcuate dorsally curved. Color uniformly yellowish brown, in some areas with cuticles darker. Surface leathery, covered with dense, very short protruding pubescence in most areas; legs and thoracic sternites mostly covered with dense setae.

Head broadly oval, rather vertical, 1.3 times shorter than pronotum, slightly wider than prothorax. Eyes large, nearly contiguous, occupying almost entire visible surface of head capsule; anterior margin cut out, covering partly of antennal insertion; vertex areas each diverging to form rather triangle shape of eyes. Antennae 11-segmented, relatively short and thick, 1.2 times as long as head and 1.5 times as long as pronotum; left antenna fully visible and observable, whereas right one partially appressed with only 7 last antennomeres clearly visible; first two antennomeres each slightly transverse, rounded, similar in shape and size, with visible part of first antennomere 1.6 times as wide as its length, of second 1.3 times; 3rd to 11th antennomeres forming flagellum with somewhat fusiform club; 3rd antennomere 1.25 times wider than second, its length almost equal to its width, next two antennomeres approximately equal to it in width, fourth 1.5 times wider than its length, fifth 2 times wider than its length, sixth 1.1 times narrower than 5th, 1.3 times wider than its length, its width slightly decreasing towards apex, seventh 1.14 times narrower than sixth and 1.17 times wider its length, 8th approximately equal in length and width, 1.14 times narrower than 7th, 9th almost same width, 1.17 times wider than its length, 10th 1.1 times narrower than 9th, 1.17 times as long as wide, 11th cone-shaped, pointed, 1.18 times narrower than 10th and 2 times as long as wide. Mouthparts partially visible. Clypeus transverse. Mandibles small, inconspicuous. Maxillary palpus with at least 7 short branched appendages (i.e., maxillary palporgan sensu Wheeler 1986), each slightly to moderately longer than penultimate maxillary palpomere.

Pronotum short, without maculation, anterior margin rounded; complete longitudinal groove along midline present, but with interruption just behind middle of furrow (see Fig. 4A, C , ipf). Mesoscutellum relatively narrow, details not visible.

Mid- and hindlegs somewhat long, slender; forelegs rather short, robust, moderately thickened, profemora, protibiae, and protarsi approximately equal in length; protibiae without spur, mesotibiae and metatibiae each with one spur (indistinct on mesotibia); procoxae thick, robust, more or less cylindrical; protibiae robust, 4 times as long as wide, protarsi 1.1 times as long as protibiae, 1.35 times as long as pronotum; mesofemora 0.38 times narrower than profemora; mid- and hindlegs long and thin; mesotibiae and metatibia 1.1 times longer than pronotum and 1.8 times as long as protibiae; metacoxae strongly projecting posteriorly, metatibiae feebly curved externally. Tarsi 5-segmented, metatarsi slightly longer than metatibiae; protarsi with 1st and 5th protarsomeres very long, 2nd and 3rd short and inconspicuous, approximately equal in length, 4th protarsomere shorter than each of them; meso- and metatarsi with first segments longest, second ones about 2.3 times shorter than them, 3rd and 5th segments slightly shorter than 2nd, 4th about 1.8 times shorter than 3rd. Claws simple, widely separated.

Elytra short, 2.45 times as long as pronotum, and 3.4 times as long as its maximum width, each nearly subparallel sided, exposing most part of abdomen; outer elytral margins gently arcuate each with very shallow median emargination (Fig. 4D , eoe), whereas inner margin straight, not extending beyond metacoxae; surface covered with dense, short obliquely protruding pubescence.

Hindwings fully developed, entire, 2.7 times as long as elytra; their apices not covering last two visible abdominal tergites; venation with details remaining unclear, but with visible veins of C-Sc-R, radial-medial (r-m), medial and cubital veins, and part of anal veins. Metaventrite distinctly elongate.

Abdomen narrowly elongate, nearly subparallel-sided, weakly tapering posteriorly, uniformly bent dorsally as preserved; abdominal segments II-VII visible. Abdominal segment VIII and ovipositor partially protruding. Styli exposed, weakly clavate, their apices with sparse long bristles.

Male unknown.

Comparison.

† Raractocetus sverlilo Nazarenko, Perkovsky & Yamamoto, sp. nov. can be assigned to Atractocerinae based on the markedly reduced brachelytrous elytra with largely exposed hindwings, large bulging eyes, and distinctly modified maxillary palporgan in the female ( Paulus 2004; Yamamoto 2019). The new fossil species is placed in the genus Raractocetus by having a rather vertical head, which is slightly wider than the pronotum, and large and subcontiguous eyes in the frontal view ( Kurosawa 1985; Paulus 2004). This new species is similar to described fossil representatives of the genus from Eocene Baltic amber and mid-Cretaceous Burmese (Kachin) amber based on the strongly projecting metacoxae, length and shape of the pronotum, elytral shape, wing venation, antennal details and appendages of the maxillary palps of the female ( Yamamoto 2019), and possibly its smaller body size as well. Interestingly, † R. sverlilo sp. nov. has a much smaller body (ca. 7.1 mm) than recent atractocerine species (less than half in body length, see Lawrence 2020), but it is similar in size to the mid-Cretaceous species † R. fossilis Yamamoto, 2019 (ca. 7.6 mm). Nevertheless, the individual size of adult ship-timber beetles may depend of larval feeding and therefore, the reason for such the small sizes in the fossil lymexylid species are still unclear. More study is needed to discuss the possible miniaturization phenomenon as seen in the extinct ship-timber beetles. The new species differs from the three extinct species currently assigned to Raractocetus mainly by the pronotal and elytral shapes, together with the structures of a longitudinal furrow along midline of the pronotal disc. It is readily distinguished from † R. balticus Yamamoto, 2019 from Baltic amber, by its markedly smaller body († R. balticus , ca. 11.1 mm body length), the presence of the interruption on the pronotal furrow (see Fig. 4A, C , ipf vs. † R. balticus in Fig. 4B ), non-linear (i.e., shallowly emarginate) outer margins of the elytra (Fig. 4D , eoe) and possibly by seemingly thicker antennae. Furthermore, † R. sverlilo sp. nov. is distinguished from † R. extinctus Yamamoto, 2019 and † R. fossilis from Kachin amber by having a smaller body size († R. extinctus ca. 13.4 mm body length), simple coloration without any types of maculation, rounded anterior margin of the pronotum, more conspicuous and deeper longitudinal pronotal furrow, the presence and straight elytral inner edges with their different shapes of elytral emargination along outer margins of elytra, additionally by seemingly thicker antennae and protibiae.

Figures

Figure 1. † Raractocetus sverlilo sp. nov., female, holotype, SIZK L- 814. A. General habitus, dorsolateral view; B. General habitus, ventrolateral view. Scale bars: 1.0 mm.

Figure 2. † Raractocetus sverlilo sp. nov., female, holotype, SIZK L- 814: forebody, dorsolateral view. Scale bar: 0.5 mm.

Figure 3. † Raractocetus sverlilo sp. nov., female, holotype, SIZK L- 814. A. Head and prothorax, dorsoventral view; B. Maxillary palporgan with branches; C. Pronotum and left elytron, dorsolateral view; D. Metaventrite and legs, ventrolateral view; E. Abdomen in posterior half, ventrolateral view; F. Abdominal terminalia with styli, lateral view. Scale bars: 0.5 mm (A, C); 0.1 mm (B); 1.0 mm (D-F). Abbreviation: r-m, radial-medial vein.

Figure 4. Morphological comparison of two Eocene Raractocetus species. † Raractocetus sverlilo sp. nov., female, holotype, SIZK L- 814 (A, C, D) and † R. balticus in Eocene Baltic amber, female, holotype, FMNHINS- 3965991 (B). A. Head, pronotum and mesoscutellum, dorsolateral view; B. Head, pronotum and mesoscutellum, dorsal view; C. Head and pronotum, frontal view; D. Elytron, dorsolateral view. Scale bars: 0.5 mm (A - D). Abbreviations: eoe, shallow median emargination along outer elytral margin; ipf, interrupted longitudinal furrow on pronotum.