Cymbiodyta samueli, Fikáček & Pražák & Short & Rion, 2023

Fikacek, Martin, Prazak, Jan Simon, Short, Andrew E. Z. & Rion, Francois, 2023, Fossil Cymbiodyta from Baltic amber confirms Euro-American ancient distribution of the genus (Coleoptera: Hydrophilidae), Arthropod Systematics & amp; Phylogeny 81, pp. 555-563 : 555

Cymbiodyta samueli sp. nov.

Figs 1 , 2

Material examined.

Holotype (deposited in the Natur-historisches Museum Freiburg, Switzerland): 1 specimen in a polished piece of Baltic amber (9 ×6× 4 mm).

Type locality and age.

Baltic amber, Lithuanian coast, 34-48 Mya ( Seyfullah et al. 2018).

Description.

Body: Body size 3.7 mm, maximum width 1.6 mm. Head dark-coloured both dorsally and ventrally, without clear paler preocular patches. Pronotum dark coloured on disc, yellow along margins, pale coloration wide laterally, narrow anteriorly, and very narrow posteriorly. Elytra dark colored, with widely yellow lateral margin. Ventral surface of thorax and abdomen yellowish. Head appendages, antennae and legs yellowish (Figs 1A-C ). - Head (Figs 1D, G ; 2A, C ) with large eyes, slightly protruding laterally; interocular distance 3.6 × the eye width in dorsal view. Frontoclypeal suture well developed. Clypeus with widely emarginate anterior margin. Dorsal punctation of clypeus and frons identical, moderately coarse. Labrum transverse, slightly bisinuate on anterior margin. Mentum transversely subrectangular, with slightly protruding bisinuate anterior margin. Gular sutures clear, moderately widely separated. Labial palpi with three palpomeres, apical palpomere relatively long. Maxillary palpi not preserved. Antenna with 9 antennomeres: long scapus, moderately long conical pedicel, three minute antennomeres, a cup-like antennomere (cupule) and 3-segmented pubescent antennal club; third antennomere of the club the longest, ca. twice as long as previous two antennomeres each. - Thorax (Figs 1D ; 2A, D ): Pronotum transverse, evenly convex, widening posteriad, posterolateral corners rounded; dorsal punctation fine, uniform. Prosternum with a transverse groove, without median carina. Mesoventrite with a large triangular projection at midwidth. Mesocoxal cavities transverse, contiguous. Metaventrite ca. 1.7 × longer than mesoventrite. Metanepisterna relatively wide throughout. Elytra narrowing posteriad, with at least 8 longitudinal series of fine puctures and a short scutellary series; the series not impressed as striae. Sutural stria present, clearly distinct in apical half of elytron. - Abdomen (Figs 1C ; 2A, C ) with five ventrites, ventrite 5 weakly emarginated at apex, with several stouter setae present (remaining setae of the series have been broken). - Legs (Figs 1E, F ; 2F, G ). Procoxae large, globular, meso- and metacoxae transverse. Profemora relatively shorter than meso- and metafemora, femoral pubescence not visible. Tibiae slender, straight, with several series of spine-like setae, apically with a few longer stout apical spurs. Protarsi with 5 tarsomeres, meso-and metatarsi with 4 tarsomeres; all tarsi without swimming hairs, with fine pubescence ventrally. Claws uniform in size and shape, arcuate.

Genus assignment.

Within the family Hydrophilidae , the 5-4-4 tarsal formula is unique for the genus Cymbiodyta in the subfamily Enochrinae . The other characters preserved in the fossil correspond with modern species of the genus as well: clypeus widely emarginate anteriorly, antenna with 9 antennomeres, prosternum with a transverse ridge, elytron with a sutural stria, and abdominal apex with an emargination and stouter setae at the apex.

Differential diagnosis.

Cymbiodyta samueli sp. nov. differs from both Asian Cymbiodyta and from most of the American species by the highly elevated triangular projection of the mesoventrite (the other species have a low transverse ridge in that position). Most species with large triangular mesoventral elevation (the American C. acuminata , C. leechi and C. vindicata ) are, however larger in body size (3.6-5.3 mm) and with rather deeply emarginate abdominal ventrite 5. The American C. minima (Figs 3B,G,H ) resembles the fossil species much more, but its mesoventral projection is much lower. The European C. marginella (Figs 3A, C-F ) is the most similar species to the fossil, but differs from C. samueli sp. nov. by dark brown to black ventral body surface (yellowish in C. samueli ) and absence of elytral series of puctures (with fine elytral series in C. samueli ).

Etymology.

The last author originally purchased the piece of amber with this species as a gift for his son Samuel Rion, but agreed to provide the specimen for the study instead when it was identified as a species important for understanding the evolution of the Hydrophilidae . To compensate Samuel for not getting the piece of amber with this specimen, we dedicate the new species to him.

Historical biogeography.

DIVALIKE was the best-performing model for analyses based on all four alternative time trees without the jump dispersal allowed, -DIVALIKE+ j performed the best among models allowing for jump dispersal. In all analyses (and under all three models), a wide North American and European distribution was estimated for MRCA of Cymbiodyta , without any significant effect of the age of MRCA of C. marginella and C. samueli sp. nov. on the reconstruction and on the likelihood of individual ancestral areas (see Table 1 for results obtained with best-performing models). The models with and without jump dispersal differed in the estimate of the origin of Asian species: DIVALIKE model estimate a widespread (North American+Asian) ancestor, whereas DIVALIKE+j model revealed the long distance dispersal of the North American ancestor. The analyses without the fossil revealed 100% probability of the wide ancestral range under both DIVALIKE and DIVALIKE+j (= best performing models).

Figures

Figure 1. Photographs of Cymbiodyta samueli sp. n. in Baltic amber. A - C general habitus (A, dorsolateral; B, dorsal; C, ventral). D detail morphology of prosternum and mesoventrite, with the large triangular mesoventral projection (see arrow). E - F detail of metatarsi with 4 tarsomeres. G head in dorsal view, with emarginate clypeus. H abdominal ventrite V with shallow emargination (see arrow). I complete view of the amber piece after polishing.

Figure 2. Cymbiodyta samueli sp. nov. from Baltic amber, drawings of characters. A ventral view; B dorsal view; C head in dorsal view; D mesoventrite (dotted line = reconstruction); E abdominal apex; F - G metatarsi.

Figure 3. Modern Cymbiodyta species most similar to C. samueli sp. n. from Baltic amber. A, C - F Cymbiodyta marginella (Fabricius, 1792) from Europe; B, G - H C. minima Notman, 1919 from northern USA and southern Canada. A-B, habitus (dorsal and dorsolateral view); C, metatarsus; D, prosternum; E, G, mesoventrite with mesoventral projection; F, H, abdominal apex.