Hydroyixia, Fikáček & Prokin & Yan & Yue & Wang & Ren & Beattie, 2014

Fikáček, Martin, Prokin, Alexander, Yan, Evgeny, Yue, Yanli, Wang, Bo, Ren, Dong & Beattie, Robert, 2014, Modern hydrophilid clades present and widespread in the Late Jurassic and Early Cretaceous (Coleoptera: Hydrophiloidea: Hydrophilidae), Zoological Journal of the Linnean Society 170 (4), pp. 710-734 : 722

HYDROYIXIA GEN. NOV.

Type species: Hydroyixia elongata sp. nov., by present designation.

Time range: Early Cretaceous.

Diagnosis: Body medium-sized; labrum partly exposed in dorsal view ( Figs 5A, D, F , 7J , 8E ); anterior margin of clypeus deeply and widely excised mesally ( Figs 5A, D, F , 7J , 8E ); prosternum moderately long; scutellar shield small, triangular; anapleural sutures well developed, weakly curved, converging anteriad; mesoventrite very narrow anteriorly ( Figs 5B, I , 7I, K , 8F ); mesocoxae very narrowly separated; abdomen with five ventrites; apical abdominal ventrite with shallow wide emargination ( Figs 5B, F–G, I , 7L–M ); elytron with deeply impressed sutural stria ( Figs 5A, C, F, I ); elytra with series of punctures ( Figs 5C, F, I ); elytral trichobothria very distinct ( Fig. 5E ); metathoracic leg with five tarsomeres; mesotarsomere 1 very short ( Fig. 5H ).

Etymology: The generic name consists of the prefix hydro - referring to the Hydrophilidae , and the ending -yixia referring to the Yixian Formation from which both species of this genus are described. Feminine.

Family assignment: The combination of large clypeus, well-developed frontoclypeal sutures reaching lateral margin of head closely before eyes, head not constricted behind eyes, and the small triangular scutellar shield assigns the fossil reliably to the family Hydrophilidae s.s. Additionally, it corresponds well with the modern aquatic groups of the Hydrophilidae in general habitus, ventral morphology, and the presence of very distinct trichobothria on elytra and pronotum.

Comparison with other hydrophilid genera: Based on the presence of the apical emargination on the posterior margin of the abdominal ventrite 5, Hydroyixia resembles the extant representatives of the Hydrobiusini , early branching Hydrophilini , Enochrinae , and Acidocerinae . In contrast to the Hydrophilini , the meso- and metaventrite of Hydroyixia are not fused into a common ventral keel and the posterior leg lacks series of swimming hairs. In contrast to the Cymbiodyta clade of the Enochrinae , the metathoracic leg bears five tarsomeres. The mesoventrite narrowing anteriad is not known in any extant genus of the Enochrinae , and in the Acidocerinae only occurs in few clades of the Helochares group (e.g. genus Peltochares Régimbart, 1907 and Helochares obscurus species group, from which Hydroyixia gen. nov. differs by the presence of the sutural stria). Angular excision of the clypeus is present in the sperchopsine clade of the Hydrobiusini (which differ from Hydroyixia by denticulate lateral margins of elytra) and in many genera of the Acidocerinae . Many aquatic taxa of the Acidocerinae are also characterized by very conspicuous trichobothria on pronotum and elytra, which corresponds to some specimens of Hydroyixia . The combination of characters present in Hydroyixia hence assigns it best to the Acidocerinae .

Figures

Figure 5. Well-preserved specimens of Hydroyixia gen. nov., Yixian Formation, China. Hydroyixia elongata sp. nov.: holotype CNU 2010005, piece and counterpiece (A, B); paratype CNU 2009075, whole specimen (C), detail of head and pronotum (D); detail of elytral apices (E). Hydroyixia latissima sp. nov.: holotype, CNU 2009074, whole specimen (F), detail of abdominal apex (G); posterior leg of paratype CNU 2010014 (H); paratype CNU 2009107 (I). Abbreviations: abem, apical emargination of abdominal ventrite 5; aes3, metanepisternum; apls, anapleural sutures; cl, clypeus; fcs, frontoclypeal suture; gs, gular suture; lb, labrum; men, mentum; msv, mesoventrite; mtv, metaventrite; mttr1, metatarsomere 1; pros, prosternum; sstr, sutural stria; trich, trichobothrium.

Figure 7. Hydrophilid fossils from Early Cretaceous Yixian Formation, China. Alegorius yixianus sp. nov.: holotype CNU 2009079, whole specimen (A), detail of meso- and metaventrite (B), detail of abdominal apex (C); paratype CNU 2009078, whole specimen (D), detail of mesoventrite (E). Hydroyixia elongata sp. nov.: paratype CNU 2009075, detail of head and pronotum (F), whole specimen (G); holotype CNU 2010005: whole beetle, piece and counterpiece (H, I), detail of head (J), detail of meso- and metaventrite (K), detail of abdominal apex (L). Hydroyixia latissima sp. nov.: holotype CNU 2009074, detail of abdominal apex (M), whole specimen in dry condition (N) and under alcohol (O). Abbreviations: apls, anapleural suture; abem, apical emargination of abdominal ventrite 5.

Figure 8. Reconstructions of the clypeus shape and meso- and metathoracic morphology of the Early Cretaceous hydrophilid genera known in adult stage (three-dimensional structure of the mesoventrite not reconstructed). Alegorius gen. nov. (A, B); Cretoxenus gen. nov. (C, D); Hydroyixia gen. nov. (E, F). Head and thorax of the respective genus not to scale.