Cretoxenus, 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 : 721

CRETOXENUS GEN. NOV.

Type species: Cretoxenus australis sp. nov., by present designation.

Time range: Early Cretaceous.

Diagnosis: Body medium-sized, elongate oval; labrum exposed in front of clypeus ( Figs 3A, B , 8C ); anterior margin of clypeus concave ( Figs 3A, B , 8C ); maxillary palpi long ( Fig. 3A, B ); prosternum moderately long ( Fig. 3A ); procoxae contiguous; scutellar shield small, triangular; anapleural sutures of mesothorax well developed, nearly straight, converging anteriad ( Figs 3A , 6I , 8D ); mesanespisterna narrowly separate by mesoventrite anteriorly ( Figs 3A , 6I , 8D ); mesocoxal cavities very narrowly separated from each other; metaventrite prolonged into a triangular process posteriorly ( Figs 3A , 6A , 8D ); metanepisternum rather narrow; elytron with punctate series ( Fig. 3A, B ); sutural stria present, deeply impressed.

Etymology: The genus name consists of the prefix creto - referring to its Cretaceous age, and the ending - xenus indicating that the fossil is in some characters similar to the extant genus Limnoxenus . Masculine.

Family assignment: The combination of the antenna with antennal club, head not constricted behind eyes, presence of long maxillary palpi, small triangular scutellar shield, and femora with anteromesal corner not facing trochanter assign the fossil reliably to the family Hydrophilidae s.s. Additionally, it corresponds well with the modern Hydrophilidae in the general habitus and ventral morphology.

Comparison with other hydrophilid genera: By the combination of exposed labrum, medium body size, long maxillary palpi, mesoventrite well separates from mesanepisterna, and moderately long simple prosternum, Cretoxenus resembles the modern genera of the subfamilies Acidocerinae and Enochrinae , and of the tribe Hydrobiusini of the subfamily Hydrophilinae . It differs from the Enochrus clade of the Enochrinae by the nearly straight anapleural sutures (S-shaped in the Enochrus clade) and from the Cymbiodyta clade of the same subfamily by short mesotarsomere 1 (basal tarsomere is reduced in meso- and metathoracic legs in all representatives of the clade). The combination of the sutural stria, distinct elytral series of punctures, and long maxillary palpi distinguishes it from modern genera of the Acidocerinae . The non-denticulate lateral margins distinguish the fossil from the ‘sperchopsine’ genera of the Hydrobiusini . However, the posteriorly projecting mesal portion of the metaventrite corresponds with that present today in many Hydrobiusini (e.g. Limnoxenus Motschulsky, 1853 ; Fig. 6K, L ), some of which ( Limnocyclus , Hybogralius Orchymont, 1942 ) even share with the fossil the nearly straight anapleural sutures of the mesothorax. The presence of long maxillary palpi and exposed clypeus also diagnoses the new genus from the Rygmodinae (which have concealed labrum and/or shorter and stouter maxillary palpi) many of which are otherwise also similar to the Hydrobiusini in general habitus. The fossil hence very likely belongs to the modern tribe Hydrobiusini of the subfamily Hydrophilinae ; the above characters as well as the age of the fossil justify its separate generic status.

For the diagnosis from the Late Jurassic Protochares gen. nov., see above under that genus. Cretoxenus differs from remaining two Early Cretaceous genera Alegorius gen. nov. and Hydroyixia gen. nov., by the combination of nearly straight anapleural sutures converging anteriad (subparallel anteriorly in Alegorius , converging but weakly angulate in Hydroyixia ), deeply arcuately excised anterior margin of clypeus (shallowly excised in Alegorius , with wide subtrapezoid excision in Hydroyixia ), and large exposed labrum (narrowly exposed in Alegorius , embedded in the clypeal excision in Hydroyixia ; see Table 2 and Fig. 8C, D for details).

Figures

Figure 3. Early Cretaceous representatives of the Hydrobiusini. Cretoxenus australis gen. nov., sp. nov., Koonwarra, Australia, NMVP 103312, piece and counterpiece (A, B). Baissalarva hydrobioides gen. nov., sp. nov. from Baissa, Russia: paratype PIN 3063/6977 (C); holotype PIN 3063/6975: piece, detail of the head (D), counterpiece, detail of the head (E), whole specimen, counterpiece (F). Abbreviations: absc, abdominal dorsal sclerite; absc4, absc7, abdominal dorsal sclerite of segment 4 or 7; aes3, metanepisternum; apls, anapleural suture; cersc, cervical sclerite; dpl8, dorsal plate on abdominal segment 8; epl, epistomal lobe; md, mandible; men, mentum; mssc, mesoscutum; mstr1, mesotarsomere 1; msv, mesoventrite; mxp, maxillary palpus; mtsc, metascutum; ns, nasale; occf, occipital foramen; pros, prosternum; sbm, submentum; scsh, scutellar shield; sstr, sutural stria; ttr, tracheal trunk.

Figure 6. Fossil and recent representatives of the Hydrobiusini and Hydrophilini. Baissalarva hydrobioides sp. nov.: holotype PIN 3063/6975, anterior part of the piece (A), detail of the head of the piece (B), detail of the head of the counterpiece (C); paratype PIN 3063/6977 (D). Larva of extant Limnoxenus niger (Hydrobiusini): whole larva in dorsal view (E), detail of head (G). Larva of extant Sternolophus rufipes (Hydrophilini): whole larva in dorsal view (F), detail of head (H). Holotype of Cretoxenus australis sp. nov. NMVP 103312, piece and counterpiece (I, J). Extant hydrobiusine Limnoxenus zealandicus in ventral view, whole beetle (K) and detail of meso- and metaventrite (L). Abbreviations: md, mandible; mtvpr, metaventral process.

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.