HYDROPHILIDAE DATED BACK TO THE LATE JURASSIC

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 : 728

HYDROPHILIDAE DATED BACK TO THE LATE JURASSIC

The fossils described in this paper represent first records of the Hydrophilidae s.s. from the Mesozoic and clearly document that the family was well established already by the Late Jurassic. In this respect, Hydrophilidae s.s. corresponds to the Helophoridae , the occurrence of which in the Late Jurassic was documented by Fikáček et al. (2012a, b). Although the phylogenetic relationships of the hydrophiloid families remain unresolved (e.g. Hansen, 1991; Archangelsky, 1998; Beutel & Komarek, 2004; Bernhard et al., 2006, 2009), the Helophoridae and the Hydrophilidae seem not to be closely related and each of them probably represents a different clade of the superfamily Hydrophiloidea (the helophorid lineage in the case of the Helophoridae , the hydrophilid lineage in the case of the Hydrophilidae ; see, for example, Hansen 1991 and Archangelsky 1998, 2007 for details). The fact that both these families were well established by the Late Jurassic hence indicates that the remaining four extant hydrophiloid families ( Georissidae , Epimetopidae , Hydrochidae , and Spercheidae ) were very likely already differentiated by the same time. In addition, extinct hydrophiloid clades were also present in the Mesozoic (Fikáček et al., 2012a, b).

The presence of the Early Cretaceous fossils that may be reliably assigned to the extant tribes is rather surprising. For example, it indicates that the tribes Hydrobiusini and Hydrophilini diverged c. 90 Myr earlier than it was supposed: the minimum age of the divergence was until now dated to the middle Eocene (47 Mya) based on the fossils from the Messel Pit locality in Germany (Fikáček et al., 2010a), and is now shifted to the Early Cretaceous (c. 140 Mya) based on the above described fossils of Baissalarva gen. nov. and Cretoxenus gen. nov. The same is true for the subfamily Acidocerinae , which may be dated back to c. 126 Mya based on the above-described fossils of Hydroyixia gen. nov. The position of Baissalarva in the extant Hydrobiusini is especially well supported by the morphology of the nasale (bearing five teeth and the left-most tooth more separated from remaining teeth). Estimated ages for hydrophilid clades on the basis of previously known fossil records have been questioned based on the current distribution of several extant clades exhibiting fingerprints of possible ancient Gondwanan distribution (see Short & Fikácˇek 2013 for details). Our present results are congruent with these indirect clues, confirming that previous age estimates were far too young for the family Hydrophilidae and its subgroups.

All adult-stage fossils described in this contribution are characterized by a rather generalized morphology, as are also extant representatives of the Hydrobiusini and Acidocerinae . Both above extant groups lack apparent clade-specific apomorphies and are mostly characterized by combinations of plesiomorphic character states (Short & Fikácˇek, 2013). This stands, together with the occurrence of these generalized forms in the Late Jurassic and Early Cretaceous, in contrast to the phylogenetic positions of both clades, which were resolved as rather deeply nested in the Hydrophilidae rather than early branching (Short & Fikácˇek, 2013). Two possible scenarios may explain this seeming contradiction: either a parallel independent ‘return’ to generalized morphology in several hydrophilid clades, or a long-term survival of the generalized stem-group taxa from which morphologically derived clades diverged sequentially. The first scenario would suppose that even the Late Jurassic fossils were already members of the extant general-looking clades, whereas the second scenario would expect them to be either stem-group taxa of representatives of modern clades. Dating of the principal divergences of the hydrophilid tree and/or discovery of better-preserved Jurassic fossils of the Hydrophilidae may help to distinguish between these two scenarios. Regardless, the occurrence of the Hydrobiusini and Acidocerinae in the Early Cretaceous implies that many morphologically derived groups (e.g. Amphiopini , Berosini , Laccobiini , Chaetarthriini ) were already present at that time and may be found in the fossil record. Their small body size, globular body (which is frequently deformed during fossilization), and the loss of some easy-to-preserve hydrophilid synapomorphies (e.g. frontoclypeal suture, long maxillary palps) may explain why they were overlooked or not yet discovered in the fossil record.