New fossil taxa and notes on the Mesozoic evolution of Liadytidae and Dytiscidae (Coleoptera)

Abstract

The diagnoses of Liadytidae Ponomarenko, 1977, Liadytiscinae Prokin & Ren, 2010, Liadytiscus Prokin & Ren, 2010 and Mesoderus Prokin & Ren, 2010 (Dytiscidae) are modified, and the following new taxa are described from Mesozoic fossils: Liadytes aspidytoides sp. n. (Liadytidae); Mesoderini trib. n., Liadyxianus kirejtshuki gen. n. et sp. n., Mesoderus punctatus sp. n., Mesoderus ovatus sp. n., Mesodytes rhantoides gen. n. et sp. n., Palaeodytes baissiensis sp. n. and Cretodytes incertus sp. n. (Dytiscidae). A summarized checklist of all Mesozoic Liadytidae and Dytiscidae known from adults is given, and an identification key to the genera of Mesozoic Dytiscidae known from adults is provided for the first time. Palaeodytes incompletus Ponomarenko, Coram & Jarzembowski, 2005 (the suffix of the specific epithet is emended from the original incompleta) is found to belong not to this genus, but to another one, which remains to be described. The fossil larva Angaragabus jurassicus Ponomarenko, 1963 from the Lower Jurassic of Irkutsk Oblast, Russia, probably belonging to Liadytidae, is re-examined. If this larva actually belongs to Liadytidae, then its morphological characters provide additional confirmation of the conclusion, based on the characters of adult liadytids, that the family is quite separate from the recent family Aspidytidae, and the similarity between the adults of both families results from parallel processes in the evolution of the superfamily Dytiscoidea. We show that the principal trends of morphological changes of Liadytidae and Dytiscidae during the Upper Jurassic and Lower Cretaceous included a consistent increase in the area of the metacoxal plates at the expense of decreasing area of the lateral lobes of the metaventrite ("wings"), flattening and loss of the lateral border of the elevated median area of the metaventrite, and shortening and dilation of the metafemur and metatibia. These changes were probably associated with an increased load of swimming taken by the hindlegs, which required, among other things, the development of swimming muscles attached to the metacoxal plates. The development of the hindlegs allowed diving deeper, thus being an adaptation to the nektonic instead of benthic lifestyle. This is confirmed by the adaptive coloration of the Liadytidae and Dytiscidae found in Shar Teg (Liadytes aspidytoides sp. n.), Yixian (Mesoderus magnus Prokin & Ren, 2010) and Baisa (Palaeodytes baissiensis sp. n.), in which the dorsum was darker than underside, providing camouflage in the depths of the water.

The peril of dating beetles

“…Our analysis dates the divergences of most principal polyphagan clades in a rather narrow window in the Late Permian and Early Triassic, around the Palaeozoic–Mesozoic boundary. These results stand in contrast to the current understanding of the Permian–Triassic fossil record of Coleoptera, in which the oldest definite representatives of all four modern suborders date to Early Triassic or earliest Middle Triassic (Ponomarenko, , , ; Lawrence, ; Chatzimanolis et al , ; Grebennikov & Newton, ; Tan et al , ; Lawrence & Ślipiński, ; Prokin et al , , ).…”

The morphological evolution of the Adephaga (Coleoptera)

“…The phylogenetic placement of Mesozoic adephagan taxa was addressed in Beutel et al (2013), but since that time a considerable number of relevant studies was published (e.g. Prokin et al, 2013aProkin et al, , 2013bKirejtshuk & Prokin, 2018;Volkov, 2013;Yan et al, 2018; see also Ponomarenko & Prokin, 2015). A time-calibrated phylogeny with reliably placed fossils and an extensive sampling of genes and 'hydradephagan' and geadephagan taxa should be the goal for the near future.…”

Molecular phylogeny of the highly disjunct cliff water beetles from South Africa and China (Coleoptera: Aspidytidae)

“…The fossil record contains species similar to Aspidytidae in the extinct †Liadytidae. The two families differ primarily by the absence of an anteromedian metaventral process (between the mesocoxae) in the latter (Prokin et al, 2013). Unfortunately, the antennal scapus and pedicellus are not visible in the available †Liadytes fossils.…”

“…Finally, new fossil evidence was presented by Prokin et al . (), and a morphology‐based phylogeny of extant and fossil Adephaga by Beutel et al . ().…”

“…Larval instars of Meruidae and Aspidytidae were described recently, with analyses of phylogenetically relevant structures (Alarie & Bilton, 2005;Alarie et al, 2011;Dressler, Ge & Beutel, 2011). Finally, new fossil evidence was presented by Prokin et al (2013), and a morphologybased phylogeny of extant and fossil Adephaga by Beutel et al (2013). Although these efforts place adephagan aquatic families among the best studied groups of beetles, our understanding of their phylogenetic relationships remains incomplete.…”