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.

Epimetopidae are a small beetle family of the superfamily Hydrophiloidea, comprising 72 described species in three genera: the American Epimetopus Lacordaire, 1854 (56 species), Asian Eumetopus Balfour-Browne, 1949 (eight species) and African Eupotemus Ji & Jäch, 1998 (eight species, of which six are described as new here). In this study we illustrate and compare the adult morphology of all three genera and generate the first DNA sequences for Eumetopus and Eupotemus. The morphological data and sequences of four genes (cox1, 16S, 18S and 28S) are used to reconstruct phylogenetic relationships among genera. Both strongly support the monophyly of Epimetopidae, reveal Eumetopus as the earliest diverging taxon and Epimetopus + Eupotemus as a strongly supported clade with numerous synapomorphies. The reciprocal monophyly of Epimetopus and Eupotemus is strongly supported by DNA data but not in the morphological analysis which reveals Epimetopus paraphyletic. Eumetopus, despite being the earliest branching clade, is characterized by many unique derived structures, e.g. by the presence of the sperm pump in males (unique in Hydrophiloidea). The available data on the biology of Epimetopidae indicate that most species inhabit sandy to muddy margins of streams or rivers. Females of all three genera carry egg cases; Epimetopidae hence are one of three independent lineages of Hydrophiloidea in which this behavior evolved. Larvae are only known for Epimetopus and are characterized by morphological adaptations for feeding by piercing and sucking, a closed tracheal system and abdominal gills; larvae of Eumetopus and Eupotemus remain unknown and further research is needed to confirm whether they show the same adaptations as Epimetopus. The taxonomy of the African genus Eupotemus is re­vised, with six species described as new: E. bilobatus sp. nov. (Nigeria), E. cameroonensis sp. nov. (Cameroon), E. ophioglossus sp. nov. (Gabon, Togo), E. smithi sp. nov. (Côte d’Ivoire), E. taianus sp. nov. (Côte d’Ivoire) and E. uluguru sp. nov. (Tanzania). Eupotemus limicola Delève, 1967 is fixed as the type species of the genus according to ICZN (1999: Art. 70.3). New records of Eumetopus species are provided (E. acutimontis Ji & Jäch, 1998 from Vietnam, E. bullatus (Sharp, 1875) from India: Maharashtra, E. flavidulus (Sharp, 1890) from India: Maharashtra and Andhra Pradesh, E. maindroni (Régimbart, 1903) from India: Maharashtra and Gujarat, and E. weigeli Skale & Jäch, 2003 from India: Uttarakhand). The habitus of all species is illustrated. An updated checklist of the Epimetopus species is provided, and records of two specimens of the E. costatus group from Zambia and Saudi Arabia are considered to result from either accidental introductions or mislabelling.