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Background  The timescale of the origins of  Daphnia  O. F. Mueller (Crustacea: Cladocera) remains controversial. The origin of the two main subgenera has been associated with the breakup of the supercontinent Pangaea. This vicariance hypothesis is supported by reciprocal monophyly, present day associations with the former Gondwanaland and Laurasia regions, and mitochondrial DNA divergence estimates. However, previous multilocus nuclear DNA sequence divergence estimates at < 10 Million years are inconsistent with the breakup of Pangaea. We examined new and existing cladoceran fossils from a Mesozoic Mongolian site, in hopes of gaining insights into the timescale of the evolution of  Daphnia .    Results  We describe new fossils of ephippia from the Khotont site in Mongolia associated with the Jurassic-Cretaceous boundary (about 145 MYA) that are morphologically similar to several modern genera of the family Daphniidae, including the two major subgenera of  Daphnia , i.e.,  Daphnia  s. str. and  Ctenodaphnia . The daphniid fossils co-occurred with fossils of the predaceous phantom midge (Chaoboridae).    Conclusions  Our findings indicate that the main subgenera of  Daphnia  are likely much older than previously known from fossils (at least 100 MY older) or from nuclear DNA estimates of divergence. The results showing co-occurrence of the main subgenera far from the presumed Laurasia/Gondwanaland dispersal barrier shortly after formation suggests that vicariance from the breakup of Pangaea is an unlikely explanation for the origin of the main subgenera. The fossil impressions also reveal that the coevolution of a dipteran predator (Chaoboridae) with the subgenus  Daphnia  is much older than previously known -- since the Mesozoic.

Mesozoic fossils (>145 Mya) suggest the antiquity of the subgenera of Daphniaand their coevolution with chaoborid predators