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

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).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.The timescale of the evolution of some of the most successful freshwater microcrustacean groups such as copepods and cladocerans is poorly known or controversial. Both groups are comprised of small and often fragile species, whose fossilized body parts might easily be overlooked. Copepods appear be predisposed to weak fossilization as they are extremely rare in the subfossil and fossil record and apparently only preserved under very unusual circumstances such as oil seeps [1]. Tasch [2], however, reasoned (after conducting drying experiments of cladocerans on pond mud) that body parts of cladocerans should be well-preserved in freshwater sediments. Of course, the subfossils of cladoceran ephippia (modified moulting exuviae containing resting eggs) and heavily chitinized body parts are common in lacustrine sediments -- fossilized cladoceran resting eggs had already been reported by 1968 from the Pliocene, the Miocene and the Eocene [3-6]. Tasch [2] "hunted thoroughly" f