Ephippia of Daphnia O.F. Müller (Crustacea: Branchiopoda) from the Late Eocene of Central Tibetan Plateau.

On a planetary scale, the Eocene is interesting for climate changes, extensive orogeny and transformation of terrestrial and aquatic communities. Mass extinction of many organisms took place in the late Eocene to the early Oligocene, but not all groups have been studied in detail. The most complete data on taxonomy, diversity, distribution, and ecology were obtained on the Eocene angiosperms, vertebrates, insects, marine molluscs and large-bodied crustaceans. At the same time, Eocene microcrustaceans from inland waters are significantly less studied. Here we present the first data on the Cladocera from the greyish-white layered mudstones of the Late Eocene in Xiede village (Shuanghu County, central Tibetan Plateau), China. In total 29 ephippia were investigated and all of them belong to the genus Daphnia O.F. Müller, 1785 (Cladocera: Daphniidae). Twenty seven ephippia belong to Daphnia sp. 1 morphotype, the most common in the investigated samples. They belong to the subgenus Daphnia (Daphnia). These ephippia are covered by hollows with meshy ornamentation similar to those of some recent species and having two eggs with axes almost perpendicular to the dorsal margin. Two other morphotypes include a sole specimen each with ephippia covered by hillocks. Daphnia sp. 2 possesses a unique orientation of locules, unknown in any recent species and presumably belongs to an extinct taxon, may be even of subgenus level. Daphnia sp. 3 belongs to the subgenus D. (Ctenodaphnia) Dybowski & Grochowski, 1895 according to the ephippium size and locule orientation. In Eocene sediments of the eastern Nima Basin, the ephippia of Daphnia coexist with remains of thermophilic plants, insects and fishes. The occurrence of these ephippia supplements data on the diversity of thermophilic freshwater organisms in the geological past of the Tibetan Plateau. Moreover, the record of D. (Ctenodaphnia) in the sediment suggests a shallow, eutrophic, well warmed water body, possibly without a stable population of planktivorous fish in the late Eocene in central Tibet and confirm a warm climate with a low elevation at that time. Further studies can reveal more Daphniidae with unique morphology and help to prepare reliable descriptions for taxa new to science.