Thalassemys

Thalassemys is a genus of extinct thalassochelydian turtle from the Late Jurassic of western and central Europe. While the genus was originally named by Rütimeyer in 1859 for a large carapace and other associated fragments from the late Kimmeridgian of the Reuchenette Formation of Switzerland, although the taxon was not validly named until 1873 when Rütimeyer designated the type species T. hugii. Rütimeyer also named T. gresslyi from the Reunchenette Formation in the same paper as T. hugii, but it cannot be differentiated from the type material of T. hugii and is therefore a junior synonym. A large assemblage of shell and postcranial material from the Reunchenette was named as a species of Eurysternum, E. ignoratum, by Bräm in 1965. While originally distinguished based on the presence of fontanelles on the plastron, the feature was later identified on T. hugii and E. ignoratum was designated a junior synonym. Additional material from the Kimmeridge Clay of the United Kingdom has also been referred to T. hugii.

The second valid species of Thalassemys is T. marina, named in 1903 by Fraas for a partial carapace and half a plastron from the late Kimmeridgian deposits of Schnaitheim, Germany. It was suggested to be a species of the related taxon Eurysternum by Maisch in 2001, who referred additional material from Schnaitheim to the species. In 2003, it was suggested that T. marina was a synonym of the related turtle Palaeomedusa, which was based on an unknown but restored region of the original carapace.