Thalassochelydia

Thalassochelydia is a clade of extinct marine turtles from the Late Jurassic and earliest Cretaceous of Europe and South America. The group is defined as including Eurysternum, Plesiochelys and Thalassemys to the exclusion of Pelomedusa, Testudo and Protostega. While a clade uniting the families Eurysternidae, Plesiochelyidae and Thalassemydidae had been supported by phylogenetic evidence, a name was not given for the clade until 2017, when Jérémy Anquetin and colleagues coined Thalassochelydia.

While inner relationships of the clade are so uncertain as to make the monophyly of the families questionable, the significant diversity of thalassochelydians makes using the potentially invalid terms useful to describe the different morphologies. The monophyly of Thalassochelydia was further supported by a later analysis by Serjocha Evers and Roger Benson in 2019, where the group, represented by Solnhofia and Plesiochelyidae was sister taxon to Sandownidae within the clade Angolachelonia, which, unlike as found by Anquetin et al. earlier, was outside Cryptodira and Testudines.