Thalassochelydia Temporal range: Late Jurassic, Oxfordian–Berriasian PreꞒ Ꞓ O S D C P T J K Pg N
Skeleton of Thalassemys bruntrutana
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Clade:	Pantestudines
Clade:	Testudinata
Clade:	Mesochelydia
Clade:	Perichelydia
Clade:	† Thalassochelydia Anquetin et al., 2017 [1]
Subgroups [1]
Aplax Cyrtura Enaliochelys Euryaspis Hispaniachelys Jurassichelon Neusticemys Owadowia Pelobatochelys Eurysternidae Plesiochelyidae Thalassemydidae Protostegidae? Sandownidae?

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. ^[1] 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. ^[2]

Phylogenetics

The phylogeny below of Thalassochelydian inter-relationships follows the results of Anquetin and colleagues in 2017. ^[1]

All species, aside from Neusticemys from the Vaca Muerta of Patagonia, are known from Europe. ^[1] An thalassochelydian skull from the Berriasian aged Purbeck Group of southern England represents the only reliable Cretaceous record of the group. ^[3]

A 2018 study found Sandownidae to represent a sister group to Thalassochelydia, under the clade Angolachelonia. ^[4] Similarly, a 2021 study found Protostegidae and Sandownidae to possibly be closely related to or even members of Thalassochelydia, although the exact affinities of the clade are still an open question. If Protostegidae is included, the clade might exist just outside Chelonoidea, which contains the extant sea turtles. ^[5]

Biology

At least some taxa developed flippers similar to those of chelonioidean sea turtles, an example of convergent evolution. The morphology of the limb girdles resembles those of modern sea turtles, suggesting that they swam in a similar manner. ^[6]

References