Thalassochelydia is a

clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English term, ...

extinct Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...

marine

turtles Turtles are an order (biology), order of reptiles known as Testudines, characterized by a special turtle shell, shell developed mainly from their ribs. Modern turtles are divided into two major groups, the Pleurodira (side necked turtles) an ...

from the

Late Jurassic The Late Jurassic is the third epoch of the Jurassic Period, and it spans the geologic time from 163.5 ± 1.0 to 145.0 ± 0.8 million years ago (Ma), which is preserved in Upper Jurassic strata.Owen 1987. In European lithostratigraphy, the name ...

and earliest Cretaceous of Europe and South America. The group is defined as including ''

Eurysternum ''Eurysternum'' is an extinct genus of thalassochelydian turtle. Its type species is ''Eurysternum wagleri'', the holotype A holotype is a single physical example (or illustration) of an organism, known to have been used when the species ...

'', ''

Plesiochelys ''Plesiochelys'' is a genus of late Jurassic European and Asian turtle. The type species is ''Plesiochelys etalloni''. Fossil records have discovered ''Plesiochelys bigleri'' and ''Plesiochelys etalloni'' from the Kimmeridge Clay of England and ...

'' and ''

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 Kim ...

'' to the exclusion of ''

Pelomedusa The African helmeted turtle (''Pelomedusa subrufa''), also known commonly as the marsh terrapin, the crocodile turtle, or in the pet trade as the African side-necked turtle, is a species of omnivorous side-necked terrapin in the family Pelomedus ...

'', '' Testudo'' and '' Protostega''. While a clade uniting the families Eurysternidae,

Plesiochelyidae The Plesiochelyidae are an extinct family of turtles in the clade Thalassochelydia originally classified within the Cryptodira suborder, mostly belonging from the Jurassic period. An alternate study placed the clade Thalassochelydia in the Angola ...

and

Thalassemydidae ''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 Kim ...

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 In cladistics for a group of organisms, monophyly is the condition of being a clade—that is, a group of taxa composed only of a common ancestor (or more precisely an ancestral population) and all of its lineal descendants. Monophyletic grou ...

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

was sister taxon to

Sandownidae Sandownidae is a family of extinct marine turtles from the Cretaceous and Paleogene distributed around the Atlantic Ocean and adjacent areas. The family is defined as all taxa closer to the type genus '' Sandownia'' than to '' Pelomedusa'', '' ...

within the clade

Angolachelonia Angolachelonia is a clade of extinct turtles from the Late Jurassic to Paleogene of Eurasia. The group is defined as all taxa derived from the ancestor of the type genus ''Angolachelys'' and '' Solnhofia'', a definition that could potentially e ...

, which, unlike as found by Anquetin ''et al.'' earlier, was outside

Cryptodira The Cryptodira ('' el, hidden neck'') are a suborder of Testudines that includes most living tortoises and turtles. Cryptodira differ from Pleurodira (side-necked turtles) in that they lower their necks and pull the heads straight back into the ...

and

Testudines Turtles are an order (biology), order of reptiles known as Testudines, characterized by a special turtle shell, shell developed mainly from their ribs. Modern turtles are divided into two major groups, the Pleurodira (side necked turtles) an ...

Phylogenetics

The phylogeny below of Thalassochelydian inter-relationships follows the results of Anquetin and colleagues in 2017. Solnhofia parsonsi

All species, aside from '' Neusticemys'' from the

Vaca Muerta The Vaca Muerta Formation, commonly known as Vaca Muerta (Spanish for ''Dead Cow''), is a geologic formation of Late Jurassic to Early Cretaceous age, located in the Neuquén Basin in northern Patagonia, Argentina. It is well known as the host ro ...

of Patagonia, are known from Europe. An thalassochelydian skull from the

Berriasian In the geological timescale, the Berriasian is an age/stage of the Early/Lower Cretaceous. It is the oldest subdivision in the entire Cretaceous. It has been taken to span the time between 145.0 ± 4.0 Ma and 139.8 ± 3.0 Ma (million years ago ...

aged

Purbeck Group The Purbeck Group is an Upper Jurassic to Lower Cretaceous lithostratigraphic group (a sequence of rock strata) in south-east England. The name is derived from the district known as the Isle of Purbeck in Dorset where the strata are exposed in ...

of southern England represents the only reliable Cretaceous record of the group. A 2018 study found

to represent a sister group to Thalassochelydia, under the clade

. Similarly, a 2021 study found

Protostegidae Protostegidae is a family of extinct marine turtles that lived during the Cretaceous period. The family includes some of the largest sea turtles that ever existed. The largest, '' Archelon'', had a head long. Like most sea turtles, they had fl ...

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.

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.

References

{{Taxonbar, from=Q96408024 Late Jurassic reptiles Tetrapod unranked clades