Thalassochelydia is a clade of

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 of reptiles known as Testudines, characterized by a special shell developed mainly from their ribs. Modern turtles are divided into two major groups, the Pleurodira (side necked turtles) and Cryptodira (hidden necked t ...

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'', ''

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 an ...

'' 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 File:2017 Events Collage V2.png, From top left, clockwise: The War Against ISIS at the Battle of Mosul (2016-2017); aftermath of the Manchester Arena bombing; The Solar eclipse of August 21, 2017 ("Great American Eclipse"); North Korea tests a se ...

, 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 gro ...

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 File:2019 collage v1.png, From top left, clockwise: Hong Kong protests turn to widespread riots and civil disobedience; House of Representatives votes to adopt articles of impeachment against Donald Trump; CRISPR gene editing first used to experim ...

, 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 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 of reptiles known as Testudines, characterized by a special shell developed mainly from their ribs. Modern turtles are divided into two major groups, the Pleurodira (side necked turtles) and Cryptodira (hidden necked tu ...

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 ag ...

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 t ...

of southern England represents the only reliable Cretaceous record of the group. A 2018 study found Sandownidae to represent a sister group to Thalassochelydia, under the clade Angolachelonia. 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 Sea turtles (superfamily Chelonioidea), sometimes called marine turtles, are reptiles of the order Testudines and of the suborder Cryptodira. The seven existing species of sea turtles are the flatback sea turtle, flatback, green sea turtle, green ...

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