''Mesodermochelys'' is an

extinct Extinction is the termination of an organism by the death of its Endling, last member. A taxon may become Functional extinction, functionally extinct before the death of its last member if it loses the capacity to Reproduction, reproduce and ...

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

sea turtle 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, green, hawksbill, leatherback, loggerh ...

known from the

Campanian The Campanian is the fifth of six ages of the Late Cretaceous epoch on the geologic timescale of the International Commission on Stratigraphy (ICS). In chronostratigraphy, it is the fifth of six stages in the Upper Cretaceous Series. Campa ...

to the

Maastrichtian The Maastrichtian ( ) is, in the International Commission on Stratigraphy (ICS) geologic timescale, the latest age (geology), age (uppermost stage (stratigraphy), stage) of the Late Cretaceous epoch (geology), Epoch or Upper Cretaceous series (s ...

(Late

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...

) of what today is

Japan Japan is an island country in East Asia. Located in the Pacific Ocean off the northeast coast of the Asia, Asian mainland, it is bordered on the west by the Sea of Japan and extends from the Sea of Okhotsk in the north to the East China Sea ...

Hirayama R. (2007). "Cranial morphology of ''Mesodermochelys'' (Chelonioidea; Testudines) from the Late Cretaceous of Japan". ''Journal of Vertebrate Paleontology'' 27 (3): 89A
online, retrieved 28 July 2008
/ref> and from the Maastrichtian of

Chile Chile, officially the Republic of Chile, is a country in western South America. It is the southernmost country in the world and the closest to Antarctica, stretching along a narrow strip of land between the Andes, Andes Mountains and the Paci ...

. One species is known, the

type species In International_Code_of_Zoological_Nomenclature, zoological nomenclature, a type species (''species typica'') is the species name with which the name of a genus or subgenus is considered to be permanently taxonomically associated, i.e., the spe ...

''M. undulatus''; it was given its

binomial name In taxonomy, binomial nomenclature ("two-term naming system"), also called binary nomenclature, is a formal system of naming species of living things by giving each a name composed of two parts, both of which use Latin grammatical forms, altho ...

by Ren Hirayama and Tsutomu Chitoku in 1996.Hirayama R, Fujii A, Takahashi K. (2006). "A dermochelyid sea turtle from the Upper Cretaceous (Late Campanian) Izumi Group of Shionoe, Takamatsu, Kagawa Prefecture, Western Japan". ''Fossils (Palaeontological Society of Japan)'' 80: 17-20
online, retrieved 28 July 2008
/ref> Studies of its skull indicate that it was a primitive member of the Dermochelyidae ( leatherback turtle family) that was closely related to the

Protostegidae Protostegidae is a family of extinct sea turtle, 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, the ...

. It has been described as the best representative of

Mesozoic The Mesozoic Era is the Era (geology), era of Earth's Geologic time scale, geological history, lasting from about , comprising the Triassic, Jurassic and Cretaceous Period (geology), Periods. It is characterized by the dominance of archosaurian r ...

dermochelyids.

Description

Like other dermochelyids, ''Mesodermochelys'' had elongated front flippers. One fossil found in Japan's

Kagawa Prefecture is a Prefectures of Japan, prefecture of Japan located on the island of Shikoku. Kagawa Prefecture has a population of 949,358 (as of 2020) and is the List of Japanese prefectures by area, smallest prefecture by geographic area at . Kagawa Pr ...

had a

carapace A carapace is a dorsal (upper) section of the exoskeleton or shell in a number of animal groups, including arthropods, such as crustaceans and arachnids, as well as vertebrates, such as turtles and tortoises. In turtles and tortoises, the unde ...

estimated to be in length, and specimen from Hokkaido had carapace up to . Largest specimen is estimated to have shell around , possibly comparable with ''

Archelon ''Archelon'' is an extinct marine turtle from the Late Cretaceous, and is the largest turtle ever to have been documented, with the biggest specimen measuring from head to tail and in body mass. It is known only from the Pierre Shale and ...

''. Only the neural or spinal

scute A scute () or scutum (Latin: ''scutum''; plural: ''scuta'' "Scutum (shield), shield") is a bony external plate or scale overlaid with horn, as on the shell of a turtle, the skin of crocodilians, and the feet of Bird anatomy#Scales, birds. The ter ...

s, or individual plates, of the carapace are well grooved.

Specimens

The holotype specimen of ''Mesodermochelys undulatus'' Hobetsu Museum (HMG) 5 was collected in 1980 from the

Hakobuchi Formation ( Yezo Group) in Inasato, Hobetsu,

Hokkaido is the list of islands of Japan by area, second-largest island of Japan and comprises the largest and northernmost prefectures of Japan, prefecture, making up its own list of regions of Japan, region. The Tsugaru Strait separates Hokkaidō fr ...

, Japan. It comprises a supraoccipital and partial postcranial skeleton and was reported in 1996 along with 14 referred specimens collected between 1977 and 1994 and also reposited at HMG. In October 2005, Yoshiyuki Hattori uncovered a skull presumed to be ''Mesodermochelys'', from an unnamed

Santonian The Santonian is an age in the geologic timescale or a chronostratigraphic stage. It is a subdivision of the Late Cretaceous Epoch or Upper Cretaceous Series. It spans the time between 86.3 ± 0.7 mya ( million years ago) and 83.6 ± 0.7 m ...

-aged interval of the Yezo Group in the

Hokkaido Prefecture is the second-largest island of Japan and comprises the largest and northernmost prefecture, making up its own region. The Tsugaru Strait separates Hokkaidō from Honshu; the two islands are connected by railway via the Seikan Tunnel. The ...

. The skull is very well preserved but is missing most of the dermal roof bones through weathering. It was donated to the defunct Historical Museum of Hokkaido, which has since been integrated into and replaced by the Hokkaido Museum.

References

{{Taxonbar, from=Q6821460 Late Cretaceous turtles of Asia Dermochelyidae Cretaceous animals of Asia Fossil taxa described in 1996 Prehistoric turtle genera Campanian genus first appearances Monotypic prehistoric reptile genera Fossils of Japan Maastrichtian genus extinctions