''Mesodermochelys'' is an

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 ...

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...

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, loggerhead, ...

known from the Campanian to the

Maastrichtian The Maastrichtian () is, in the ICS geologic timescale, the latest age (uppermost stage) of the Late Cretaceous Epoch or Upper Cretaceous Series, the Cretaceous Period or System, and of the Mesozoic Era or Erathem. It spanned the interval from ...

(Late

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of th ...

) of what today is

Japan Japan ( ja, 日本, or , and formally , ''Nihonkoku'') is an island country in East Asia. It is situated in the northwest Pacific Ocean, and is bordered on the west by the Sea of Japan, while extending from the Sea of Okhotsk in the north ...

.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> One species is known, the

type species In 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 species that contains the biological type specimen ...

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

binomial name In taxonomy, binomial nomenclature ("two-term naming system"), also called nomenclature ("two-name naming system") or binary nomenclature, is a formal system of naming species of living things by giving each a name composed of two parts, bot ...

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 The leatherback sea turtle (''Dermochelys coriacea''), sometimes called the lute turtle or leathery turtle or simply the luth, is the largest of all living turtles and the heaviest non-crocodilian reptile, reaching lengths of up to and weights ...

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, t ...

. It has been described as the best representative of

Mesozoic The Mesozoic Era ( ), also called the Age of Reptiles, the Age of Conifers, and colloquially as the Age of the Dinosaurs is the second-to-last era of Earth's geological history, lasting from about , comprising the Triassic, Jurassic and Cretaceo ...

dermochelyids.

Description

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

Kagawa Prefecture is a prefecture of Japan located on the island of Shikoku. Kagawa Prefecture has a population of 949,358 (as of 2020) and is the smallest prefecture by geographic area at . Kagawa Prefecture borders Ehime Prefecture to the southwest and Tok ...

had a

carapace A carapace is a Dorsum (biology), 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 tor ...

estimated to be 1.3 metres in length. Only the neural or spinal scutes, 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 The Hakobuchi Formation is a geological formation in Hokkaido, Japan. It is the uppermost unit of the Yezo Group, being early Maastrichtian in age. It consists of bioturbated glauconitic sandstones, siltstones and conglomerates with coaly muds ...

(

Yezo Group The Yezo Group is a stratigraphic group in Hokkaido, Japan and Sakhalin, Russia which is primarily Late Cretaceous in age (Aptian to Earliest Paleocene). It is exposed as roughly north–south trending belt extending 1,500 kilometres through centr ...

) in Inasato,

Hobetsu was a town located in Yūfutsu (Iburi) District, Iburi Subprefecture, Hokkaido, Japan. As of 2004, the town had an estimated population of 3,727 and a density of 6.82 persons per km2. The total area is 546.48 km2. On March 27, 2006, Hobets ...

Hokkaido is Japan's second largest island 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 the undersea railway Seikan Tunnel. The la ...

, 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 mya. The ...

-aged interval of the

in the Hokkaido Prefecture. 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 opened in Sapporo, Hokkaidō, Japan in 2015. Located within Nopporo Shinrin Kōen Prefectural Natural Park, the permanent exhibition is dedicated to the nature, history, and culture of Hokkaido. Also known as , the museum integrates and replaces ...

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