|Similar Eosphargis, Protosphargis, Cratochelone, Gigantatypus, Glarichelys|
Mesodermochelys is an extinct genus of sea turtle known from the Santonian to the Campanian (Late Cretaceous) of what today is Japan. One species is known, the type species M. undulatus; it was given its binomial name by Ren Hirayama and Tsutomu Chitoku in 1996. Studies of its skull indicate that it was a primitive member of the Dermochelyidae (leatherback turtle family) that was closely related to the Protostegidae. It has been described as the best representative of Mesozoic dermochelyids.
Like other dermochelyids, Mesodermochelys had elongated front flippers. One fossil found in Japan's Kagawa Prefecture had a carapace estimated to be 1.3 metres in length. Only the neural or spinal scutes, or individual plates, of the carapace are well grooved.
In October 2005, Yoshiyuki Hattori uncovered a skull of Mesodermochelys, in the Santonian-age Yezo Group 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 Hokkaido Museum, where formic acid etching revealed characteristics distinguishing it from related chelonioids and enabling its taxonomic placement in relation to them. Other, less complete specimens of Mesodermochelys skulls and jaws have been found in the Maastrichtian type locality of the Hokkaido Prefecture. A total of fifteen specimens have been uncovered, including an almost-complete shell.