Proganochelys

Proganochelys quenstedti is a primitive stem-turtle that has been hypothesized to be the sister taxon to all other turtles creating a monophyletic group, the Casichelydia.[1][better source needed] Proganochelys was named by Georg Baur in 1887 as the oldest turtle in existence at the time. The name Proganochelys comes from the Greek word ganos meaning brightness, combined with prefix pro, and Greek base chelys meaning "turtle". Proganochelys is believed to have been around 1 meter in size and herbivorous in nature. Proganochelys was known as the most primitive stem-turtle for over a century, until the novel discovery of Odontochelys in 2008.[2]Odontochelys and Proganochelys share unique primitive features that are not found in Casichelydia, such as teeth on the pterygoid and vomer and a plate-like coracoid.[2]

Proganochelys is the oldest stem-turtle species with a complete shell discovered to date, known from fossils found in Germany, Greenland, and Thailand in strata from the late Triassic, dating to approximately 210 million years ago.[3] The location of these fossils suggest that Proganochelys was active throughout the continent of Laurasia. There are only two known species of Proganchelys, with little information as a result of a small fossil record. All Proganochelys quentesti fossils were discovered in Germany, while Proganochelys ruchae fossils were found in Thailand.

Psammochelys, Stegochelys, and Triassochelys are junior synonyms of Proganochelys. Chelytherium von Meyer, 1863 has been considered a synonym of Proganochelys by some authors, but Joyce (2017) considers it a nomen dubium given the fragmentary nature of the syntype material. Joyce (2017) also considered North American genus Chinlechelys to be a junior synonym of Proganochelys, though the author maintains the type species of the former genus, C. tenertesta, as a distinct species within the genus Proganochelys.[4]