Mesochelydia (from Greek mesos "middle" and chelys "turtle") is a clade within Pantestudines, more inclusive than Perichelydia, but less than Testudinata. The clade is known from the Early Jurassic to the Present, and contains all Jurassic representatives of Testudinata aside from Australochelys. The ancestral condition for Mesochelydia is thought to be aquatic, as opposed to terrestrial for Testudinata. They are distinguished from more basal testudinatans by the presence of the following characters: strap like pectoral girdle, supramarginals absent, reduced posterior entoplastral process, eleven pairs of peripherals, elongate processus interfenestralis, paired basioccipital tubercles, fully formed cavum tympani and antrum postoticum, single vomer, confluent external nares, lacrimals and supratemporals absent.
Taxonomy
Condorchelys Sterli, 2008 Cañadón Asfalto Formation, Toarcian, Argentina Eileanchelys Anquetin et al. 2009 Kilmaluag Formation, Bathonian, Scotland Heckerochelys Sukhanov, 2006V. B. Sukhanov. 2006. An archaic turtle, Heckerochelys romani gen. et sp. nov. from the Middle Jurassic of Moscow region, Russia. Fossil Turtle Research 1:112-118 Moskvoretskaya Formation, Bathonian, Russia Indochelys Datta et al., 2000 Kota Formation, Toarcian, India Kayentachelys Gaffney et al. 1987 Kayenta Formation, Sinemurian, United States Perichelydia Joyce, 2017 Middle Jurassic-Present, Worldwide
Additionally, an indeterminate basal non-perichelydian mesochelydian taxon is known from the Early Cretaceous Batylykh Formation in Russia.
Cladogram after Tong et al. 2022:
References