BackgroundMost turtles from the Middle and Late Jurassic of Asia are referred to the newly defined clade Xinjiangchelyidae, a group of mostly shell-based, generalized, small to mid-sized aquatic froms that are widely considered to represent the stem lineage of Cryptodira. Xinjiangchelyids provide us with great insights into the plesiomorphic anatomy of crown-cryptodires, the most diverse group of living turtles, and they are particularly relevant for understanding the origin and early divergence of the primary clades of extant turtles.ResultsExceptionally complete new xinjiangchelyid material from the ?Qigu Formation of the Turpan Basin (Xinjiang Autonomous Province, China) provides new insights into the anatomy of this group and is assigned to Xinjiangchelys wusu n. sp. A phylogenetic analysis places Xinjiangchelys wusu n. sp. in a monophyletic polytomy with other xinjiangchelyids, including Xinjiangchelys junggarensis, X. radiplicatoides, X. levensis and X. latiens. However, the analysis supports the unorthodox, though tentative placement of xinjiangchelyids and sinemydids outside of crown-group Testudines. A particularly interesting new observation is that the skull of this xinjiangchelyid retains such primitive features as a reduced interpterygoid vacuity and basipterygoid processes.ConclusionsThe homology of basipterygoid processes is confidently demonstrated based on a comprehensive review of the basicranial anatomy of Mesozoic turtles and a new nomenclatural system is introduced for the carotid canal system of turtles. The loss of the basipterygoid process and the bony enclosure of the carotid circulation system occurred a number of times independently during turtle evolution suggesting that the reinforcement of the basicranial region was essential for developing a rigid skull, thus paralleling the evolution of other amniote groups with massive skulls.

Small terrestrial non-eusuchian mesoeucrocodylians are common components of Cretaceous assemblages of Gondwanan provinces with notosuchians and araripesuchids as flagship taxa in South America, Africa and Madagascar, well into the Late Cretaceous. On the other hand, these are exceedingly rare in Laurasian landmasses during the Late Cretaceous. Small terrestrial mesoeucrocodylians from Europe were often referred to the genus Theriosuchus, a taxon with stratigraphic range extending from the Late Jurassic to the late Early Cretaceous. Theriosuchus is abundantly reported from various European localities, although Asiatic and possibly North American members are also known. It has often been closely associated with the first modern crocodilians, members of the Eusuchia, because of the presence of procoelous vertebrae, a widespread key character diagnosing the Eusuchia. Nevertheless, the relationships of Theriosuchus have not been explored in detail although one species, Theriosuchus pusillus, has been extensively described and referred in numerous works. Here, we describe a new basal mesoeucrocodylian, Theriosuchus sympiestodon sp. nov. from the Maastrichtian of the Haţeg Basin, Romania, suggesting a large temporal gap (about 58 myr) in the fossil record of the genus. Inclusion of the new taxon, along with Theriosuchus guimarotae, in a phylogenetic analysis confirms its referral to the genus Theriosuchus, within a monophyletic atoposaurid clade. Although phylogenetic resolution within this clade is still poor, the new taxon appears, on morphological grounds, to be most closely related to T. pusillus. The relationships of Atoposauridae within Mesoeucrocodylia and especially to Neosuchia are discussed in light of the results of the present contribution as well as from recent work. Our results raise the possibility that Atoposauridae might not be regarded as a derived neosuchian clade anymore, although further investigation of the neosuchian interrelationships is needed. Reports of isolated teeth referable to a closely related taxon from the Upper Cretaceous of Romania and France, together with the presence of Doratodon and Ischyrochampsa, indicate a previously unsuspected diverse assemblage of non-eusuchian mesoeucrocodylians in the Late Cretaceous European archipelago.

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BackgroundTurtles (Testudinata) are a successful lineage of vertebrates with about 350 extant species that inhabit all major oceans and landmasses with tropical to temperate climates. The rich fossil record of turtles documents the adaptation of various sub-lineages to a broad range of habitat preferences, but a synthetic biogeographic model is still lacking for the group.ResultsWe herein describe a new species of fossil turtle from the Late Jurassic of Xinjiang, China, Sichuanchelys palatodentata sp. nov., that is highly unusual by plesiomorphically exhibiting palatal teeth. Phylogenetic analysis places the Late Jurassic Sichuanchelys palatodentata in a clade with the Late Cretaceous Mongolochelys efremovi outside crown group Testudines thereby establishing the prolonged presence of a previously unrecognized clade of turtles in Asia, herein named Sichuanchelyidae. In contrast to previous hypotheses, M. efremovi and Kallokibotion bajazidi are not found within Meiolaniformes, a clade that is here reinterpreted as being restricted to Gondwana.ConclusionsA revision of the global distribution of fossil and recent turtle reveals that the three primary lineages of derived, aquatic turtles, including the crown, Paracryptodira, Pan-Pleurodira, and Pan-Cryptodira can be traced back to the Middle Jurassic of Euramerica, Gondwana, and Asia, respectively, which resulted from the primary break up of Pangaea at that time. The two primary lineages of Pleurodira, Pan-Pelomedusoides and Pan-Chelidae, can similarly be traced back to the Cretaceous of northern and southern Gondwana, respectively, which were separated from one another by a large desert zone during that time. The primary divergence of crown turtles was therefore driven by vicariance to the primary freshwater aquatic habitat of these lineages. The temporally persistent lineages of basal turtles, Helochelydridae, Meiolaniformes, Sichuanchelyidae, can similarly be traced back to the Late Mesozoic of Euramerica, southern Gondwana, and Asia. Given the ambiguous phylogenetic relationships of these three lineages, it is unclear if their diversification was driven by vicariance as well, or if they display a vicariance-like pattern. The clean, primary signal apparent among early turtles is secondarily obliterated throughout the Late Cretaceous to Recent by extensive dispersal of continental turtles and by multiple invasions of marine habitats.Electronic supplementary materialThe online version of this article (doi:10.1186/s12862-016-0762-5) contains supplementary material, which is available to authorized users.