Wednesday, November 9, 2016

[Paleontology • 2016] Sichuanchelys palatodentata • A Toothed Turtle from the Late Jurassic of China and the Global Biogeographic History of Turtles

Sichuanchelys palatodentata  
 Joyce, Rabi, Clark & Xu, 2016

  DOI:  10.1186/s12862-016-0762-5   


Turtles (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.

We 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.

A 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.

Keywords: Testudinata, Sichuanchelyidae, Helochelydridae, Meiolaniformes, Sichuanchelys palatodentata, Jurassic, Xinjiang, China, Phylogeny, Biogeography

Reconstruction of the well preserved palate of the previously unknown Sichuanchelys palatodentata with remnants of teeth.
Illustration: Lida Xing     DOI:  10.1186/s12862-016-0762-5  

Systematic paleontology

TESTUDINATA Klein, 1760.
SICHUANCHELYIDAE Tong et al., 2012.

SICHUANCHELYS Ye and Pi, 1997.

Sichuanchelys palatodentata sp. nov.

Etymology: In reference to the presence of palatal teeth. The species epithet is here formed and used explicitly as a noun in apposition and therefore does not have a gender.

Locality and horizon: All specimens herein referred to the new taxon were collected from the early Upper Jurassic (Oxfordian) upper part of the Shishugou Formation at the Wucaiwan Locality in Xinjiang Uygur Autonomous Region, China. The holotype and referred specimens IVPP V18094–18096 were found in close association to one another, along with nearly complete, articulated skeletons of a squamate and a shartegosuchid crocodyliform. IVPP V18097 was recovered 1.2 km to the north of the type locality and IVPP V18102 an additional 2.2 km northward. The type locality is positioned between two tuffs (T-2 and T-BW of [20]) and can thereby be dated securely to the early Oxfordian. V18102 and V18097 were recovered from sediments slightly higher in the formation, just above the T-BW tuff dated 159.7+/-0.3 million years ago, but still thought to be Oxfordian in age considering locally calculated sedimentation rates. Precise locality information is unavailable for V18101 and V18103 within Wucaiwan, but they are likely from the upper part of the Shishugou Formation, and therefore Oxfordian as well.

Fig. 10: The vicariant origin of the primary clades of turtles. a the early or complete fossil record of the clades Pan-Chelidae, Pan-Pelomedusoides, Paracryptodira, Pan-Cryptodira and Cryptodira imposed upon a paleogeographic reconstruction for the Late Jurassic highlighting the biogeographic areas of southern Gondwana, northern Gondwana, Euramerica and Asia, respectively. b the complete fossil record of the clades Meiolaniformes, Helochelydridae, and Sichuanchelyidae imposed upon the paleogeographic reconstruction for the Late Jurassic illustrating the biogeographic areas of southern Gondwana, Euramerica and Asia, respectively 
  DOI:  10.1186/s12862-016-0762-5   

 Walter G. Joyce, Márton Rabi, James M. Clark and Xing Xu. 2016. A Toothed Turtle from the Late Jurassic of China and the Global Biogeographic History of Turtles. BMC Evolutionary Biology.  

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