[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
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Abstract
Background
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.
Results
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.
Conclusions
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
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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.
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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