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An ancestral turtle from the Late Triassic of southwestern China

Abstract

The origin of the turtle body plan remains one of the great mysteries of reptile evolution. The anatomy of turtles is highly derived, which renders it difficult to establish the relationships of turtles with other groups of reptiles. The oldest known turtle, Proganochelys from the Late Triassic period of Germany1, has a fully formed shell and offers no clue as to its origin. Here we describe a new 220-million-year-old turtle from China, somewhat older than Proganochelys, that documents an intermediate step in the evolution of the shell and associated structures. A ventral plastron is fully developed, but the dorsal carapace consists of neural plates only. The dorsal ribs are expanded, and osteoderms are absent. The new species shows that the plastron evolved before the carapace and that the first step of carapace formation is the ossification of the neural plates coupled with a broadening of the ribs. This corresponds to early embryonic stages of carapace formation in extant turtles, and shows that the turtle shell is not derived from a fusion of osteoderms. Phylogenetic analysis places the new species basal to all known turtles, fossil and extant. The marine deposits that yielded the fossils indicate that this primitive turtle inhabited marginal areas of the sea or river deltas.

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Figure 1: Holotype (IVPP V 15639) of Odontochelys semitestacea gen. et sp. nov.
Figure 2: Paratype (IVPP V 13240) of Odontochelys semitestacea gen. et sp. nov.
Figure 3: Referred specimen (IVPP V 15653) of Odontochelys semitestacea gen. et sp. nov.

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Acknowledgements

We thank Z. Tang for his help in collecting the specimens; J. Ding and H. Zhou for preparing the specimens; and W. Gao for taking the photos. C.L. and L.-T.W. were supported by the Major Basic Research Projects (2006CB806400) of the Ministry of Science and Technology of China, the National Natural Science Foundation (40772015, 40121202) of China (NNSFC) and a special grant for fossil excavation and preparation of the Chinese Academy of Sciences. X.-C.W. was supported by grants from the Canadian Museum of Nature (RS 34), NNSFC 40772015 and the CAS/SAFEA International Partnership Program for Creative Research Teams. O.R. was supported by a stipend from M. Tang.

Author Contributions C.L. designed the project. C.L., X.-C.W., O.R., L.-T.W. and L.-J.Z. performed the research. C.L., X.-C.W. and O.R. contributed to the writing.

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Correspondence to Chun Li or Xiao-Chun Wu.

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Li, C., Wu, XC., Rieppel, O. et al. An ancestral turtle from the Late Triassic of southwestern China. Nature 456, 497–501 (2008). https://doi.org/10.1038/nature07533

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