[Paleontology • 2015] Xiaochelys ningchengensis • A Sinemydid Turtle from the Jehol Biota provides insights into the Basal Divergence of Crown Turtles
|Xiaochelys ningchengensis in its freshwater environment, preying on a small fish Lycoptera found in the same layer as the turtle|
Reconstruction: W. S. Wang
|The newly-described fossil species, Xiaochelys ningchengensis,
from northeastern China’s Jehol Biota; right: an illustration of the
skeleton incorporating positive and negative impressions preserved in
the layers of stone. |
Morphological phylogenies stand in a major conflict with molecular hypotheses regarding the phylogeny of Cryptodira, the most diverse and widely distributed clade of extant turtles. However, molecular hypotheses are often considered a better estimate of phylogeny given that it is more consistent with the stratigraphic and geographic distribution of extinct taxa. That morphology fails to reproduce the molecular topology partly originates from problematic character polarization due to yet another contradiction around the composition of the cryptodiran stem lineage. Extinct sinemydids are one of these problematic clades: they have been either placed among stem-cryptodires, stem-chelonioid sea turtles, or even stem-turtles.
A new sinemydid from the Early Cretaceous Jehol Biota (Yixian Formation, Barremian-Early Aptian) of China, Xiaochelys ningchengensis gen. et sp. nov., allows for a reassessment of the phylogenetic position of Sinemydidae. Our analysis indicates that sinemydids mostly share symplesiomorphies with sea turtles and their purported placement outside the crown-group of turtles is an artefact of previous datasets. The best current phylogenetic estimate is therefore that sinemydids are part of the stem lineage of Cryptodira together with an array of other Jurassic to Cretaceous taxa. Our study further emphasises the importance of using molecular scaffolds in global turtle analyses.
Testudinata Klein, 1760
Testudines Batsch, 1788
Pan-Cryptodira Joyce, Parham, and Gauthier, 2004
Sinemydidae sensu Rabi, Sukhanov, Egorova, Danilov, and Joyce 2014
Xiaochelys ningchengensis gen. et sp. nov.
Holotype: Articulated skeleton of a potentially juvenile or subadult individual obtained in two slabs, PMOL-AR00210 (Figs. 2, 3, 4, 5; see Supplementary Note for a discussion on the ontogenetic status of the specimen).
Type locality and horizon: The fossil site is near to Yang-shu-wan-zi Village, Bi-si-ying-zi Township, Ningcheng County, Chifeng City, Inner Mongolia; Early Cretaceous, Yixian Formation, Barremian-Early–Aptian.
Etymology: Chinese Pinyin ‘Xiao’, refers to the small size of the taxon, ‘chelys’ Greek ‘turtle’; the specific epithet refers to the type locality.
Chang-Fu Zhou and Márton Rabi. 2015. A Sinemydid Turtle from the Jehol Biota provides insights into the Basal Divergence of Crown Turtles. Scientific Reports 5, Article number: 16299. DOI: 10.1038/srep16299