[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
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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.
DOI: 10.1038/srep16299
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Abstract
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.
Systematic Palaeontology.
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
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