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New mixosaurid ichthyosaur specimen from the Middle Triassic of SW China: further evidence for the diapsid origin of ichthyosaurs

Published online by Cambridge University Press:  14 July 2015

Jun Liu ,
Jonathan C. Aitchison ,
Yuan-Yuan Sun ,
Qi-Yue Zhang ,
Chang-Yong Zhou  and
Tao Lv
Jun Liu
Affiliation:
1Chengdu Center of China Geological Survey, Chengdu, China 2Department of Earth Sciences, The University of Hong Kong, Hong Kong SAR, China,
Jonathan C. Aitchison
Affiliation:
2Department of Earth Sciences, The University of Hong Kong, Hong Kong SAR, China,
Yuan-Yuan Sun
Affiliation:
1Chengdu Center of China Geological Survey, Chengdu, China
Qi-Yue Zhang
Affiliation:
1Chengdu Center of China Geological Survey, Chengdu, China
Chang-Yong Zhou
Affiliation:
1Chengdu Center of China Geological Survey, Chengdu, China
Tao Lv
Affiliation:
1Chengdu Center of China Geological Survey, Chengdu, China
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Abstract

Recent cladistic analyses have all suggested a diapsid origin of ichthyosaurs. However, an intermediate evolutionary stage of the lower temporal region of ichthyosaurian skull between basal diapsids and derived ichthyosaurs has been absent from the fossil record. Here we describe the cranial skeleton of a new mixosaurid ichthyosaur specimen with a well-preserved lower temporal region from the Anisian Guanling Formation of eastern Yunnan. It is characterized by the most primitive lower temporal region within known ichthyosaurs. The primitive characters of the lower temporal region include both external and internal separation between the jugal and the quadratojugal, an anterior process of the quadratojugal, an apparent posteroventral process of the jugal, and a large lower temporal opening surrounded by the jugal, the postorbital, the squamosal, and the quadratojugal. The lower temporal region of this specimen provides the most direct evidence to the diapsid origin of ichthyosaurs. It also suggests that the disappearance of the lower temporal fenestra is caused initially by the reduction of the lower temporal arcade rather than the enlargement of the surrounding bones.

Type
Research Article
Information
Journal of Paleontology , Volume 85 , Issue 1 , January 2011 , pp. 32 - 36
Copyright
Copyright © The Paleontological Society 

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