Ichthyosaurs Are Diapsid Reptiles

The oceans of the Mesozoic contained species that we know today only from their fossil remains, species which looked quite unlike anything we are familiar with today. Ichthyosaurs may have looked slightly familiar, because despite the fact they were reptiles, they had dolphin- or fish-shaped bodies. The “fish lizards” first appear in the fossil record about 225 million years ago (MYA) and disappear about 90 MYA. While most species were in the 2 to 4 meter range, some grew larger, and a few of the earliest forms were less than 2 m long. Some specimens have been exceptionally well preserved and provide information on soft tissue, diet and reproduction. Their relationship to other tetrapods has been controversial and there has been much speculation on their origin but a consensus has been building that ichthyosaurs are indeed diapsid reptiles. The absence of a lower temporal region has been one of the sticking points to accepting them as diapsids. Liu et al. (2011) have now described the cranial skeleton of a new mixosaurid ichthyosaur specimen with a well-preserved lower temporal region from the Anisian Guanling Formation of easternYunnan. It is has the most primitive lower temporal region known in ichthyosaurs, and it was well preserved. The specimen provides definite direct evidence for the diapsid origin of ichthyosaurs. It also gives strong support to the hypothesis that the lower temporal fenestra in ichthyosaurs is lost due to the reduction of the jugal and the quadratojugal that comprise the primitive lower temporal arcade in diapsids. Given that these marine reptiles are in fact diapsids, the question remains what clade did they arise from? They may have shared an ancestor with the lizards.

Liu, J., J. C. Aitchison, Y.-Y. Sun, Q-Y Zhang, C.-Y. Zhou, and T. Lv, 2011. New mixosaurid Ichthyosaur specimen from the middle Triassic of SW China: further evidence for the diapsid origin of ichthyosaurs. Journal of Paleontology, 85(1):32-36. 2011

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