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
Labels: diapsids, evolution, ichthyosaurs