Ichthyosaurs are fascinating
Most paleontologists are still trying to figure out what ichthyosaurs are (Liuh et al. 2011). The situation is akin to the mystery surrounding bat, dinosaur, turtle and pterosaur origins. All these problems stem from a lack of a published large gamut study, like the large reptile tree.
The early diapsid reptiles,
characterized by two pairs of temporal fenestrae, rather rapidly evolved variations on this theme that involved loss of the lower temporal bar and, at times, the loss of the upper temporal fenestra, among other changes.
Mesosaurs are widely considered anapsids, characterized by no temporal fenestrae, but recent advances have shown that some mesosaurs had diapsid fenestrae, or remnants thereof. In any case, mesosaurs nest near the base of the Diapsida, fenestrae present or not. Araeoscelis, a basal diapsid, lost the lateral temporal fenestrae. Now we’ll look at an ichthyosaur that lost, or practically lost, the upper temporal fenestrae, Mixosaurus (Figs. 1, 2).
Discussing a new Mixosaurus (basal ichthyosaur) Liu et al. 2011(Fig. 2) reported, “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.” This might be true if one were defining diapsids strictly by their temporal fenestra, instead of their phylogenetic nesting. In like fashion, snakes have no temporal bars, yet no one doubts they are related to lepidosaurs with temporal fenestrae. Earlier, Jiang et al. (2006) described a very similar Mixosaurus (Fig. 1) that also had a jugal posterior process, with an overlooked portion broken off during taphonomy.
Even so, it’s great to see the vestigial remnants of the lower temporal bar in a basal ichthyosaur. That bar is not needed to document a diapsid affinity. A large reptile tree also does the trick.
Losing the upper temporal fenestra
Mixosaurus has a very small temporal region, less than half the width of the orbit and scleral ring. The upper temporal fenestra is correspondingly very small — but still present. On the other side of the spectrum, another mixosaur ichthyosaur, Contectopalatus (Fig. 1) had a similarly small upper temporal fenestra, but a hyper-expanded upper temporal fossa, extending anteriorly nearly to the naris and creating a median crest.
Again, the point is
these changes reflect the remarkable plasticity in the temporal region of basal diapsids, including mesosaurs, like Stereosternum (Fig. 1) and ichthyosaurs, like Mixosaurus. I still find it remarkable how similar the skull of Mixosaurus is to that of Stereosternum. Even more so to Wumengosaurus. And I wonder why this similarity has gone largely unnoticed and untested in the literature.
Jiang DY, Schmitz L, Hao W-C and Sun Y-L 2006. A new mixosaurid ichthyosaur from the Middle Triassic of China. Journal of Vertebrate Paleontology 26:60–69. BioOne
Liuh J, Aitchison JC, Sun Y-Y, Zhang Q-Y, Zhou C-Y and Lv T 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.