Which ichthyosaur is the most primitive? Part 1

A while back Maisch and Matzke (2002) discussed a new Early Triassic ichthyosaur skull (actually just the temple region, SVT 331, Fig. 1) and its implications for the origin of the Ichthyosauria. Unfortunately, to this day this nesting remains in contention, despite the results of the large reptile tree, where the mesosaur, Wumengosaurus, nests as the proximal outgroup taxon to hupehsuchians + ichthyosaurs.

Figure 1. Basal ichthyosaur skulls compared to one another from Maisch and Matzke 2002. I have added the proximal sister, Wumengosaurus, a sister to the Mesosauria.

Figure 1. Basal ichthyosaur skulls compared to one another from Maisch and Matzke 2002. I have added the proximal sister, Wumengosaurus, a sister to the Mesosauria. Click to enlarge. The variation in temporal morphology is striking.

To be fair,
Wumengosaurus was not known in 2002, but mesosaurs were and they should have been considered.

So if we look at Wumengosaurus
the temporal region includes a gracile jugal without a quadratojugal process, a triangular postorbital, a sliver of a squamosal with a small descending process, a vestigial quadratojugal bordering the quadrate and a supratemporal rimming the upper temporal fenestra. Note, the squamosal contacts the jugal at the postorbital process.

Which of the above candidates is closest?
While Utatsusaurus has been promoted as the most basal ichthyosaur, both SVT 331 and Contectopalatus are the only ichthyosaurs that retain a contact between the jugal and squamosal. Both have a relatively small quadratojugal. Only SVT 331 retains the squamosal arch over the lateral temporal fenestra. All ichthyosaurs have more robust temporal elements, which brings us back to…

Figure 3. Stereosternum skull, This basal mesosaur is the proximal outgroup taxon to Wumengosaururs + hupehsuchids + ichthyosaurs.

Figure 3. Stereosternum skull, This basal mesosaur is the proximal outgroup taxon to Wumengosaururs + hupehsuchids + ichthyosaurs. Here the quadratojugal is longer than the jugal and the squamosal does not contact the jugal. The basic diapsid openings are apparent here. Only the SVT specimen among all the ichthyosaurs retains a vestige of the jugal’s quadratojugal process. The postorbital is broad here, more like that of basal ichthyosaurs.

Mesosaurs,
especially Stereosternum (Fig. 3).  Wumengosaurus is a derived mesosaur. Mesosaurs are derived pachypleurosaurs. Most mesosaurs have such robust temporal bones that the temporal fenestrae have been closed off, or nearly so. As you can see in ichthyosaurs, this area is very plastic with regard to morphology. In basal ichthyosaurs the postorbital and supratemporal border the upper temporal fenestra. In derived forms the postfrontal and supratemporal contact one another. The SVT specimen even retains a bit of the old jugal quadratojugal process and a robust postorbital.

Remember, 
Wumengosaurus is not a perfect transition from Stereosternum to ichthyosaurs. Rather it was a Middle Triassic reptile that kept evolving along its own path. The transitional taxon would have been a latest Permian, earlier Triassic form.

Maisch 1997 argued against a diapsid origin for ichthyosaurs due to the presence of a large quadratojugal, but this is a derived state. They report, “All this evidence indicates that in the grundplan ichthyosaurs possessed a small quadratojugal, much smaller than the squamosal, of styliform shape, bordering a large jugal-quadratojugal notch posteriorly. The fact that a jugal-quadratojugal contact is lacking in SVT 331 of course re-opens the debate about ichthyosaur origins. We are therefore inclined to accept, for the time being, the hypothesis that ichthyosaurs developed from tetrapods with both an upper and a lower temporal fenestra. Nevertheless, there is still no diapsid known which could have given rise to the ichthyosaurs or which appears a likely candidate as ichthyosaur sister-taxon. There is no evidence that sauropterygians and placodonts are particularly closely related to ichthyosaurs, as envisaged by Mazin (1982). The search for ichthyosaur ancestors is therefore far from over. It appears, however, that many possibilities of ichthyosaur origins can now be positively excluded, and that the idea that ichthyosaurs are stem-group diapsids is the least far-fetched and most convincing hypothesis presently to hand.”

This is all done without phylogenetic analysis, as you can see. Phylogenetic analysis solves all these mysteries.

Tomorrow we’ll look at a new most basal ichthyosaur that was not recognized as such when published.

References
Maisch MW and Matzke AT 2002. The skull of a large Lower Triassic ichthyosaur from Spitzbergen and its implications for the origin of the Ichthyosauria. Lethaia 35:250256.

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