Choristoderes are a varied clade of mostly aquatic, often croc-like reptiles descending from certain long-snouted younginids, like the BPI-2871 specimen (Fig. 2). Doswellia (Fig. 2) is closely related to choristoderes, splitting off at the base to form its own clade. Diandongosuchus likewise split off early, giving rise to later parasuchids (phytosaurs), proterochampsids and chanaresuchids.
Many from this varied clade of pararchosauriformes had nostrils shifted back from the snout tip, reaching an acme with parasuchians. Champsosaurus (Figs. 1, 2) was different. The naris was located at the very tip, probably to act as a snorkel, in order to breathe without surfacing. Given the unusual morphology of the snout tip, it’s very possible that the reversion to the tip was a secondary adaptation.
The identification of the rostral bones in Champsosaurus is controversial. Here we’ll look at some heretical labels for traditional paradigms.
In Champsosaurus (Fig. 1) the dorsal medial bone is traditionally considered the nasal and the paired bones following it are considered the prefrontals. However if you look at all the closest kin to Champsosaurus it becomes clear that the paired bones remain traditional nasals. The prefrontals are simply missing, likely due to fusion with the nasals. That means the tooth-bearing portions of the premaxilla wrapped completely around the rostrum and nares until they came into contact with the ascending process of the premaxilla, which extends beyond the naris in many related taxa.
Did Champsosaurus once have an antorbital fenestra?
Related taxa, including Diandogosuchus and Doswellia had an antorbital fenestra and there are signs of a nascent or vestigial antorbital fenestra in certain Champsosaurus (Fig. 3). If it’s there, it’s tiny, but worth searching for.
Choristoderes are underrepresented in the fossil record.
So are doswellids and basal parasuchians. What this means, with present data, is basal taxa appear to be large forms, which goes against the grain of evolutionary patterns. These named taxa are all quite derived at their first appearance. I’m guessing we’re likely to find smaller basal and transitional forms, more like the BPI-2781 specimen (Fig. 2) as they become known.
It’s also interesting that the largest choristodere, Simoedosaurus (Fig. 1) has the more primitive skull, with a dorsal set of nares and more laterally-oriented lateral temporal fenestra. The smaller Champsosaurus has the more derived snout tip and more dorsally open lateral temporal fenestrae.
Popping paradigms is what we do here.
If you have data that supports other positions, please send them forward.