Phytosaurs and their Sisters

The nesting of phytosaurs (parasuchians) has moved around quite a bit.
Gauthier (1984) nested Parasuchia between Proterochampsa and AetosauriaBenton and Clark (1985) nested Parasuchia between Ornithosuchidae and Gracilisuchus.  Bennett (1996) found Parasuchia nested with Suchia between Euparkeria and Ornithosuchia + Pterosauria. Sereno (1991) nested Parasuchia between Proterochampsa, Ornithosuchia and Dinosauromorpha. Benton (1999) nested Phytosauridae between Proterochampsidae and Stagonolepidae and Scleromochlus. Benton (2004) nested Phytosauridae with Gracilisuchus. Norrell (2006) and Nesbitt (2007) nested Parasuchia between Stagonolepidae and Pterosauria. (These were all referenced and illustrated n Nesbitt 2011, see below.)

A selection of phytosaurs (parasuchians).

Figure 1. A selection of phytosaurs (parasuchians).

The Latest Nesting
Nesbitt (2011) provided the latest and largest published tree. He nested phytosaurs as derived from Euparkeria and basal to two clades within the Archosauria. Ornithosuchidae was at the base of one branch and pterosaurs were at the base of the other.

It’s puzzling how aetosaurs, pterosaurs and Scleromochlus made earlier lists. They bear little to no resemblance to phytosaurs. It’s also noteworthy that there has been little consensus in prior studies.

It’s a shame that so many earlier studies eschewed generic taxa for suprageneric taxa. The mysteries remained mysteries. Sister taxa did not share many traits. The large study sheds light on the relationships of phytosaurs and all of the members of the Reptilia.

The sisters of Phytosaurs,

Figure 2. The sisters of Phytosaurs, including Cerritosaurus, Proterochampsa and the RC91 specimen of Youngoides. Click to enlarge.

Given All These Choices Where Does the Large Study Nest Phytosaurs?
Unfortunately only skulls are known for the closest sisters of phytosaurs. Those who nested Proterochampsa with phytosaurs were correct, as confirmed by  the large study. However, nobody mentioned Cerritosaurus, the phytosaurs’ other sister taxon. At the base of all three nests Youngoides minor. It lacks an antorbital fenestra. The Choristodera are also sisters and Doswellia is at the base of that large and varied clade. Chanaresuchus and its sisters are related beyond Cerritosaurus.

Phytosaur Family Tree
I have not attempted a phytosaur family tree. Paleorhinus (Fig. 2) and Parasuchus are generally considered basal phytosaurs.

Shared Traits?
A pair of dorsal and displaced nares distinguish phytosaurs from all euarchosauriformes (Euparkeria, Ornithosuchidae, Gracilisuchus, Dinosauromorpha, etc.) and pterosaurs. Proterochampsa shares that trait. Cerritosaurus and the chanaresuchids do too, but to a lesser extent.

A skull that is wider than tall is also a shared trait. The rostrum is elongated. The top of the skull is narrower than the base at the jaw joint. The top of the skull is flat across the upper temporal fenestra. The orbits are elevated or on top of a flattened skull. The postfrontal and postorbital are fused. An antorbital fenestra developed independently in this clade. Phytosaurs share with chanaresuchids a skull longer than half the presacral length of cervicals. Certainly many of these traits find convergence elsewhere on a tree where convergence runs rampant. Even so, the rules of parsimony place phytosaurs with these pararchosauriformes and the clade is a sister to the euarchosauriformes.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

Nesbitt SJ 2011. The early evolution of archosaurs: relationships and the origin of major clades. Bulletin of the American Museum of Natural History 352: 292 pp.

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