Archosauria: Watch out how you use this taxon!

According to EvoWiki, Archosauria (Cope 1869 emended by Gauthier 1984 “ruling reptiles”) sensu stricto, or sensu Gauthier, is the crown clade of Archosauromorpha and is defined as “the common ancestor of birds and crocodiles and all descendants thereof.

In older, traditional smaller reptile trees, pterosaurs are included in the Archosauria as they were once, and are still traditionally considered close to dinosaurs, despite the fact that the two share very few traits. No series of taxa within or just outside the Archosauria demonstrate a gradual accumulation of pterosaurian traits. Rather, that gradual accumulation of traits is found in a series of now extinct tritosaur lizards, once and traditionally mistakenly considered prolacertiformes or protorosaurs.

Figure 1. Segment of the large reptile tree showing the Euarchosauriformes. The Archosauria is highlighted at the bottom.

The large reptile tree (Fig. 1) demonstrates that the outgroup for the dinosaurs (including birds) is the crocodylomorpha. So birds and crocs share last common ancestor around Turfanosuchus and Decuriasuchus. So the Archosauria is now restricted to just the Crocodylomorpha and the Dinosauria and the smaller clades and taxa they include. Note the basal placement of rauisuchoids preceding the archosauria.

Because prior studies did not include so many basal taxa, they included many additional ‘by default’ mismatches nesting between crocs and dinos. Let’s have a look at them.

Figure 2. A tree recovered from Gauthier 1986 (form Wiki) Pararchosauriformes are in green boxes. Pterosaurs are included within Ornithodira. Crocs and dinos here are greatly separated, so all basal forms back to a last common ancestor are included within the Archosauria.

Gauthier 1986
This very early smaller study included Proterochampsidae and Parasuchia, two suprageneric clades now known to nest outside the Euarchosauriformes when more taxa, like various Youngina and Choristodera, are included. It also employed suprageneric taxa instead of generic taxa (which always invites trouble), and certainly two few taxa to demonstrate a gradual accumulation of traits in derived forms. Gauthier employs the Ornithodira, which includes pterosaurs. Not sure how you would score such a taxon, with wings or without?

Figure 3. The Archosauria according to Sereno 1991 from Wiki. Light red arrows shows that crocs and dinos are still far apart here.

Sereno 1991
Five years later, but still early in the world of PAUP, Sereno (1991) moves Euparkeria outside of the Archosauria. Crocs nest within Suchia here. Ornithodira is divided into three clades. Pterosauria is highlighted because it doesn’t belong here, but in lizards (not employed). As in Gauthier (1986) Proterochampsidae and Parasuchia are also not correctly nested within these Euarchosauriformes, but nest here by default.

Figure 4. Benton 2004 diagrams the Archosauria but includes two lepidosauromorphs and separates the crocs into three widely divided clades.  Lagerpeton is more closely related to Proterochampsidae than dinos when Tropidosuchus is employed, which is not the case here. Clearly there needed to be more attention paid to basal crocodylomorphs that nest in three widely varied nodes here. Suprageneric taxa, like Crocodylomorpha, are probably the cause of the problem.

Benton 2004
Thirteen years later, and four years after Peters (2000), Benton (2004, Fig. 4) expanded on earlier studies, still employing suprageneric taxa, but moving towards employing more generic taxa. The topology was largely the same, though, as in prior studies (Figs. 2, 3). Yes, unbelievably, that’s  Hyperdapedon as a basal taxon.

Figure 5. Archosauria according to Brusatte 2010. Various clades found in the large reptile tree are identified by color boxes. Yes, there appear to be problems here as compared to the large reptile tree results. 

Brusatte et al. 2010
Six years later, and a full ten years after Peters (2000), Brusatte et al. (2010) employed still more generic taxa, keeping Pterosauria and Phytosauria but dropping Proterochampsidae. Here Gracilisuchus and Erpetosuchus correctly nested with Crocodylomorpha, but Scleromochlus did not. Taxa found to nest as poposaurids in the large reptile tree are divided, unresolved and widely separated here. Note that no taxa are shown to be basal to the Archosauria (which is always a problem!) and the basal taxa in each branch (Scleromochlus, Pterosauria, Phytosauria (=Parasuchia) and Aetosauria do not resemble one another — but they should if they truly reflect and model actual evolutionary paths.

Only the large reptile tree provides the gradual accumulation of traits in all derived taxa from more primitive taxa. So when you talk about archosaurs, it would be a good idea to restrict your discussion to crocs and dinos, which by definition, make up this clade. Keep the poposaurs within the Dinosauria. The others are all outliers.

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.

References
Brusatte SL , Benton MJ , Desojo JB and Langer MC 2010. The higher-level phylogeny of Archosauria (Tetrapoda: Diapsida), Journal of Systematic Palaeontology, 8:1, 3-47.
Cope ED 1869. Synopsis of the extinct Batrachia, Reptilia and Aves of North America. Transactions of the America Philosophical Society 14: 1–252.
Gauthier J1984. A cladistic analysis of the higher systematic categories of the Diapsida. [dissertation]. Available from University Microfilms International, Ann Arbor, #85-12825, vii + 564 pp.
Gauthier J 1986. Saurischian monophyly and the origin of birds. Memoirs of the California Academy of Sciences 8: 1-55.
Sereno PC 1991. Basal archosaurs: phylogenetic relationships and functional implications. J Vert Paleo 11 (Supp) Mem 2: 1–53.