Strange Bedfellows – Nesbitt (2011) – Part 9 – The Wrap-Up: Rauisuchians, Aetosaurs and Crocs

Sometimes we miss the big picture. 
Here then, for your approval and disapproval are comparisons between closest kin found by the Nesbitt (2011) tree versus those found by the large reptile tree.

The origin of the Aetosauria, Rauisuchia and Crocodylomorpha is today’s topic.
Nesbitt (2011) recovered the clade Loricata to include rauisuchians and their descendants, the crocodylomorpha (Fig. 1). Nesbitt (2011) mentioned Vjushkovia along with other erythrosuchids, but neglected to include it in his family tree. That was unfortunate. The large reptile tree found Vjushkovia a key taxon at the base of the Rauisuchia (Fig. 4), the Ticinosuchus/Aetosaur clade (Fig. 3) and the Crocodylomorpha (Fig. 5). Euparkeria and the Ornithosuchidae were recovered as outgroups for Vjushkovia in the LRT. Nesbitt (2011) nested the two pre-dinosaur plant-eaters together, Revueltosaurus with the Aetosauria, but otherwise there is little the two clades share.

Figure 1. Loricata according to Nesbitt (2011). Here Nesbitt recovers basal crocs arising from derived rauisuchians. The large reptile tree found both crocs and rauisuchians to be derived from a derived erythrosuchid, Vjushkovia, which was not listed by Nesbitt (2011). As a rule, major clades typically arise from generalized basal members, not derived taxa. Apparent exceptions, like the origin of pterosaurs from Longisquama, actually follow this pattern as pterosaurs originated from a basal longisquamid without several of the derived traits seen only on Longisquama.

The Nesbitt Tree Illustrated with Taxa
Here (Fig. 2) are many of the taxa recovered by Nesbitt (2011) in phylogenetic order. Much of this matches the large reptile tree. For instance, all of the rauisuchids nested together. However, derived rauisuchids did not give rise to crocs in the LRT. The morphological leap between Postosuchus and Hesperosuchus in the Nesbitt (2011) tree has a completely different pattern in the LRT.

Figure 2. The lineage of crocodylomorphs as recoverd by Nesbitt (2011). That’s a pretty big morphological jump between Postosuchus and Hesperosuchus (still waiting on data for CM73372, hence the place saver oval.) Even so, given the included taxa,  I can see the logic of Nesbitt’s tree, but the shared traits of these two are convergent when more taxa are added. There’s less of jump in the large reptile tree where Postosuchus is among the most derived of rauisuchians and Hesperosuchus and Dromicosuchus have many predecessor taxa in the Crocodylomorpha and basal Archosauria.

The Large Reptile Tree Illustrated with Taxa
In phylogenetic order, taxa within the large reptile tree (Figs. 3-5) appear to form more gradual transitions, have a better chronological order, and the most derived taxa in the various extinct clades actually lead to extinction. Generalized basal taxa give rise to derived forms. For instance, in the LRT a sister to Vjushkovia gave rise to the fish-eaters, Ticinosuchus and Yarasuchus + Qianosuchus and a sister to amored Ticinosuchus gave rise to heavily armored aetosaurs, both taxa with toothless premaxillae. Unfortunately Nesbitt (2011) did not reconstruct the skull of Ticinosuchus. Otherwise the aetosaur connection would have been more obvious.

Figure 3. Vjushkovia, Ticinosuchus and the base of the Aetosauria (Stagonolepidae). There’s still a pretty big jump here between Vjushkovia and Ticinosuchus, ameliorated by Qianosuchus (Fig. 4).

The Rauisuchia (Fig. 4) arose from a sister to the small derived erythrosuchid, Euparkeria. Vjushkovia was a descendant taxon that gave rise to several clades as is readily apparent here (details at reptileevolution.com):

Figure 4. The lineage of Rauisuchians, crocs and kin according to their skulls. Here the gradual accumulation of derived traits is easier to demonstrate.

Vjushkovia: basal to Aetosaurs, Crocs and Rauisuchids
The modifications that evolved in the descendants of Vjushkovia produced a lineage of decreasing size that ultimately produced tiny bipedal crocs (Figs. 4-5). There is no indication of a link between Postosuchus and Hesperosuchus (Nesbitt 2011) when you add these taxa.

Figure 1. Ten basal bipedal crocodylomorphs descending from a sister to Decuriasuchus.

Larger Studies Brings Greater Resolution
The LRT recovered different branching for the aetosaurs and crocs simply by adding more taxa and, in the case of Ticinosuchus, by more fully describing the formerly enigmatic skull. Images of these taxa demonstrate gradual transitions that are confirmed by 228 character scores leading to complete resolution.

Several notes in the dinosaur blog called these traits convergences, but convergence is defined by the initial phylogenetic distance and only a few traits are shared. Here large suites of traits were shared by sister taxa, the definition of homology.

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
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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