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.

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).

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).

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.

Vjushkovia, Ticinosuchus and the base of the Stagonolepidae (aetosaurs)

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.

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.

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.

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

  1. Nesbitt isn’t saying that large, derived rauisuchians gave rise to/are the direct ancestor of things like Hesperosuchus, only that they share a common ancestor (which likely would have looked similar to a blend of the two taxa).
    As to reconstructing a skull to get characters out of it – I am not familiar with the skull of Ticinosuchus and the shape it is in. Perhaps you could go into more detail with that as to the preserved state of the skull etc.

  2. Did bipedality evolve independently and in parallel in the various lines of crocs, or was there a common bipedal ancestor? Also, which were the first and last bipedal crocs?

    Thanks!

      • A common bipedal ancestor between dinos and crocs? Not sure. If you think they started large, like Pseudhesperosuchus derived from Decuriasuchus, then there you go. However, often tiny taxa make the big break, and if so, then Lewisuchus is closer to your common ancestor.

  3. Do you have plain photos of the skull of Ticinosuchus? It looks pretty crushed and beat up but I can’t tell about the original preservation with the overlays on there.
    You said, “That’s a pretty big morphological jump between Postosuchus and Hesperosuchus” Perhaps so, but there is a pretty big morphological jump between Homo sapiens and Gorilla – one is a large quadrupedal herbivore while the other is a slender, hairless bipedal omnivore. No one disputes that we are closely related to Gorilla (and other great apes). With the imperfect fossil record we will never recover all possible taxa. So Nesbitt is not saying that Hesperosuchus descended from rauischuians. Rauisuchia and Hesperosuchus (okay, techinicall CM37732) arose from a common ancestor that likely (but does not have to) looked something like a mix between both clades. Again, this hypothetical ancestor does not have to look exactly like Fasolasuchus, Saurosuchus, or anything else further down the line since we don’t have (at least from the data presented here) any indication of the morphological distance between the ancestor and the preserved fossils.
    What confidences are you getting for your clades vs. the clades that Nesbitt recovered?

    • My tree is completely resolved at every branch. So high confidence there. The Nesbitt tree has loss of resolution at the origin of crocs, following CM73372, and elsewhere. I understand what you are saying, and, as I mentioned, there are similarities between Hesperosuchus and Postosuchus that are not present in other crocs and other rauisuchians. Even so, there are better, more parsimonious sisters that remain untested and excluded by Nesbitt’s study. On the other hand, what does CM73373 look like? Is it a good transitional taxon? I still don’t know, but am curious to find out.

  4. Vjushkovia and some other ‘Erythrosuchid/Basal Rauisuchians’ like Vjushkovisaurus, Asperoris, Dongusuchus, Tsylmosuchus etc seem to have had a more Sprawling type of stance like the Proterosuchids due to the lack of some kind of bone over the Supra Ace-tibulum or something and a flatter or elingated Pelvis aswell is this True? Also is Vjushkovia and Vjushkovisaurus more or less the same creature?

    • Sorry that was meant to say Elongated pelvis…. Just want to know if these where Sprawlers or say Semi-Sprawlers like Lizards…

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