Updated December 09, 2014 with Fugusuchus instead of Postosuchus as the closest sister to Revueltosaurus.
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. First we started at the base with Mesosuchus. and several days and parts later, today we look at Revueltosaurus surrounded by Turfanosuchus, Gracillisuchus and Aetosaurus in the Nesbitt (2011) tree (Figs. 1, 2). Revueltosaurus was derived from a sister to Postosuchus in the large reptile tree.
Here (Fig. 2, lower three taxa) are the selected Nesbitt (2011) sisters for visual comparison. That’s quite a variety between the long neck, the sharp teeth and the heavy armor. Postosuchus (Fig. 2, upper taxon) is closer overall and in every detail according to the results of the large reptile tree. Not a match. Just closer. That means the herbivorous diet was arrived at by convergence in Revueltosaurus and Aetosaurus.
Similar or different?
The large reptile tree found Revueltosaurus close to Postosuchus. It found Aetosaurus close to Ticinosuchus. Gracilisuchus and Turfanosuchus were closer to basal crocs and you can see the similarities shared in their two skulls. The long neck of Turfanosuchus points to a possible relationship with dinosaurs, and it is a sister to the basal dinosaur Trialestes. It could also be a convergence, which appears more likely.
It’s too bad that Nesbitt (2011) did not reconstruct the skull of Ticinosuchus. Nor did he score more than a few characters for it. That would have proved valuable in establishing relationships. What Nesbitt (2011) did do was score traits that are harder to see than are the traits scored for the large reptile tree. And he omitted several key taxa. Hence the difference in recovered sisters.
According to the large reptile tree, Revueltosaurus evolved from smaller, quadrupedal ancestors of Postosuchus and it retained many of those traits. The high dorsal spines of Postosuchus are derived and appear to be correlated to the trend toward a larger hind limb and smaller forelimb strengthening that long lever arm with a fulcrum at the acetebulum. The development of the longer pubis and pubic boot in Postosuchus likewise added mechanical strength to the increasingly bipedal hind limbs. Sans the derived teeth, Revueltosaurus provides clues as to what the ancestors of Postosuchus may have looked like: smaller, more quadrupedal and with a relatively smaller pelvis.
In real evolution, sister taxa should look alike (Fig. 3) with slight variation. In real evolution, one should be able to trace a gradual accumulation of character traits, as we do with fossil hominids and Revueltosaurus. The rules don’t change with reptiles. PAUP take the shortest path, the one with the fewest changes, to nest sister taxa. One look at these results brings immediate understanding that there’s something wrong in the Nesbitt (2011) matrix that produces such strange bedfellows.
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.