A new paper by Kubo and Kubo (2012) discussed bipedalism in archosaurs. They found basal dinosaurs and Poposaurus to be definite bipeds. Possible bipeds in their study included Euparkeria, Crocodylomorpha, Gracilisuchus, Ornithosuchidae and Pterosauromorpha (pterosaurs + Scleromochlus). Seeking to find a phylogenetic connection, Kubo and Kubo (2012) recovered over 12,000 MPTrees. They avoided pterosaurs and most of the bipdal croc-types without comment. They considered Poposaurus a “crurotarsan,” which is traditional thinking that has come into question here.
The large reptile tree (Fig. 1) includes 300+ taxa and recovers a single tree (when the two poorly known taxa are excluded). Several taxa were facultative bipeds. Others were obligate bipeds (highlighted in white).
In the new Lepidosauromorpha, only one clade was bipedal, the Fenestrasauria (pterosaurs and kin). However 19 living lizards (not listed) are facultative bipeds.
In the new Archosauromorpha several clades were at least faultatively bipedal, from Eudibamus and Lagerpeton + Tropidosuchus to Smok + Postosuchus and basal archosaurs, most of which (excepting theropods) had descendants that reverted to a quadrupedal configuration. In the large reptile tree the only two ornithischian dinosaurs reverted to quadrupedalism and both were armored. Lotosaurus had a finback, reason enough.
Kubo and Kubo (2012) considered Silesaurus a quadruped. I think it was rarely a quadruped. They discussed comparisons with certain mammals, which developed more propulsive powers of the forelimb, such as a mobilized scapula. Dinosaurs, with their heavy tails, had a center of balance further back than in mammals. All good thoughts.
By the large reptile tree it appears that reptiles like Lewisuchus and Gracilisuchus originated bipedal locomotion in the Archosauria. Bipedalism conferred certain advantages. Those advantages include breathing while running (avoiding Carrier’s restraint), elevating the skull above obstructions in order to see further, enabling the forelimbs to perform other tasks (such as flapping in Cosesaurus). Increased speed is not associated with bipedality. That many clades reverted to quadrupedalism is just in the nature of evolution, like losing limbs after they had evolved from fins.
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.
Kubo T and Kubo MO 2012. Associated evolution of bipedality and cursoriality among Triassic archosaurs: a phylogenetically controlled evaluation. Paleobiology 38(3):474-485.