Poposaurus gracilis (Mehl 1915) Late Triassic ~210 mya was recently (Gauthier et al. 2011) considererd a basal crocodilian close to Arizonasaurus. Neither is correct. Here Poposaurus nested as a basal pseudornithischian. Derived from a sister to Turfanosuchusand Daemonosaurus, Poposaurus phylogenetically preceded Shuvosaurus and Effigia.
Distinct from Turfanosuchus, Poposaurus had a shorter torso and five sacral vertebrae with longer and taller neural spines. The caudals also had taller spines.
The dorsal scapula was further expanded. The fore limb was shorter. The hand was smaller with smaller fingers and unguals. Digits 4 and 5 lost two phalanges.
The anterior ilium was elongated. The longer pubis included a "boot" at its ventral tip. The pes was relatively larger overall. The tube of the calcaneum was enlarged. Metatarsal 4 was shorter than mt3. Digit 4 included 5 phalanges. Digit 5 was absent.
In consideration of the shorter torso, the length of the pelvis, the incorporation of more sacral vertebrae and the disparate lengths of the fore and hind limbs, Poposaurus was clearly a biped. Its toes could be placed beneath the center of balance.
Gauthier et al. (2011) and others consider archosaurs with an enlarged tuber on the calcaneum to be closer to crocodiles than to dinosaurs. Here Poposaurus demonstrates the secondary enlargement of the tuber within the Dinosauria.
|
