Anatomical reconstructions of respiratory morphology and hindlimb musculature in Poposaurus gracilis (Archosauria: Poposauroidea) and related dinosauriformes

With the Extant Phylogenetic Bracket (EPB) method, patterns observed between specific regions of the skeleton and the corresponding soft tissues of extant archosaurs and squamates are used to generate inferences of the myology and respiratory biology of extinct taxa. Reconstructions of soft tissue structures in extinct taxa are important for three reasons: (1) the form of skeletal tissues is often controlled by adjacent soft tissues; (2) soft tissue form and function are generally the bases for paleobiological reconstructions in an extinct taxon; and (3) soft tissue trains are often used as characters in systematic analyses.;The EPB was used to reconstruct the pelvic and hindlimb musculotendinous system of Poposaurus gracilis (Archosauria: Poposauroidea), an obligate parasagittally erect bipedal pseudosuchian archosaur. This is the first phylogenetically based reconstruction of appendicular myology for a non-dinosaurian archosaur. The osteological data were derived from a largely complete, articulated specimen of Poposaurus from the Chinle Formation of Utah, USA. The results include a series of inferences on the morphology of 26 individual muscles or muscle groups, three pelvic ligaments, and two connective tissue structures in the pelvis and hindlimb of Poposaurus. This data set serves as a foundation for investigation into the myological and postural changes association with the evolution of bipedalism in crocodile-line archosaurs.;Detailed examination of the dorsal vertebrae and corresponding ribs of select well preserved extinct dinosauriform archosaurs allowed for predictions of morphology of lungs and ventilation mechanics. When the results are mapped onto a phylogeny for Archosauria, a clear trend becomes apparent indicating that the highly derived respiratory morphology of the avian and crocodilian lineages are a function of very different evolutionary pressures, and that neither likely represents the plesiomorphic condition for Archosauria. These data suggest that dorsocranially rigid, unidirectionally ventilated lungs were already present in basal dinosauriform archosaurs and that the crocodilian-style hepatic-piston ventilation system is a derived mechanism likely unique to Crocodyliformes.