Pelvic and hindlimb myology of the basal archosaur Poposaurus gracilis (archosauria: Poposauroidea)

Abstract

The discovery of a largely complete and well preserved specimen of Poposaurus gracilis has provided the opportunity to generate the first phylogenetically based reconstruction of pelvic and hindlimb musculature of an extinct nondinosaurian archosaur. As in dinosaurs, multiple lineages of basal archosaurs convergently evolved parasagittally erect limbs. However, in contrast to the laterally projecting acetabulum, or "buttress erect" hip morphology of ornithodirans, basal archosaurs evolved a very different, ventrally projecting acetabulum, or "pillar erect" hip. Reconstruction of the pelvic and hindlimb musculotendinous system in a bipedal suchian archosaur clarifies how the anatomical transformations associated with the evolution of bipedalism in basal archosaurs differed from that of bipedal dinosaurs and birds. This reconstruction is based on the direct examination of the osteology and myology of phylogenetically relevant extant taxa in conjunction with osteological correlates from the skeleton of P. gracilis. This data set includes a series of inferences (presence/absence of a structure, number of components, and origin/insertion sites) regarding 26 individual muscles or muscle groups, three pelvic ligaments, and two connective tissue structures in the pelvis, hindlimb, and pes of P. gracilis. These data provide a foundation for subsequent examination of variation in myological orientation and function based on pelvic and hindlimb morphology, across the basal archosaur lineage leading to extant crocodilians.

Disparity and convergence in bipedal archosaur locomotion

“…While noting differences in the magnitude of muscle leverage ( figures 3 and 4), we infer that stance phase lateral limb stability in bipedal suchians was achieved in the same manner as inferred for basal theropods, specifically a support phase abduction moment generated by the IF group (figure 7), represented by IFM in Poposaurus [18]. Given the triradiate arrangement of the pelvis, and the likely more upright posture and caudofemoralis-driven limb retraction, it is likely that femoral abduction predominantly countered the adduction GRF moment in Poposaurus, rather than medial rotation as in extant birds [10], which is greatly aided by enhanced muscle leverage for this action (figures 3 and 4).…”

“…Furthermore, differences in specific muscles are also evident in Poposaurus, notably the adductor group (ADD1 and 2) and CFB. A raised process midway down the caudal aspect of the femoral shaft and a vertically oriented ridge that extends distally from this process are reconstructed as the insertion sites of ADD1 and 2 in Poposaurus [18]. These insertion sites of the adductor complex in Poposaurus are significantly larger than those in any other archosaur, extinct or extant (see electronic supplementary material).…”

“…In particular, we provide a detailed biomechanical investigation of pelvic and hindlimb muscle mechanics in a bipedal poposauroid (a sub-clade of extinct pseudosuchian archosaurs that includes many bipedal taxa), with the aim of understanding the similarities and differences between the evolution of upright bipedal posture and locomotion in non-dinosaurian archosaurs and the theropod lineage that gave rise to extant birds (figure 1). Our analysis is facilitated by the discovery of a new, largely complete and articulated specimen of Poposaurus gracilis (YPM 57100; figure 1b), which represents the most complete poposauroid skeleton known to date, and is probably the most complete bipedal basal (non-ornithodiran) archosaur yet discovered [8,18]. The exceptional preservation of this specimen has conserved the majority of the muscle attachment sites on the pelvic and hindlimb skeletal elements, enabling a phylogenetically based reconstruction of hindlimb myology [18] (figure 2a).…”

Reconstructing pectoral appendicular muscle anatomy in fossil fish and tetrapods over the fins‐to‐limbs transition

“…Reconstructions of soft tissue anatomy are equally important as those of the skeleton because they can provide crucial information about locomotion, mode of life, and ecology of extinct animals (Witmer, 1995). For example, muscle reconstructions have been used to infer characteristics such as posture, maximum speed, and even possible gaits in extinct vertebrates (Russell, 1972;Sumida, 1989;Hutchinson & Garcia, 2002;Sellers et al, 2004Sellers et al, , 2009Hutchinson et al, 2005;Sellers & Manning, 2007;Bates et al, 2010;Schachner, Manning & Dodson, 2011). In addition, the ancestral configuration of the limbs is critical for understanding how constraints and adaptations led to the morphological diversity of modern tetrapod limbs.…”

Bases ósseas e musculares dos cortes comerciais do tronco de jacaré-do-Pantanal (Caimanyacare Daudin, 1802)

“…Essa descrição assemelha-se ao verificado nesta pesquisa no jacaré-do-Pantanal. Para Schachner et al (2011), o m. puboisquiofemoral interno apresenta diferentes divisões em todos os sáurios existentes, sendo que os crocodilianos e a maioria dos lepidosauros têm dois componentes, ao passo que as aves e alguns esquamatas possuem três. Essa afirmação está de acordo com o descrito neste trabalho em C. yacare.…”