Disparity and convergence in bipedal archosaur locomotion

Abstract

This study aims to investigate functional disparity in the locomotor apparatus of bipedal archosaurs. We use reconstructions of hindlimb myology of extant and extinct archosaurs to generate musculoskeletal biomechanical models to test hypothesized convergence between bipedal crocodile-line archosaurs and dinosaurs. Quantitative comparison of muscle leverage supports the inference that bipedal crocodile-line archosaurs and non-avian theropods had highly convergent hindlimb myology, suggesting similar muscular mechanics and neuromuscular control of locomotion. While these groups independently evolved similar musculoskeletal solutions to the challenges of parasagittally erect bipedalism, differences also clearly exist, particularly the distinct hip and crurotarsal ankle morphology characteristic of many pseudosuchian archosaurs. Furthermore, comparative analyses of muscle design in extant archosaurs reveal that muscular parameters such as size and architecture are more highly adapted or optimized for habitual locomotion than moment arms. The importance of these aspects of muscle design, which are not directly retrievable from fossils, warns against over-extrapolating the functional significance of anatomical convergences. Nevertheless, links identified between posture, muscle moments and neural control in archosaur locomotion suggest that functional interpretations of osteological changes in limb anatomy traditionally linked to postural evolution in Late Triassic archosaurs could be constrained through musculoskeletal modelling.

Musculoskeletal modelling of an ostrich (Struthio camelus) pelvic limb: influence of limb orientation on muscular capacity during locomotion

“…Studies using models to estimate muscle moment arms need to consider not only this, but also how or whether those parameters actually matter for particular muscles, joints, behaviours or species (see also Bates & Schachner, 2012; Bates et al, 2012; Maidment et al, 2013). …”

“…10). Bates & Schachner (2012) appear to have avoided these errors.A third point of discordance between this study and Gangletal’s (2004) is that we consider the latter study’s “Mm. femorotibiales externus et medius” to be two parts (superficial and deep) of M. femorotibialis lateralis (FMTL; vide Zinoviev, 2006), because this avian muscle typically originates on the lateral surface of the femur, deep to M. iliotibialis lateralis (IL), as the former two parts do.…”

Computational modelling of locomotor muscle moment arms in the basal dinosaur Lesothosaurus diagnosticus: assessing convergence between birds and basal ornithischians

“…2005, 2008; Bates et al. in press; Bates & Schachner, in press), thereby decreasing muscular capacity to support the hip in these postures. Various osteological and myological changes occurred during avian evolution that may have plausibly acted to alleviate this decrease in the magnitude of hip extensor moment arm (i.e.…”

“…in press) using the same procedures employed here for Lesothosaurus : two on non‐avian theropod dinosaurs ( Allosaurus fragilis , Struthiomimus sedens ); and one on the extant ostrich ( Struthio camelus ). Data gleaned from these taxa include hip flexion–extension, adduction–abduction and long axis rotation moment arms across a wide spectrum of limb postures (Bates & Schachner, in press; Bates et al. in press).…”

A phenology of the evolution of endothermy in birds and mammals

“…For animals efficiently to exploit terrestrial environments the transition from a sprawling gait -limbs held lateral to the body -to a fully upright stance -limbs held directly underneath the body -was requisite for a terrestrial existence (Sereno, 1991;Benton, 2000;Hutchinson, 2006;Kielan-Jaworowska & Hurum, 2006;Kubo & Benton, 2007;Bates & Schachner, 2012;Iijima & Kobayashi, 2014). The transition involved profound changes to the postcranial skeleton, especially to the pelvic girdle (hip joint or acetabulum and ilium), pectoral girdle (scapular and clavicle), hind limb (femur, tibia, fibula, tarsals, metatarsals and digits) and forelimb (humerus, ulna, radius, carpals, metacarpals and phalanges) (Sereno, 1991;Benton, 2000;Blob, 2001;Kielan-Jaworowska & Hurum, 2006;Kubo & Benton, 2007).…”