Biomechanical analysis of sauropod dinosaur appendicular anatomy using three-dimensional musculoskeletal modelling and finite element analysis

Abstract

<p>The massive size of some sauropod dinosaurs imposed many biomechanical constraints on their locomotion and behaviour. Postural transitions within Sauropoda reflect these constraints including the development of a quadrupedal stance from the bipedal stance of more basal sauropodomorphs, as well as the transition to a wide-gauge stance in titanosaurs from the narrow-gauge stance of other sauropods. In this thesis muscle moment arms and bone mechanical performance in the forelimb and hind limb of sauropods have been analysed using three-dimensional musculoskeletal modelling and finite element analysis. Taxa chosen for study represented a diversity of postures and gaits. <i>Plateosaurus engelhardti</i> from the Late Triassic is a basal and bipedal sauropodomorph. <i>Diplodocus carnegii</i> from the Late Jurassic is a narrow-gauge sauropod with a gracile bone morphology and posteriorly located centre of mass. <i>Giraffatitan brancai</i>, also from the Late Jurassic, is a titanosauriform sauropod with an intermediate stance and more centrally located centre of mass. It had gracile a bone morphology and greatly elongated forelimbs. Finally, the Australian Late Cretaceous titanosaur <i>Diamantinasaurus matildae</i> displayed a fully wide-gauge stance and robust bone morphology. It is also thought to have a more anteriorly located centre of mass, as has been previously suggested for titanosaurs.</p><p> By examining changes in the shift from a bipedal to a quadrupedal posture, I found that the bipedal sauropodomorph <i>Plateosaurus</i> had higher knee extension moment arms compared with the sauropod taxa analysed. I proposed this was indicative of leverage for maintaining a more flexed hind limb stance and more variable locomotor behaviours in contrast to the development of a columnar stance in sauropods and the resulting reduction in locomotor capabilities. More derived sauropods in this analysis (<i>Giraffatitan</i> and <i>Diamantinasaurus</i>) displayed a reduction in hip extensor leverage, possibly associated with an anterior shift in centre of mass. This shift, combined with higher bone stress magnitudes observed in the humeri and femora of these taxa, indicated they were not well-adapted to maintain a bipedal pose for long periods of time; however, the much more posteriorly located centre of mass and the low stress magnitudes in the proximal limb bones of <i>Diplodocus</i> suggested that this taxon was well-adapted for more habitual bipedal rearing. This was partially supported by hind limb moment arm results for <i>Diplodocus</i> which showed high leverage in hip mediolateral rotation and knee flexion and extension. There were also some interesting results uncovered in the forelimb of <i>Giraffatitan</i> and <i>Plateosaurus</i> during the course of this project. The elongated forelimb of <i>Giraffatitan</i> increased moment arm magnitudes in shoulder flexion thought to reduce peak forces acting on the forelimb during locomotion. This was supported by finite element analysis results which showed high relative stress in the humerus of <i>Giraffatitan</i> compared with the other sauropod taxa. This suggested that an increase in mechanical advantage of the muscles in the forelimb would have been advantageous for protecting the limb against peak stresses. Low stress magnitudes in the humerus of <i>Plateosaurus</i> suggested its bone morphology was still well-adapted for a weight-bearing role even though it is considered to be bipedal. This indicated that it might have used its forelimbs during locomotion as hatchlings or when traversing unstable substrates as adults, or in other non-locomotor behaviours like feeding or defence.</p><p> <p>In assessing the biomechanical impact of the transition to a wide-gauge stance in titanosaurs, I found moment arm results for <i>Diamantinasaurus</i> showed increased action in both shoulder adduction and hip adduction, presumably for stabilising limbs less well-aligned with ground reaction force. Bone stress magnitudes were also lower when counteracting adductor rather than abductor loads in <i>Diamantinasaurus</i>, indicating an adaptation for resisting these forces. In the forelimb of Diamatinasaurus, high leverage in elbow flexion and extension was found when compared with the other taxa studied. This result, combined with an increased role for the forelimb in forward propulsion indicated by an anteriorly shifted centre of mass, suggested that the forelimb in titanosaurs was adapted for increased manoeuvrability. I proposed this would have allowed titanosaurs to adopt a broader range of behaviours. In the hind limb however, results were a little different. Knee moment arm results showed low leverage about the knee in <i>Diamantinasaurus</i>, possibly indicating a preference for high velocity movements for improved stabilisation rather than manoeuvrability in the hind limb. In addition to this, the lateral shift of the limbs from the body with the development of a wide-gauge stance was also observed to have increased bone stress in both the humerus and femur of <i>Diamantinasaurus</i>. This increase in stress magnitude suggested that improved manoeuvrability in titanosaurs compared with other sauropods might have been less likely than previously thought. I therefore suggested that the transition to a wide-gauge stance in titanosaurs might not be related to the development of a broader range of behaviours, but might instead be related to an increased need for stability (and manoeuvrability in the forelimb) for inhabiting different palaeoenvironments.</p>