Comparative Three-Dimensional Moment Arm Analysis of the Sauropod Forelimb: Implications for the Transition to a Wide-Gauge Stance in Titanosaurs

Abstract

The evolution of extraordinarily large size among Sauropoda was associated with a number of biomechanical adaptations. Changes in muscle moment arms undoubtedly accompanied these adaptations, but since muscles rarely fossilize, our ability to understand them has been restricted. Here, we use three-dimensional (3D) musculoskeletal modeling to reconstruct and quantitatively assess leverage of forelimb muscles in the transition from the narrow-gauge stance of basal sauropods to a wide-gauge stance in titanosaurs. A comparative analysis is conducted on three neosauropods: the narrow-gauge diplodocid <i>Apatosaurus louisae</i>, the intermediate-gauge titanosariform <i>Giraffatitan brancai</i>, and the wide-gauge titanosaur <i>Diamantinasaurus matildae</i>. In this study, moment arm magnitudes and corresponding morphological evidence indicates multiple changes across the narrow-gauge to wide-gauge transition in sauropods. High shoulder adduction was found in <i>Diamantinasaurus</i>, suggesting functional changes for supporting a wider stance and a limb less aligned with ground reaction force. High leverage in shoulder extension of <i>Diamantinasaurus</i> and <i>Giraffatitan</i> is possibly related to the increased use of the forelimb in forward propulsion with an anterior shift in center of mass. In addition, the prominence of the olecranon process in <i>Diamantinasaurus</i> produced high moment arm leverage in elbow flexion and extension, suggesting titanosaurs might have maintained a more flexed forelimb posture and displayed an increased degree of maneuverability. Other results are more variable between taxa but still indicate smaller scale changes. A sensitivity analysis was also conducted to measure the reliability of our models and test specific uncertainties within the modeling process, as well as other uncertainties uncovered during analysis.