PhD Candidate, University of Queensland
I aim to elicit pedal flexibility and posture of the foot of a sauropod dinosaur
Within the sauropod lineage, the trend toward gigantism is assumed to have been associated with changes in foot morphology in order to withstand tremendous load-bearing and locomotor forces. To date, the biomechanics of the sauropod hind foot and the extent to which it may have influenced pedal posture have been poorly understood. Six different hypotheses have been proposed for the evolution of sauropod pedal posture, each requiring a unique set of changes to the configuration of the pedal bones, associated, in some instances, with the acquisition of a ‘heel’ pad. However, these anatomically-based interpretations have incorporated minimal biomechanical inference. To address this, we reconstructed the range of motion of the foot of Rhoetosaurus brownei, a basal gravisaurian sauropod from the Lower Cretaceous of Queensland, using three-dimensional modelling to elicit pedal flexibility and posture. The biomechanics of this specimen suggest the retention of digitigrady, with a likely capacity for ‘mid-subunguligrady’ in gravisaurians. Additionally, the data support the presence of a compliant soft tissue pad that extends caudally and likely provided support for the elevated proximal portions of the pedal autopodium. Unlike prior hypotheses, this scenario implies minimal changes to the underlying skeletal posture. Instead, it emphasizes that the evolutionary increase in body size may have lead to the substantially modified pedal soft tissue anatomy, commencing in basal sauropods. In the future, methods such as loading regimes and ichnological data will be combined to further inform and test long-standing hypotheses about sauropod pedal evolution.
Abstract: Locomotion over deformable substrates is a common occurrence in nature. Footprints represent sedimentary distortions that provide anatomical, functional, and behavioral insights into trackmaker biology. The interpretation of such evidence can be challenging, however, particularly for fossil tracks recovered at bedding planes below the originally exposed surface. Even in living animals, the complex dynamics that give rise to footprint morphology are obscured by both foot and sediment opacity, which conceals animal-substrate and substrate-substrate interactions. We used X-ray reconstruction of moving morphology (XROMM) to image and animate the hind limb skeleton of a chicken-like bird traversing a dry, granular material. Foot movement differed significantly from walking on solid ground; the longest toe penetrated to a depth of ∼5 cm, reaching an angle of 30° below horizontal before slipping backward on withdrawal. The 3D kinematic data were integrated into a validated substrate simulation using the discrete element method (DEM) to create a quantitative model of limb-induced substrate deformation. Simulation revealed that despite sediment collapse yielding poor quality tracks at the air-substrate interface, subsurface displacements maintain a high level of organization owing to grain-grain support. Splitting the substrate volume along "virtual bedding planes" exposed prints that more closely resembled the foot and could easily be mistaken for shallow tracks. DEM data elucidate how highly localized deformations associated with foot entry and exit generate specific features in the final tracks, a temporal sequence that we term "track ontogeny." This combination of methodologies fosters a synthesis between the surface/layer-based perspective prevalent in paleontology and the particle/volume-based perspective essential for a mechanistic understanding of sediment redistribution during track formation.
Pub.: 10 Dec '14, Pinned: 31 Aug '17
Abstract: Titanosauria is an exceptionally diverse, globally-distributed clade of sauropod dinosaurs that includes the largest known land animals. Knowledge of titanosaurian pedal structure is critical to understanding the stance and locomotion of these enormous herbivores and, by extension, gigantic terrestrial vertebrates as a whole. However, completely preserved pedes are extremely rare among Titanosauria, especially as regards the truly giant members of the group. Here we describe Notocolossus gonzalezparejasi gen. et sp. nov. from the Upper Cretaceous of Mendoza Province, Argentina. With a powerfully-constructed humerus 1.76 m in length, Notocolossus is one of the largest known dinosaurs. Furthermore, the complete pes of the new taxon exhibits a strikingly compact, homogeneous metatarsus-seemingly adapted for bearing extraordinary weight-and truncated unguals, morphologies that are otherwise unknown in Sauropoda. The pes underwent a near-progressive reduction in the number of phalanges along the line to derived titanosaurs, eventually resulting in the reduced hind foot of these sauropods.
Pub.: 19 Jan '16, Pinned: 31 Aug '17