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Major transformations in the locomotor system of archosaurs (a major clade of reptiles including birds, crocodiles, dinosaurs, and pterosaurs) were accompanied by significant modifications to ankle anatomy. How the evolution of such a complex multi‐joint structure is related to shifts in ankle function and locomotor diversity across this clade remains unclear and weakly grounded in extant experimental data. Here, we used X‐ray Reconstruction of Moving Morphology to reconstruct skeletal motion and quantify the sources of three‐dimensional ankle mobility in the American alligator, a species that retains the ancestral archosaur ankle structure. We then applied the observed relationships between joint excursion and locomotor behaviors to predict ankle function in extinct archosaurs. High‐resolution reconstructions of Alligator skeletal movement revealed previously unseen regionalized coordination among joints responsible for overall ankle rotation. Differences in joint contributions between maneuvers and steady walking parallel transitions in mobility inferred from the ankle structure of fossil taxa in lineages with more erect hind limb postures. Key ankle structures related to ankle mobility were identified in the alligator, which permitted the characterization of ancestral archosaur ankle function. Modifications of these structures provide morphological evidence for functional convergence among sublineages of bird‐line and crocodylian‐line archosaurs. Using the dynamic insight into the internal sources of Alligator ankle mobility and trends among locomotor modes, we trace anatomical shifts and propose a mechanistic hypothesis for the evolution of ankle structure and function across Archosauria. [ABSTRACT FROM AUTHOR]

Paleobiological reconstructions of joint mobility are an essential component of functional analyses of extinct animals. Over the past half-decade, the methods underlying mobility studies have advanced rapidly in three main areas: increasing complexity of virtual joint manipulation, formalizing pose viability criteria, and constructing more rigorous quantitative frameworks. Here we contextualize and review the recent history of this field, and call attention to remaining challenges and potential future directions. Additionally, we make available and describe a set of user-friendly scripts for the animation software Autodesk Maya. In doing so, we aim to make many of the latest approaches for virtual mobility reconstruction more easily accessible to other researchers, encouraging their broader adoption and collaborative improvement. [ABSTRACT FROM AUTHOR]

Early Mesozoic theropods sank down, and forward, and extracted the foot with convergent toes, according to evidence provided by dinosaur trackways in the Orsted Dal Member of the uppermost Fleming Fjord Formation of Jameson Land, East Greenland. These elongate tracks show complete toe convergence, which is retained in birds as a primitive feature. It is possible that the Greenlandic trackways were created by cretosaur feet in which the hallux was somewhat abducted and flexed, but not reversed.

Haramiyid dentaries have been discovered in the Upper Triasic of East England. Features shared by the haramiyids with other mammals based on the find are differentiated teeth and multirooted molariforms. Haramiyids are found to be highly specialized mammals with a unique pattern of puncture-crushing occlusion that is different from the grinding or shearing mechanisms of other Early Mesozoic mammals.

Extant archosaurs exhibit highly divergent articular soft tissue anatomies between avian and crocodilian lineages. However, the general lack of understanding of the dynamic interactions among archosaur joint soft tissues has hampered further inferences about the function and evolution of these joints. Here we use contrast‐enhanced computed tomography to generate 3D surface models of the pelvis, femora, and hip joint soft tissues in an extant archosaur, the American alligator. The hip joints were then animated using marker‐based X‐Ray Reconstruction of Moving Morphology (XROMM) to visualize soft tissue articulation during forward terrestrial locomotion. We found that the anatomical femoral head of the alligator travels beyond the cranial extent of the bony acetabulum and does not act as a central pivot, as has been suggested for some extinct archosaurs. Additionally, the fibrocartilaginous surfaces of the alligator's antitrochanter and femoral neck remain engaged during hip flexion and extension, similar to the articulation between homologous structures in birds. Moreover, the femoral insertion of the ligamentum capitis moves dorsoventrally against the membrane‐bound portion of the medial acetabular wall, suggesting that the inner acetabular foramen constrains the excursion of this ligament as it undergoes cyclical stretching during the step cycle. Finally, the articular surface of the femoral cartilage model interpenetrates with those of the acetabular labrum and antitrochanter menisci; we interpret such interpenetration as evidence of compressive deformation of the labrum and of sliding movement of the menisci. Our data illustrate the utility of XROMM for studying in vivo articular soft tissue interactions. These results also allow us to propose functional hypotheses for crocodilian hip joint soft tissues, expanding our knowledge of vertebrate connective tissue biology and the role of joint soft tissues in locomotor behavior. [ABSTRACT FROM AUTHOR]

Dinosaur locomotion can be interpreted via functional analyses of skeletons, study of fossil trackways, and comparisons with living animals. Since dinosaurs included the largest land animals of all time, both quadrupeds and bipeds, consideration of their locomotor abilities relates to theoretical concerns about relationships between body size and locomotor performance. Because of evidence that birds are a subclade of carnivorous dinosaurs, the diversity of theropod locomotor adaptations is emphasized. A phylogenetic survey of pelvic, hindlimb and tail characters across different theropod groups permits interpreting changes in functional terms; special attention is given to the effects of body size on locomotor adaptations. Non-avian theropods are characterized by bipedal striding reflected in very narrow trackways. The pes is markedly tridactyl, with an elongated metatarsus, and greatly reduced digit I. Over the course of theropod evolution the tail became relatively shorter, with stiffened distal elements. Hip architecture altered, presumably in tandem with changes in pelvic musculature. Avian walking further modifies typically theropod locomotor features in more horizontal carriage of the femur and more symmetrical weight distribution on the pedal digits. Evolving avian flight involved coopting and modifying ancestral features originally unrelated to volant locomotion.