Cranial kinesis in dinosaurs: intracranial joints, protractor muscles, and their significance for cranial evolution and function in diapsids

Abstract

ABSTRACT Different forms of intracranial mobility, including streptostyly, pleurokinesis, and prokinesis, have been postulated for many dinosaurs. The basis for inferring kinesis typically has included the presence of presumably synovial intracranial joints (otic and basal joints) and various ad hoc ‘sliding joints’ (many without modern parallels), whereas the protractor musculature that would have powered movement at these joints has received little attention. No study has reviewed the evidence underlying these inferences, and the functional mechanisms and evolution of kinesis among dinosaurs have remained unclear. We analyzed the relevant musculoskeletal structures in extant diapsids and extinct dinosaurs to evaluate in general the morphological support for inferences of cranial kinesis in dinosaurs. Four criteria (synovial otic joints, synovial basal joints, protractor muscles, and permissive kinematic linkages) were considered necessary but individually insufficient for the inference of kinesis. Assessing these criteria across dinosaurs reveals that synovial otic and basal joints are almost universally present (even in widely acknowledged akinetic taxa), and most taxa retained protractor musculature. However, unlike fully kinetic extant birds and squamates, almost all dinosaurs lacked the kinematic linkages that would have permitted movement (reduced palatal and temporal articulations, additional flexion zones). Thus, synovial basal and otic joints and protractor musculature are diapsid plesiomorphies, and, in the absence of permissive kinematic linkages, most formulations of nonavian dinosaur kinesis are currently problematic. Alternatively, persistent synovial joints may simply be cartilaginous sites that facilitate cranial growth during ontogeny.