Abstract
Extinct archosaurs, including many non-avian dinosaurs, exhibit relatively simply shaped condylar regions in their appendicular bones, suggesting potentially large amounts of unpreserved epiphyseal (articular) cartilage. This “lost anatomy” is often underappreciated such that the ends of bones are typically considered to be the joint surfaces, potentially having a major impact on functional interpretation. Extant alligators and birds were used to establish an objective basis for inferences about cartilaginous articular structures in such extinct archosaur clades as non-avian dinosaurs. Limb elements of alligators, ostriches, and other birds were dissected, disarticulated, and defleshed. Lengths and condylar shapes of elements with intact epiphyses were measured. Limbs were subsequently completely skeletonized and the measurements repeated. Removal of cartilaginous condylar regions resulted in statistically significant changes in element length and condylar breadth. Moreover, there was marked loss of those cartilaginous structures responsible for joint architecture and congruence. Compared to alligators, birds showed less dramatic, but still significant changes. Condylar morphologies of dinosaur limb bones suggest that most non-coelurosaurian clades possessed large cartilaginous epiphyses that relied on the maintenance of vascular channels that are otherwise eliminated early in ontogeny in smaller-bodied tetrapods. A sensitivity analysis using cartilage correction factors (CCFs) obtained from extant taxa indicates that whereas the presence of cartilaginous epiphyses only moderately increases estimates of dinosaur height and speed, it has important implications for our ability to infer joint morphology, posture, and the complicated functional movements in the limbs of many extinct archosaurs. Evidence suggests that the sizes of sauropod epiphyseal cartilages surpassed those of alligators, which account for at least 10% of hindlimb length. These data suggest that large cartilaginous epiphyses were widely distributed among non-avian archosaurs and must be considered when making inferences about locomotor functional morphology in fossil taxa.
Highlights
Most vertebrate movement is dependent on articulations that join bony elements together, and these joints are generally located at the ends of long bones
Whereas the extent of the cartilaginous cap can be directly assessed among extant animals, decomposition, fossilization, and other taphonomic processes strip away this functional information, and paleontologists are left to hypothesize the limb and joint anatomy of extinct taxa without what could be a substantial part of the functional limb of the organism
Crocodylians, palaeognaths, and neognaths are considerably different with regard to ossification of the ends of their limb bones
Summary
Most vertebrate movement is dependent on articulations that join bony elements together, and these joints are generally located at the ends of long bones. In the process of skeletonization, whether in nature or the lab, these terminal cartilaginous caps are lost. The dried bony elements are not the same functional elements used by an animal, but rather just the mineralized portion. Whereas the extent of the cartilaginous cap can be directly assessed among extant animals, decomposition, fossilization, and other taphonomic processes strip away this functional information, and paleontologists are left to hypothesize the limb and joint anatomy of extinct taxa without what could be a substantial part of the functional limb of the organism
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