Cartilaginous Epiphyses in Extant Archosaurs and Their Implications for Reconstructing Limb Function in Dinosaurs

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.

Regeneration of the Epiphysis Including the Articular Cartilage in the Injured Knees of the Lizard Podarcis muralis

“…In adult mammalian epiphyses, the accidental injury or the progressive chronic degeneration of the articular cartilage does not generally elicit the neo-formation of new cartilage [16,17]. The different outcome between the repair ability between reptilian-amphibian versus mammalian-avian articular cartilage is therefore related to the different biological conditions that characterize the epiphyses of many reptiles and amphibians versus those of birds and mammals [18,25]; the former is conducive to cartilage regeneration after injury. Therefore repopulating human articular cartilage with chondrogenic stem/progenitor cells or feeding the cartilage with these cells appears a valuable attempt to be pursued in clinical trials.…”

Three‐dimensional skeletal kinematics of the shoulder girdle and forelimb in walking Alligator

“…The proximal radius articulates on the lateral condyle of the humerus and with the proximal ulna. These complex surfaces of the articular cartilages are not reflected in the underlying bones (Fujiwara, 2009;Holliday et al 2010;.…”

Articulated bone sets of manus and pedes of Camarasaurus (Sauropoda, Dinosauria)

“…Such sesamoid fibrocartilage is also present in human feet, reducing stress concentrations (Shaw and Benjamin, 2007). Furthermore, recent studies of extant archosaur long bones show that these can be capped with a significant layer of cartilage, adding up to approximately 10% of total bone length in certain cases (Bonnan et al, 2010;Holliday et al, 2010). This implies that also the corresponding elements in dinosaurs probably exhibited thick layers of articular cartilage, as was previously shown in a sauropod humerus (Schwarz et al, 2007b).…”

“…Taking FL without the claw imprints, h would increase to 3.6 FL. Although a large cartilage covering of the long bones as supposed by Holliday et al (2010), Schwarz et al (2007b), and Bonnan et al (2010) might increase h by up to 10%, it would still result around 3 FL. The ratio of metatarsal length to footprint length is also much lower in the 3D model than proposed by Thulborn (1990;24% in the model including claw prints, 46% in Thulborn).…”