Meeting Abstract
The cartilage cone is a convex extension of the epiphyseal hyaline cartilage that inserts into the metaphyseal growth plate of long bones. Among extant archosaurs, the cartilage cone results from delayed endochondral ossification relative to perichondral ossification during embryological development, and is absent in neonates. In contrast, the proximal femora of many post-neonatal fossil archosauromorphs bear evidence of cartilage cones. This study investigated the evolution, function, and ontogenetic significance of the cartilage cone in archosauromorphs. Femora of 140 taxa were studied and digitized. Key phylogenetic transitions in cartilage morphology were estimated using likelihood ancestral state reconstruction on the osteological correlates, and analyzed using phylogenetically corrected correlation to reveal trends in body size evolution. The cartilage cone arose independently in multiple archosauromorph lineages but was secondarily reduced in sauropods, theropods, and crocodylomorphs. Although adult body size does not predict the presence of the cartilage cone, it is often absent in large adults but present in locomotor patent conspecific juveniles. The cartilage cone likely provided mechanical support to the thick epiphyseal hyaline cartilage by increasing metaphyseal contact. In sauropods, reduction of the cone coincides with highly rugose growth plates; whereas reduction of the cone in theropods coincides with smooth growth plates. These divergent adaptations are hypothesized to associate with transitions in cartilage thickness and loading regimes. These results indicate that archosauromorphs used uncalcified hyaline cartilage as load bearing tissues on par with subchondral bones, illustrating a key innovation in locomotor tissues.
