The Development of Appendicular joint cartilages in Alligator mississippiensis: Evolutionary and Biomechanical Implications for Archosauria

Abstract

Crocodylians and birds are the two surviving lineages within Archosauria, a clade of morphologically and ecologically diverse vertebrates that evolved highly disparate locomotor postures. The appendicular joints of crocodylians and birds differ substantially, thus complicating inferences of ancestral morphology and evolutionary transitions. Recent studies have indicated that the American alligator, an extant crocodylian, use a wide spectrum of cartilaginous and other connective tissues to construct the hip joint. However, little is known about the distribution and use of cartilage throughout other appendicular joints, as well the ontogenetic transition of the crocodilian chondro‐osseous interface during early development. In particular, the hyaline cartilage on the proximal femur of late embryonic alligators possesses a convex, cone‐like extension that inserts deep into the metaphysis. This morphology is not seen in juvenile and adults, but is superficially similar to the cartilage cone‐metaphyseal trough articulation inferred in early members of multiple archosaurian groups. The method by which alligators and other archosaurs reduced the cartilage cone during ontogeny and evolution, as well as the functional significance of the cartilage cone itself, are poorly understood.This study examined the ontogenetic transitions of proximal appendicular joints in the American alligator, spanning from in ovo stages to the first year of life. We used dissection, histology, and imaging techniques to assess the articular soft tissue anatomy at the chondro‐osseous junction of the proximal‐ and distal epiphyses of the femur and humerus. Additionally, osteological correlates for joint cartilage were identified on fully skeletonized limb bone elements and compared with bony morphology seen in fossil archosauromorphs.At the onset of hatching, the alligator did not reduce the cartilage cone via decreasing the “depth” of cartilaginous extension into the metaphysis, as inferred for early archosaurs. Instead, neonatal alligators regress the cartilage cone via multiple invasion fronts of trabecular bones, such that the subchondral growth plate of hatchlings presents a convex surface perforated by numerous cartilage canals, similar to those found in young birds. These results suggest that the ontogenetic loss of cartilage cone in the alligator is different from the evolutionary loss of cartilage cone hypothesized for early archosaurs. Instead, the role of cartilage thinning, fibrocartilage sleeve development and calcified epiphyseal cartilage may be highly associated within different sauropsid lineages in response to evolutionary changes in joint loading regimes.Support or Funding InformationThis research is supported by the Missouri State University Faculty Research Grant