Growth and textural ageing in long bones of the American alligator Alligator mississippiensis (Crocodylia: Alligatoridae)

Abstract

Growth series of femora, tibiae, and humeri of the American alligator Alligator mississippiensis were examined to assess the relationship between bone surface textures and relative skeletal maturity. Element texture types were compared with both size-based and size-independent maturity estimates. Selected elements were thin sectioned to observe the histological structures underlying various surface textures. Results suggest little to no relationship between bone textures and skeletal maturity in Alligator. Controlling for additional factors suspected to affect textural variation - sexual dimorphism, seasonally interrupted growth, wild vs. captive habitat, and geographical range - provides little resolution. Indeterminate growth is almost certainly a factor; however, this alone cannot explain all observed variability. Histological analyses reveal that highly porous surface textures are often associated with zones composed of fibrolamellar bone; smoother textures are generally underlain by lamellar zones or annuli. Textures of intermediate porosity may be associated with more than one histological pattern. Until the factors affecting bone texture changes in modern crocodylians are better understood, it is recommended that the textural ageing method be applied with caution to studies of fossil archosaurs with crocodylian-like or unknown growth regimes.