Bone surface textures as ontogenetic indicators in extant and fossil archosaurs: Macroscopic and histological evaluations

It has been widely noted that bones of immature vertebrates are often more porous than mature elements. The use of bone surface textures may be useful for assessing relative age among fossil vertebrates; however, patterns of ontogenetic textural change have not been rigorously documented. Detailed investigation of textural change in extant taxa is critical, in order to evaluate whether universal patterns exist, determine the underlying causes of different textural types, and elucidate the effect of growth regime on patterns of textural change. This study begins such testing by examining the horned dinosaur Centrosaurus apertus, as well as members of Aves (Branta canadensis) and Crocodylia (Alligator mississippiensis), the extant groups phylogenetically closest to Dinosauria. Long bones (femur, tibia, and humerus) were studied in all taxa; the cranial frill (parietal and squamosal bones) was also examined in Centrosaurus. Surface textural patterns for all bones were defined based on gross examination and ordered based on decreasing porosity. Textural sequences were then compared with both size-based and size-independent maturity estimates to determine whether traceable patterns of ontogenetic texture change exist in these taxa. Selected elements were thin sectioned to evaluate growth regime and histological features underlying textural types. A regular pattern of decreasing surface porosity occurs during ontogeny in Branta, with juvenile, subadult, and adult age classes distinguishable by long bone textures. No consistent pattern of textural change is apparent in long bones of Alligator. Adult Centrosaurus are distinguished from juveniles and subadults by both postcranial and cranial textures; cranial textures may distinguish juveniles from subadults if a sufficient portion of the element is preserved. Results suggest textural aging may be applied with varying success to taxa with determinate growth regimes, but is not useful for taxa with indeterminate and/or interrupted growth. Thus application to fossil taxa without prior understanding of growth regime is risky at best. Surface textures observed in the three taxa were not directly comparable, suggesting that establishment of universal textural aging criteria may not be possible.