INTEGRATING HISTOLOGY AND MORPHOLOGY TO ASSESS THE SKELETAL MATURITY OF EARLY-DIVERGING DINOSAUROMORPHS

The Upper Triassic sediments of North America recorded the evolution of many extant vertebrate clades, including many new forms of dinosauromorphs¬—dinosaurs and their close relatives. Although growth is a universal biological process, ontogenetic patterns of early-diverging dinosauromorphs are relatively unexplored because of a lack of well-preserved growth series. Additionally, our knowledge of the relationship between histological and morphological changes during ontogeny in this group is lacking. Bone scars increase in number and size during ontogeny in many extant reptiles, and Dromomeron romeri, an early-diverging dinosauromorph from the Late Triassic of New Mexico, is reported to lack large ossified bone scars in the proximal portion of the femur that are present in many other archosaurs (e.g., anterior trochanter, trochanteric shelf). In this study, we tested whether this absence of bone scars is because the reported specimens represent skeletally immature individuals, or if the lack of scarring is an evolutionary novelty by integrating histological and morphological data from a growth series of D. romeri specimens. We observed changes in bone scars in a hypothesized growth series of six femora (96.9 mm – 136.6 mm in length). One femur was histologically sampled. Only the largest individual possesses a trochanteric shelf. The histology of the sampled femur, which lacks the anterior trochanter and trochanteric shelf, showed characteristics of a skeletally immature individual, with a vascularized subperiosteal surface and one annual line of arrested growth (LAG). This suggests that most D. romeri individuals recovered thus far represent immature individuals, and that D. romeri did deposit at least some of these femoral bone scars during ontogeny, but mature individuals have rarely been recovered. Based on these data, we hypothesize that other early-diverging dinosauromorphs lacking these femoral bone scars also represent immature individuals. Because bone scars are phylogenetically important characters, understanding how they change during ontogeny is necessary to correctly reconstruct evolutionary relationships. Integrated knowledge of growth patterns of early dinosauromorphs is necessary to reconstruct the early paleobiology of this highly successful clade.