Comparative osteohistology of some embryonic and perinatal archosaurs: developmental and behavioral implications for dinosaurs

Histologic studies of embryonic and perinatal longbones of living birds, no n-avian dinosaurs, and other reptiles show a strong phylogenetic signal in the distribution of tissues and patterns of vascularization in both the sha fts and the bone ends. The embryonic bones of basal archosaurs and other re ptiles have thin-walled cortices and large marrow cavities that are sometim es subdivided by erosion rooms in early stages of growth. The cortices of b asal reptiles are poorly vascularized, and osteocyte lacunae are common but randomly organized. Additionally, there is no evidence of fibrolamellar ti ssue organization around the vascular spaces. Compared with turtles, basal archosaurs show an increase in vascularization, better organized osteocytes , and some fibrolamellar tissue organization. In dinosaurs, including birds , vascularization is greater than in basal archosaurs, as is cortical thick ness, and the osteocyte lacunae are more abundant and less randomly organiz ed. Fibrolamellar tissues are evident around vascular canals and form organ ized primary osteons in older perinates and juveniles. Metaphyseal ("epiphyseal") morphology varies with the acquisition of new fe atures in derived groups. The cartilage cone, persistent through the Reptil ia (crown-group reptiles, including birds), is completely calcified in orni thischian dinosaurs before it is eroded by marrow processes; cartilage cana ls, absent in basal archosaurs, are present in Dinosauria; a thickened calc ified hypertrophy zone in Dinosauria indicates an acceleration of longitudi nal bone growth. Variations in this set of histological synapomorphies overlap between birds and non-avian dinosaurs. In birds, these variations are strongly correlate d with life-history strategies. This overlap, plus independent evidence fro m nesting sites, reinforces the hypothesis that variations in bone growth s trategies in Mesozoic dinosaurs reflect different life-history strategies, including nesting behavior of neonates and parental care.