Jr. Horner et al., Comparative osteohistology of some embryonic and perinatal archosaurs: developmental and behavioral implications for dinosaurs, PALEOBIOL, 27(1), 2001, pp. 39-58
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.