After observation of detailed structural evidence for the origin of birds f
rom dinosaurs(1), and in light of evidence that dinosaur bone tissue resemb
les the histology in mammals(2), the histology of bone has become one of th
e focal points in discussions of the physiology of dinosaurs and Mesozoic b
irds(3-10). Most of this microstructural information has focused on feature
s related to the vascular organization and the amount of remodelled bone ar
ound vascular canals. However, the finer structures have received less atte
ntion, although differences in such structures have been observed among mod
ern vertebrates(10,11). Here we present evidence that canaliculi-the submic
rometre-sized channels that interconnect bone cells and vascular canals-and
the collagen fibre bundles in bone are differently organized among certain
dinosaur lineages. Ornithomimid dinosaurs(12) are more like birds than mam
mals in these features. In canalicular structure, and to some extent in fib
re bundle arrangement, ornithischian dinosaurs are more like mammals. These
differences in both canalicular and lamellar structure are probably linked
to differences in the process and rate(13) of bone formation.