Primary compact bone is ignored in some recent discussions of claims that dinosaurs were endotherms, but forms the basis of one of the arguments from bone, and part of the basis of another. This paper explains its histology and discusses its possible significance. In dinosaurs the primary compact bone was commonly fibre-lamellar bone, resembling bone seen in many large mammals, and implying a capacity to sustain rapid growth to large sizes. This probably indicates some physiological difference between dinosaurs and modern types of reptiles; but similar bone is present in early therapsids, which were probably not endotherms, and bone with typical reptilian ‘growth rings’ was sometimes formed. Endothermy is also unlikely in most kinds of dinosaurs, if its evolution requires a trend to small sizes; but perhaps they were ‘failed endotherms’.
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