Dinosaur bone contains many significant features including fibro-lamellar bone, dense vascularization, and secondary haversian bone Such features are found in living endotherms, and have been cited as supporting the concept that dinosaurs were endothermic. Recently the view has emerged that all tissue types are found in both endotherms and ectotherms. Specific histological types of bone may indicate elevated growth rate. Combined with other approaches such as skeletochronology, some insight into metabolic rate may be obtained. Progress was made by careful investigation of the histology of series of femora that span the ontogeny of two Early Jurassic dinosaurs from Zimbabwe, Massospondylus carinatus and Syntarsus rhodesiensis. Both dinosaurs show fibrolamellar bone but neither shows all features of living endotherms. Massospondylus shows growth rings that indicate it took 15 years to attain a body size f 250 kg. Growth rate was indeterminate as in modern ectotherms. In Syntarsus growth was determinate as in living endotherms, but it took 7 years to attain the size of 20 kg. Each of these dinosaurs occupies a unique position on a physiological continuum between endothermy and ectothermy. The proposed study will extend the careful techniques worked out on the Zimbabwean dinosaurs to ontogenetic series of Triassic, Jurassic and Cretaceous dinosaurs from North America and Mongolia in order to obtain a phylogenetic, stratigraphic and geographic perspective on dinosaurian growth rates and the potential development of endothermy.
