Scientists studying the physiology of dinosaurs have long debated whether dinosaurs were warm- or cold-blooded. Those who suspect they were warm-blooded point out that dinosaur bone is generally fibro-lamellar in nature; because fibro-lamellar bone is formed quickly, the bone fibrils, or filaments, are laid down haphazardly. Consistent with their rapid growth rate, warm-blooded animals, such as birds and mammals, tend to produce fibro-lamellar bone, whereas reptiles, which are slow-growing and cold-blooded, generally produce bone in which fibrils are laid down parallel to each other. Moreover, like the bone of birds and mammals, dinosaur bone tends to be highly vascularized, or filled with blood vessels. These characteristics, first recognized in the 1930's, were documented in the 1960's by de Ricqlès, who found highly vascularized, fibro-lamellar bone in several groups of dinosaurs. In the 1970's, Bakker cited these characteristics as evidence for the warm-bloodedness of dinosaurs. Although de Ricqlès urged caution, arguing for an intermediate type of dinosaur physiology, a generation of paleontologists has come to believe that dinosaur bone is mammalianlike. In the 1980's, however, Bakker's contention began to be questioned, as a number of scientists found growth rings in the bones of various dinosaurs that are much like those in modern reptiles. Bone growth in reptiles is periodic in nature, producing a series of concentric rings in the bone, not unlike the growth rings of a tree. Recently, Chinsamy investigated the bones of two dinosaurs from the early Jurassic period (208-187 million years ago), and found that these bones also had growth rings; however, they were also partially fibro-lamellar in nature. Chinsamy's work raises a question central to the debate over dinosaur physiology: did dinosaurs form fibro-lamellar bone because of an innately high metabolic rate associated with warm-bloodedness or because of periods of unusually fast growth that occurred under favorable environmental conditions? (Although modern reptiles generally do not form fibro-lamellar bone, juvenile crocodiles raised under optimal environmental conditions do.) This question remains unanswered; indeed, taking all the evidence into account, one cannot make a definitive statement about dinosaur physiology on the basis of dinosaur bone. It may be that dinosaurs had an intermediate pattern of bone structure because their physiology was neither typically reptilian, mammalian, nor avian.