Dinosaurs evolved from cold-blooded animals (the reptiles) and evolved into warm-blooded animals (the birds). All dinosaurs, however, were not the same, and perhaps their physiologies differed also. The huge dinosaurs and the tiny dinosaurs might have used different heat-regulation strategies, just as they used different strategies for other aspect of living. A good argument for this is found among modern mammals. Although warm-blooded, there are some mammals (monotremes, the egg-laying mammals like the duck-billed platypus) whose metabolisms are close to being cold-blooded.

Scipionyx, a Cold-Blooded Theropod?
Scipionyx samniticus was a Cretaceous period theropod (from about 113 million years ago), perhaps a maniraptor. It is known from a single, extremely detailed specimen of a hatchling that includes fossilized soft tissues, including muscles and internal organs. It was 9.5 inches (24 cm) long and had a very large head.

John A. Ruben, a vertebrate paleobiologist from Oregon State University, used an 80-watt ultraviolet (UV) lamp to help reveal outlines of Scipionyx's fossilized internal organs. Ruben found that the position of Scipionyx's colon (intestines) and liver were similar to that of modern crocodilians (which are cold-blooded), and unlike that of birds (which are warm-blooded). The position of the liver also gives information about the lungs, since a muscle that runs by the liver helps the lungs to expand and contract in crocodilians. Scipionyx probably had reptilian-style lungs (and not highly efficient bird lungs).

Although Ruben's work is not absolutely conclusive, it looks like the small theropod Scipionyx may be cold-blooded. For more information on Ruben's work, see the January 22; 283: 514-516 (1999) issue of the magazine Science.

A Fossilized Dinosaur Heart Found
Recently, a fossilized dinosaur heart was discovered. This powerful, four-chambered pump belonged to Thescelosuarus, a lated Cretaceous Period ornithopod (a plant-eater). This 66 million year old heart has recently been analyzed -- it may have four chambers and a single aorta, which is quite different from existing reptiles, but is similar to modern warm-blooded animals. However, Tim Rowe, Earl McBride and Paul Sereno think that the "heart" is an ironstone concretion.

If true, the discovery of a four-cha,bered heart suggests not only that Thescelosaurus may have been warm-blooded, but that many other dinosaurs may have been warm-blooded as well. This is because we now perhaps have evidence of warm-bloodedness in both Ornithischia and Saurischia (the two major groups of dinosaurs, which diverged early in the Mesozoic Era). Thescelosaurus was an ornithischian; birds, which are known to be warm-blooded, are saurischians.

Heat-Regulating Devices: Some dinosaurs seem to have had heat regulating structures on their bodies. For example, Spinosaurus and Ouranosaurus had large sails on their backs, and Stegosaurus had numerous plates. These devices were probably used for the collection and dispersion of heat. This suggests that they needed these structures to regulate their body heat and that they were cold-blooded.