A recent Discover article (December 2000) describes a study of the fossilized heart of a herbivorous dinosaur. According to conventional thought, all dinosaurs were cold-blooded because they were reptiles and all reptiles today are cold-blooded. But this study suggested that the dinosaur heart had four chambers instead of three, which is most common with cold-blooded reptiles of today. The four chambered heart is more like the heart of warm-blooded mammals.
This information is important to gaining a picture of what dinosaurs were like when they were alive. Cold-blooded creatures, because of their dependence on the environmental temperature to maintain their body temperature, tend to be slow and sluggish. Their behavior is often dictated by the time of day and the season of the year. Many modern reptiles, such as lizards and snakes, spend most of their time moving from one location to another in an attempt to keep their body temperature regulated.
If dinosaurs were warm-blooded then we could expect that their lives were more like mammals than reptiles. They may have been active hunters, migrated in large herds, engaged in complex courtship displays, and used more parental care. In short, the results of this research could drastically change the way we think of life on Earth millions of years ago. | 
2002 McGraw-Hill Higher Education