Passage V

Two scientists discussed the evolution of 3 animal groups: crocodiles, nonavian dinosaurs (all dinosaurs except modern birds), and modern birds. The scientists characterized the animals in each group by their resting metabolic rate, RMR (the rate at which an animal uses caloric energy when at rest), and according to whether they were poikilothermic (had body temperatures that varied significantly) or homeothermic (had body temperatures that remained relatively constant).

Scientist 1

Crocodiles and nonavian dinosaurs evolved from a common ancestor that had a low RMR and was poikilothermic. Crocodiles retained these traits. However, nonavian dinosaurs had a high RMR and were homeothermic. Modern birds inherited these traits when the group first evolved from nonavian dinosaurs, and both traits have been retained by all modern birds.

The bones of nonavian dinosaurs had a porous structure, no growth rings, and extensive vascularization (channels containing blood vessels). Extensive vascularization, which is also seen in modern birds, is evidence of an efficient circulatory system and a high RMR. Efficient circulation distributes heat evenly throughout the body. Thus, nonavian dinosaurs were homeothermic and could have survived in cold climates.

Scientist 2

Crocodiles and nonavian dinosaurs evolved from a common ancestor that had a low RMR and was poikilothermic. Crocodiles retained these traits. Nonavian dinosaurs also had a low RMR and were poikilothermic. In contrast, modern birds have had a high RMR ever since the group first evolved from nonavian dinosaurs. Modern birds only became homeothermic after nonavian dinosaurs went extinct 65 million years ago.

The bones of nonavian dinosaurs were dense and had growth rings. Each of these features indicates a low RMR, and neither is seen in the bones of modern birds. Nonavian dinosaur bones also lacked extensive vascularization, suggesting an inefficient circulatory system that would not have been able to distribute heat evenly throughout the body. Thus, nonavian dinosaurs were poikilothermic and could not have survived in cold climates.