relatives of dinosaurs.The skeletons of many small dinosaurs resemble those of
birds (Fig. 136). Many large bipedaled dinosaurs assumed similar appearances
simply because of the need to balance themselves on two feet, which required
a huge tail and small forearms. A two-legged sprint would also have been the
fastest way to travel to run down prey.
Some dinosaur species might have acquired a certain degree of body
temperature control similar to mammals and birds. When the dinosaur age
began, the climates of southern Africa and the tip of South America where the
early dinosaurs roamed experienced cold winters, during which large, cold-
blooded animals could not have survived without migrating to warmer
regions. The stamina needed for such long-distance migration would have
required sustained energy levels that only warm-blooded bodies could pro-
vide.Warm-blooded animals mature more rapidly than cold-blooded reptiles,
which continue growing steadily until death.
An analysis of the bones of dinosaurs, crocodiles, and birds, all of which
had a common ancestor, shows a similarity between bird and dinosaur
bones—another sign of possible warm-bloodedness. The complex social
behavior of dinosaurs that requires a high rate of metabolism appears to be an
evolutionary advancement that results from being warm-blooded. Yet at the
end of the Cretaceous, when the climate is thought to have grown colder, the
warm-blooded mammals survived while the dinosaurs did not.
The carnivorous dinosaurs were cunning and aggressive, attacking prey
with unusual voracity. The cranial capacity of some carnivores suggests they
Figure 136Many small
dinosaurs such as
Mononykuswere built
much like birds.TRIASSIC DINOSAURS