had relatively large brains and were fairly intelligent (Fig. 137), able to react to
a variety of environmental pressures. The velociraptors, meaning “speedy
hunters,” had sharp claws and powerful jaws. They were vicious killing
machines whose voracious appetites suggest they were warm-blooded.
Many dinosaur species were swift and agile, requiring a high rate of
metabolism that only a warm-blooded body could provide. The anatomy of
some dinosaurs implies they were active, upright, and constantly feeding, a
behavior that suggests a high metabolic rate commensurate with warm-
blooded animals. The comparison of ratios of oxygen 18 to oxygen 16 in
dinosaur teeth with those of cold-blooded crocodiles suggests dinosaurs had a
constant body temperature regardless of the outside temperature.They
breathed with diaphragm-driven lungs for the extra respiration needed for
rapid movement much like modern mammals. However, when at rest, they
might have s witched to rib-based respiration similar to reptiles, giving them a
metabolism functioning unlike that of any living animal.
A comparison of the density of pelvic bones in fossilized dinosaur
embryos with the bones of modern crocodiles and birds indicated that
dinosaur juveniles were highly mobile, requiring the energy only a warm-
blooded body could supply. Evidence for rapid juvenile growth, which is
common among mammals, also exists in the bones of some dinosaur species,
possibly providing another sign of warm-bloodedness. Newborn dinosaurs
apparently were not defenseless dependents as with most birds but leapt from
their shells ready to run and protect themselves from a dangerous world. The
infant dinosaurs had the bone and muscle strength to move rapidly and scam-Figure 137
Stenonychosaurushad a
large brain weight to body
weight ratio.
Historical Geology