The giant herbivorous dinosaurs such as apathosaurs and stegosaurs (Fig.
153) developed a large stomach to digest the tough, fibrous fronds, requiring
an enormous body to carry it around.The dinosaurs grew to such giants prob-
ably for the same reasons that large ungulates such as the rhinoceros and ele-
phant are so big. Most large dinosaurs were herbivores that consumed huge
quantities of coarse cellulose that required much time to digest.The digestive
juices further broke down the rough material, and the long fermentation
process required a large storage capacity.
The large size of the herbivores spurred the evolution of giant carnivo-
rous dinosaurs to prey on them such as Tyrannosaurus rex,perhaps the fiercest
carnivore of them all. The giant dinosaurs were prevented from growing any
larger due to the force of gravity.When an animal doubles its size, the weight
on its bones is four times greater. The only exceptions were dinosaurs living
permanently in the sea.As with modern whales, some of which are even larger
than the biggest dinosaurs, the buoyancy of seawater kept the weight off their
bones. If the animal accidentally beached itself, as whales sometimes do, it suf-
focated because its bones were unable to support the weight of its body, crush-
ing its lungs.Figure 153Stegosaurs
were ornithischian
dinosaurs with strongly
developed dorsal plates
and spikes and known
from upper Jurassic rocks
of Colorado and
Wyoming.
Historical Geology