living today.They were very common and ranged over wide spaces like mod-
ern-day sheep.The smallest known dinosaur footprints are only about the size
of a penny. The smaller dinosaurs had hollow bones similar to those of birds.
Some had long, slender hind legs;long,delicate forelimbs;and a long neck.If
not for a lengthy tail, their skeletons would have closely resembled those of
modern ostriches.
Many early small dinosaurs reared up on their hind legs and were among
the first animals to establish a successful permanent two-legged stance. They
are descendants of the diapsid reptiles, which ran on two legs, the fastest way
to travel. The diapsids were one of the most primitive reptile lineages. They
gave rise to dinosaurs, birds, and most living reptiles, including crocodiles,
lizards, and snakes. Bipedalism increased speed and agility. It also freed the
forelimbs for foraging and other tasks. The back legs and hips supported the
entire weight of the animal, while a large tail counterbalanced the upper por-
tions of the body. The dinosaur walked much like birds. Therefore, dinosaurs
are classified either as ornithischians with a birdlike pelvis or as saurischians
with a lizardlike pelvis.
The ornithischians probably arose from the same group of thecodont
reptiles that gave rise to crocodiles and birds. Indeed, birds are the only livingFigure 135The small
plant eater camptosaur
was an ancestor of many
later dinosaurs.
(Photo courtesy National
Park Service)
Historical Geology