evolution, the animals had to compete with huge crocodiles, flying reptiles
called pterodactyls, and other fierce reptiles. In the middle Triassic about 230
million years ago, when mammal-like reptiles dominated the land, dinosaurs
represented only a minor percentage of all animals. Several reptile species liv-
ing at the time of the early dinosaurs still far outweighed them.
In just 10 million years, however, dinosaurs rapidly became the domi-
nant species.They evolved from moderate-sized animals less than 20 feet long
to the giants for which they are famous. Dinosaurs arose to become the dom-
inant terrestrial species for the next 150 million years. By the end of the Juras-
sic, dinosaurs were the largest predators ever to roam Earth. Their fossils are
found in relative abundance in Triassic to Cretaceous formations (Fig. 134).
The dinosaurs ventured to all major continents. Their distribution
throughout the world is strong evidence for continental drift (Table 8). At the
time the dinosaurs came into existence, all continents were assembled into the
supercontinent Pangaea. Early in the Jurassic, it began to rift apart, and the con-
tinents drifted toward their present locations. Except for a few temporary land
bridges, the oceans that filled the rifts between the newly formed continents
provided a barrier to further dinosaur migration. At this time, almost identical
species lived in North America, Europe, and Africa.Figure 134The Carmel
Formation, Uinta Range,
Utah,where dinosaur
fossils are found.
(Photo courtesy of the
author)
Historical Geology