Dinosaurs evolved from a reptile called a thecodont, which seems to have been the ancestor of birds as well. Thecodonts were medium-size predators that lived in the Permian and Triassic periods on land that was thawing after a recent ice age, a time of cold temperatures and widespread glaciers, or layers of ice.
Dinosaurs diversified in many different directions. Camptosaurus was a 25-foot herbivore (plant-eater), but many other early dinosaurs were much smaller—even as small as a chicken. Some were carnivorous (meat-eating) predators.
A number of early dinosaurs began walking on two legs rather than four, thus becoming the earliest bipedal creatures.
Within 10 million years, dinosaurs of all varieties reigned over Earth.
The carnivorous Tyrannosaurus rex was huge, the largest meat-eating animal known. Tyrannosaurus bones were first discovered in 1902 in Montana.
Imagine Earth inhabited by nothing but centipedes and spiders!