on dry land. They successfully inhabited desert areas in the interiors of conti-
nents and other desolate places where they could flourish on small amounts of
food. Many reptiles walked four footed but reared up on their hind legs when
running down prey, thus freeing the forelimbs for attacking other animals.
The earliest mammals saw their beginning in the Triassic. At the start of
the dinosaur era, mammals lived alongside the great beasts. At this time, mam-
mals were small, ratlike creatures of little consequence. However, in the Meso-
zoic mammals underwent some remarkable adaptations that in many ways
dwarfed those of their larger reptilian neighbors. Mammals developed special-
ized teeth, highly sensitive hearing and sight, and enlarged brains that would
later help them outlive the dinosaurs.
Teeth are often the only remains of tiny, extinct mammals. However,
they r eveal much about the animals such as their diet over the past 220 mil-
lion years. Chief among these early mammals were the multituberculates,
which first appeared in the upper Triassic 210 million years ago. Unfortu-
nately, after having survived the dinosaur extinction at the end of the Creta-
ceous, they all died out in the Eocene more than 30 million years ago.
Insects can easily claim the title of the world’s most prosperous creatures.
Ever since animals left the oceans and took up residence on dry land, insects
and their arthropod relatives have ruled the planet. Paleontologists discoveredFigure 132Petrified
trees at the Petrified
Forest National
Monument, Apache
County, Arizona.
(Photo by N. H. Darton,
courtesy USGS)
Historical Geology