Dinosaurs survived for an astounding 160 million years (humans, in contrast, have existed for less than one million years). Ranging in size from animals no bigger than pigeons to lumbering giants the size of a truck, they were reptiles, but very different from modern-day reptiles.
■ Lived on land
■ Built nests and laid eggs
■ Most had scaly skin (some had feathers)
■ Long tails, held off the ground
■ Walked on upright, pillarlike legs.
■ Holes (“windows”) in the skulls of larger dinosaurs, making them lighter. (Only the armored dinosaurs had solid skulls.)
■ Walked on their toes
■ Claws on fingers and toes
One way in which dinosaurs differed from today’s reptiles and lizards is that they could stand with their legs straight, in the same way as mammals. Some dinosaurs walked on two legs, some on four. Some may have done both.
DINOSAURS walked on upright, pillarlike legs.
CROCODILES walked with knees and elbows bent.
LIZARDS hold their legs at rightangles to the body.
Also called armored dinosaurs, members of this group of plant-eaters were large, walked on four feet, and had armor plates and spikes that protected them from attack. Some of these dinosaurs even had armored eyelids!
All meat-eating dinosaurs were saurischians, and they formed a group called the theropods. This is the group from which birds are descended. They ranged in size from the chicken-sized Compsognathus to monsters such as the mighty Spinosaurus.
This group contained the heaviest and longest animals ever to walk on Earth. They were herbivores (plant-eaters) and would have had to graze constantly to obtain the energy they needed.
These plant-eaters all roamed on two legs, so were able to use their forelimbs to grasp their food. They were immensely successful and very common. Fossils of these types of dinosaur have been found all over the world.
This group of plant-eaters had heads that sported bony frills. (Marginocephalian means “fringed head.”) Some walked on two legs, some on four. They were common dinosaurs in the Cretaceous and included the well-known Triceratops.