Thursday, January 31, 2008

Dinosaur Classification

There were thousands of different types of dinosaurs. Scientists have divided these types into a number of groups, based on the ways the different species were related to one another.
Dinosaurs are divided into two major groups. This distinction is based on the shape of the hipbones. Saurischians have hipbones similar to modern reptiles, such as crocodiles and crawling lizards. Ornithischians, however, have hipbones more like birds.

Confusingly, bird-hipped dinosaurs are only distantly related to birds. The ancestors of modern birds are actually the saurischians. Ornithopods were all plant-eaters. The basic bird-hipped dinosaur dinosaur was a nimble herbivore which walked on two legs. Some, however, walked on four legs and developed heavy armor, like the stegosaurus and ankylosaurus.

The saurischian dinosaurs are further divided into two groups: the theropods and the sauropods. Theropods were bipedal meat-eaters dinosaurs with powerful jaws and grasping hands with sharp claws. Everybody's favorite, Tyrannosaurus Rex, is the most famous theropod. The sauropods, however, were plant-eaters. They had long necks and heavy, lumbering bodies.
While ornithischians and saurischians are distinguished by the shape of their hips, theropods and sauropods are classified by the shape of the animal's foot. Theropods had feet like modern mammals, and sauropods had feet more like a lizard's.

All the meat-eating dinosaurs belonged to the theropod group. They had compact bodies and walked on two legs. The head was held out in front, and was balanced by a heavy tail. This is the ideal shape for a hunter: having the teeth and jaws up front means they are the first parts to make contact with its prey.
The legs are muscular and fast for short distances. We know that at least some of the theropods were warm-blooded.

As a group of dinosaurs, the sauropods include the largest land animals ever to walk the earth. These were the big plant-eaters of dinosaur times, with heavy bodies, long necks, and whip-like tails.
The sauropod's heart had to be especially strong, to be able to pump blood up through the long neck. The teeth were purely for plant-eating: they were small and serrated, like a vegetable shredder. Some of the sauropods weighed more than an entire herd of elephants.

For my previous posts on dinosaurs, click here.

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