Iguanodon

Stegosaurus was the largest of the stegosaur group. Its fossils were found mainly in present-day Colorado, Utah, and Wyoming.

Like most of its group, Stegosaurus lived toward the end of the Jurassic period, about 150 mya.

Stegosaurus was about 25-30 feet long from nose to tail and probably weighed más than 2 tons.

Its most striking feature was the row of large, roughly triangular bony plates along its back.

The name Stegosaurus means "roof reptile". This is because it was first thought that its bony plates lay flat on its back, overlapping like the tiles on a roof.

It is now thought that the back...

A small, wormlike creature called Pikaia is thought to be the ancestor of all backboned animals.

Its fossil remains were found in the 530- million-year-old mudstone deposits of the Burgess Shale in Canada.

Pikaia was the first known chordate. All backboned animales belong to this group as well as marine animales called tunicates and acraniates.

Pikaia was 2 inches long, with a notochord running around its body-- a kind of primitive spine that gave its body flexibility.

The notochord also allowed the animal's simple muscles to work against it, and the animal's body organs to hang from it.

Pikaia is...