The most abundant organisms in terms of number of species are the arthropods, including insects, spiders, scorpions, and crustaceans. Most of the fossils shown here are trilobites, which were abundant throughout the Paleozoic but then became extinct. Echinoderms include starfish, sand dollars and sea urchins. Crinoids are something like starfish on a stalk. They were abundant in the Paleozoic and still live in some deep ocean areas.
Chordates are organisms with a stiffening rod along one side.
They include mammals, birds, reptiles, amphibians, several classes of fish, and several classes of simpler organisms, including things one would scarcely guess at first sight were chordates. A few chordate fossils. If you find a piece of rock with a white lump in it, you can be absolutely certain it is not a bone Real fossil bones are dark brown because of mineralization. Dinosaur tracks near Denver, Colorado. Probably the most dramatic intermediate fossil form.