Hi Kids! Carl Coati here once again!

If you know how, rocks can be like history books. You just need to know how to read them. There are rocks in Arizona that are 1400 to 1800 million years old!

Microscopic creatures can be found in these. In rocks which are 1200 million years old, researchers can find colonies ofalgae which is like moss that lives in the waters. In rocks 570 to 240 million years old, sharks teeth, trilobites, corals, clams and oyster remains can be found.

Scientists know from the rock record that by 150 to 90 million years ago (mya), the waters receded (moved back away from) the Arizona area. Tucson Mountain rocks of this age show lizard tracks and horsetail (a tall reed) remains as well as pieces of petrified wood. Fossils of dinosaurs and the oldest known mammals of Arizona have been found from about these times in the Santa Rita Mountains.

As time went by, the reading of the rocks show the emergence (coming out of) ancient mammals like the ancestors of the horses, the camels, the pigs and early giant rhinoceroses. By 10 to 2 mya there is evidence in the rocks of lots of grasses and the animals that fed on the grasses and the animals that fed on the animals who ate the grasses -- camels, bison, near-modern horses, mastodons, mammoths, wolves, lions, giant beavers and short-faced bears. Eventually the record shows dart points imbedded in fossil animal remains left by the early North American people.

There are two kinds of fossils, the actual remains of plants and animals and things like tracks or imprints or scat (ca-ca) left over by those that lived way back then. It is exciting for people today to discover these ancient remains because that is the way we can know what lived before our times.

It is not easy to find fossils. Most animals plants that lived millions of years ago have disintegrated (broken apart and disappeared). Only a very few remain to show us what the world was like back then. Many are very fragile (easily broken) and all are precious and expensive. We would not want to have these fossils broken or lost. So today people make casts or molds of the original fossils so that people can study and hold them while not ruining the originals which can be put away for safe keeping.

Go Behind-the-Scenes to see how the Desert Museum makes fossil casts