Figure 25. This fossil was formed from a starfish that lived about 300 million years ago in the Pennsylvanian Period.
Fossils form only under certain conditions. Very few plants and animals are fossilized. Dead plants and animals must be buried quickly and not exposed to harsh weather and running water. Deltas, marshes, swamps, and shallow seas are ideal places for fossils to form. Because Kansas has been covered with marshes and seas several times in the past, it has a good supply of fossils.
The most common Kansas fossils are marine animals found in rocks formed in the Pennsylvanian, Permian, and Cretaceous periods. Few fossils of plants and animals that lived before the Mississippian Period have been found in Kansas. As the rocks changed over time, many of the fossils were destroyed. Also, many early animals had soft bodies. Most animal parts that are preserved are hard, such as bones, teeth, and shells.
Figure 26. A clam-shell fossil from the Pennsylvanian Period.
But the main reason for few early fossil finds in Kansas is that no rocks formed before the Mississippian Period are found at the surface in Kansas. Fossils of tiny single-celled plants and animals from earlier periods have been found in rocks brought up from hundreds, even thousands, of feet underground when oil and water wells were drilled. The chance of finding larger fossils by this method is not good because the rock brought up from underground during drilling is usually crushed into small pieces. Also, only a narrow cylinder of rock-about two to four inches in diameter-is ever removed from the hole. Much of what we know about life before the Mississippian Period comes from fossils found in areas outside of Kansas.
Figure 27. Brachiopods, small-shelled marine animals, were abundant in Kansas seas more than 250 million years ago.
Nonmarine plant and animal fossils also have been found in Kansas, but they are not as common because plants and animals often decay on dry land before they can be fossilized. Usually only fragments of bones or teeth are fossilized, and only a small number survive over millions of years.
Paleontologists compare new fossil finds to other fossils and to living animals. This helps them determine what animals looked like millions of years ago.