I.

Introduction

Fossil, remains or traces of prehistoric plants and animals, buried and preserved in sedimentary rock, or trapped in organic matter. Fossils representing most living groups have been discovered, as well as many fossils representing groups that are now extinct. Fossils range in age from 3.5-billion-year-old traces of microscopic cyanobacteria (blue-green algae) to 10,000-year-old remains of animals preserved during the last ice age.

Fossils are most commonly found in limestone, sandstone, and shale (sedimentary rock). Remains of organisms can also be found trapped in natural asphalt, amber, and ice. The hard, indigestible skeletons and shells of animals and the woody material of plants are usually preserved best. Fossils of organisms made of soft tissue that decays readily are more rare. Paleontologists (scientists who study prehistoric life) use fossils to learn how life has changed and evolved throughout earth’s history (see Paleontology).