33.6 : The Fossil Record

The fossil record documents only a small fraction of all organisms that have ever inhabited Earth. Fossilization is a rare process, and most organisms never become fossils. Moreover, the fossil record only exhibits fossils that have been discovered. Nevertheless, sedimentary rock fossils of long-lived, abundant, hard-bodied organisms dominate the fossil record. These fossils offer valuable information, such as an organism's physical form, behavior, and age. Studying the fossil record helps scientists to place fossils into geological (e.g., Paleozoic era; 250-570 million years ago) and evolutionary (e.g., first tetrapod organism) contexts.

Whale evolution, for example, is one of the most well-studied examples of evolutionary change in the fossil record. Modern whales descended from a terrestrial, tetrapod ancestor that transitioned from land, back to water. Ancestral whales' forelimbs later evolved into flippers to aid swimming, while their hindlimbs disappeared. The fossil record reveals whales' terrestrial (e.g., Indohyus), semi-aquatic (e.g., Ambulocetus), and aquatic (e.g., Dorudon) ancestors throughout the early Cenozoic era—nearly 50 million years ago. Both modern and extinct organisms can inform scientists' understanding of life on Earth.

In addition to showing evolutionary changes in organisms themselves, the fossil record captures changes in biodiversity as well. Fossils throughout the Paleozoic era record the gradual emergence of animals (e.g., marine arthropods like trilobites), plants (e.g., Gilboa trees), and fungi (e.g., Prototaxites). Fossil evidence also reflects mass extinctions of species over evolutionary time. Scientists recognize five major extinction events in which over 75% of early species vanished. For instance, a mass extinction event in the late Paleozoic era wiped out the aforenoted organisms.

Fossils enable scientists to reconstruct accounts of life on Earth. For example, extinction events tend to result in radiation of diverse species with a common ancestor. After the late Paleozoic era's mass extinction event, fossil evidence supports that the age of dinosaurs began and persisted for nearly 180 million years (i.e., Mesozoic era; 65-250 million years ago). Another mass extinction event occurred in the late Mesozoic era, at which point the age of mammals began and continues to the present day (i.e., Cenozoic era; 65 million years ago-present). Thus, the fossil record supports the origins of species and serves as an essential tool for understanding evolution.