33.6:
The Fossil Record
The dinosaur Triceratops ate plants and stood approximately 8 feet tall. Scientists inferred its shape, size, and behavior from fossils, preserved remains or imprints, such as footprints, bones, and teeth.
Most organisms do not fossilize after death. Decomposers, such as bacteria and fungi, and scavengers, such as flies and beetles, typically destroy tissues before fossilization can occur. Soft tissues, such as muscles, feathers, and fur, often disappear, while hard tissues—including bones, teeth, and shells—more commonly remain.
Fossils, like a mammoth encased in ice or a spider preserved in amber, offer an extremely rare glimpse of intact, early life-forms.
Sedimentary rocks yield the most fossils, particularly as fragments rather than complete organisms.
Sedimentary fossils form when sediment, such as sand or mud, buries an organism or its tracks. Pressure and heat transform the layers into rock, creating a fossil. These sedimentary rock layers, or strata, accumulate fossils over time.
Stratigraphy—the study of strata deposited through time and space—allows scientists to determine the relative age of fossils.
For example, strata form horizontally, with newer layers forming above the older layers. Thus, scientists regard fossils resting in more recently deposited strata as younger than fossils that occur at greater depths.
Radiometric dating allows scientists to determine a fossil’s actual age by using the known half-life of radioactive isotopes.
For instance, all living organisms accumulate both carbon-12 and the radioactive isotope carbon-14. When an organism dies, the amount of carbon-12 remains constant, while the amount of carbon-14 declines. Scientists can measure a fossil’s carbon-14 to carbon-12 ratio to estimate its actual age.
The overall fossil collection, or fossil record, documents the history of life and provides evidence of evolution. Significant events such as the mass extinction of dinosaurs, the subsequent explosion in mammalian diversity, and even human evolution are reflected in the fossil record.
The fossil record is, however, incomplete and biased as short-lived, scarce, and soft-bodied species are poorly represented. Fossils represent only a small fraction of all species that have ever inhabited Earth.
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.
Suggested Reading
Jablonski, David, and Neil H. Shubin. 2015. "The Future of the Fossil Record: Paleontology in the 21st Century." Proceedings of the National Academy of Sciences 112 (16): 4852–58. [Source].
Knoll, Andrew H., and Martin A. Nowak. 2017. "The Timetable of Evolution." Science Advances 3 (5): e1603076. [Source].