Fossils can give an idea of when the surrounding rock formed.
The process of dating rocks and rock layers has been an evolving science for hundreds of years. From the simple science of counting rock layers by height to the mechanics of radiometric rock dating, myriad techniques for dating our planet's features exist. One of the most common ways to date rocks and rock layers is through the use of index fossils: specific types of fossils that existed during short, specific prehistoric eras.
Ammonites
Ammonites can range from 6 inches to 6 feet wide.
Ammonites are, perhaps, the most common index fossils used to date periods of rock formation. These ancient relatives of the squid lived between the Devonian and Upper Cretaceous periods and maintained differing forms specific to these periods. For instance, if an archaeologist were to discover a Clydonitaceae ammonite fossil, he would know that it was from the Ceratitida family of ammonites that existed specifically during the Permian and Triassic periods. This would put the layer of rocks he was digging in between 300 and 250 million years old.
Trilobites
Trilobites are another popular index fossil used to date ancient sea beds and rock layers. Trilobites extend further into prehistory than typical ammonites, running from Precambrian (between the formation of the earth and 550 million years old), to the Permian era (around 250 million years ago). Though dating trilobite layers of rock are not as exact as ammonite-dating, trilobite fossil finds indicate rock layers that are extremely old and formed as part of ancient seafloor sedimentation. Trilobites are popular fossil finds because of their unique shapes and identifiable anatomy like legs and antennae.
Graptolites
Graptolites are small ancient animals that have been confused with plants, hieroglyphs and even natural rock formations. Over the years, these odd fossils have been whittled down to encompass under 20 species of plankton-like animals that existed between 550 and 300 million years ago. Though not as hard-shelled as trilobites and ammonites, graptolites are excellent index fossils that indicate very deep parts of ancient sea beds. The shale rock that usually houses graptolite fossils help archaeologists and geologists determine the movements of ancient plate tectonics and landmass shapes.
Brachiopods
Brachiopods are a clam-like ancient species that are not completely extinct. Though a non-relative to clams and other mollusks, brachiopods are shelled creatures that lived on sea beds and filtered food. Running between the Cambrian and Triassic periods, these ancient shelled sea creatures were fossilized in large groups that can effectively date rocks based on the species found in those groups. Brachiopods are also an excellent indicator of both climate changes and mass extinctions during prehistoric eras because of their temperature preferences: when the temperatures of the world climate changed, brachiopods moved to more suitable water latitudes and were fossilized there.
Tags: rock layers, million years, index fossils, that existed, ancient beds
