8.4: Fossils and Age Dating

Dating Rocks Using Fossils

Although the recognition of fossils goes back hundreds of years, the systematic cataloguing and assignment of relative ages to different organisms from the distant past—paleontology—only dates back to the earliest part of the 19th century. The oldest undisputed fossils are from rocks dated around 3.5 Ga, and although fossils this old are tiny, typically poorly preserved and are not useful for dating rocks, they can still provide important information about conditions at the time. The oldest well-understood fossils are from rocks dating back to around 600 Ma; the sedimentary record from that time forward is rich in fossil remains that provide a detailed record of the history and evolution of life on Earth. However, as anyone who has gone hunting for fossils knows, that does not mean that all sedimentary rocks have visible fossils, or that they are easy to find. Fossils alone cannot provide us with numerical ages of rocks, but over the past century geologists have acquired enough isotopic dates (explored elsewhere in this chapter) from rocks associated with fossil-bearing rocks (such as igneous dikes cutting through sedimentary layers, or volcanic layers between sedimentary layers) to be able to put specific time limits on most fossils.

If we understand the sequence of evolution on Earth, we can apply knowledge to determining the relative ages of rocks. This is William Smith’s principle of faunal succession, although of course it doesn’t just apply to “fauna” (animals); it also applies to fossils of plants and other organisms.

Geologic Range

If we can identify a fossil to the species level, or at least to the genus level, and we know the time period when the organism lived, we can assign a range of time to the rock. That range might be several million years because some organisms survived for a very long time. If the rock we are studying has several types of fossils in it, and we can assign time ranges to several of them, we might be able to narrow the time range for the age of the rock considerably.

Here’s an example. Some organisms survived for a very long time, and are not particularly useful for dating rocks. Sharks, for example, have been around for over 400 million years. This is not a useful range: if you find a shark fossil in a sedimentary layer, it only tells you, “This layer is 400 million years old, or younger.” However, the great white shark has a range of 16 million years, so far, which is a much shorter amount of time. Organisms that lived for relatively short time periods are particularly useful for dating rocks, especially if they were distributed over a wide geographic area, and so can be used to compare rocks from different regions. These are known as index fossils. There is no specific limit on how short the time span has to be to qualify as an index fossil. Some lived for millions of years, and others for much less than a million years.

Figure \(\PageIndex{1}\): Geologic Range Chart. The application of bracketing to constrain the age of a rock based on several fossils. In this diagram, the colored bars represent the time range during which each of the four species (A, B, C, D) existed on Earth. Although each species lived for several million years, we can narrow down the likely age of the rock to just 700,000 years during which all four species coexisted. (CC BY 4.0; Steven Earle. From: https://opentextbc.ca/physicalgeolog...using-fossils/)

A geoscientist can also date rocks using the geologic ranges of an fossil assemblage from a single sedimentary layer. A fossil assemblage (or faunal assemblage) is a group of organisms that all found fossilized together in that layer. This application involves the bracketing of overlapping ranges to constrain the age of a rock based on several fossils. Consider Figure \(\PageIndex{1}\)–four species of fossils (A, B, C, D) were collected from one stratum. The geologic ranges of each species are plotted side by side in a geologic range chart. Although each species lived for several million years, we can narrow down the likely age of the rock to just 700,000 years during which all four species coexisted. Comparison of these geologic ranges shows only a short time interval (from 7-7.7 Ma) when all four species overlap. Therefore, the age of the sedimentary layer from which these fossils were collected is constrained to the interval when all four species coexist.

Biozones

Some well-studied groups of organisms qualify as biozone fossils (a stratigraphic interval that can be defined on the basis of a specific fossil) because, although the genera and families lived over a long time, each species lived for a relatively short time and can be easily distinguished from others on the basis of specific features. For example, ammonoids have a distinctive feature known as the suture line—where the internal shell layers (septae) that separate the individual chambers meet the outer shell wall. These suture lines are sufficiently variable to identify species. These can be used to estimate the age of the rocks in which they are found. Generally, the more complex the suture pattern, the younger the fossil. Compare the sutures in the following fossil specimens. The fossil ammonoid in Figure \(\PageIndex{2}\) is Triassic in age, specifically living in the range between 251-237 Ma. The fossil ammonoid in Figure \(\PageIndex{3}\) is Cretaceous in age, specifically living in the range between 113-100 Ma. Select the play button to explore these fossils in 3D.

Figure \(\PageIndex{2}\): Fossil specimen of the ceratite ammonoid Gymnotoceras beachi from the Triassic Toad Formation of British Columbia (PRI 43629). Specimen is from the collections of the Paleontological Research Institution. Maximum diameter is approximately 4.5 cm. (CC0 Public Domain; Model by Emily Hauf.)

Figure \(\PageIndex{3}\): Fossil specimen of the ammonoid cephalopod Cleoniceras besairiei from the Cretaceous period. Specimen is from the collections of the Paleontological Research Institution, Ithaca, New York. Maximum diameter of specimen is approximately 12 cm. (CC0 Public Domain; Model by Emily Hauf.)

Foraminifera are small, carbonate-shelled marine organisms that originated during the Triassic and are still around today. They are extremely useful biozone fossils. As shown in the figure below, numerous different foraminifera lived during the Cretaceous. Some lasted for over 10 million years, but others for less than 1 million years. If the foraminifera in a rock can be identified to the species level, we can get a good idea of its age. The following is an abbreviated geologic time scale displaying select index foraminifera and age range from the Cretaceous and Paleogene time periods. The gigapixel photo that follows is of Cribratina texana, collected at “Fossil Hill,” near the Chihuahua (Mexico) / Texas / New Mexico triple point border. Being that this is an index fossil, we can accurately date the age of the rock that hosts the fossil as being between 112-93.5 Ma.

Figure \(\PageIndex{4}\): Select index foraminifera from the Cretaceous and Paleogene. (For educational purposes from the Geologic Timescale Foundation, https://timescalefoundation.org/. Modified by Shelley Jaye.)

Figure \(\PageIndex{5}\): Fossils of the benthic foraminiferid Cribratina, an index fossil for the Albian-Cenomanian age, collected at “Fossil Hill,” north of the Cristo Rey laccolith at the Chihuahua (Mexico) / Texas / New Mexico triple border. (Robin Rohrback, Mid-Atlantic Geo-Image Collection (M.A.G.I.C.).)

Watch this video to learn about the discovery of conodonts and their use as index fossils.

Key Terms

• index fossil - organisms that lived for relatively short time periods and distributed over a wide geographic area
• paleontology - the study of the history of life on Earth as based on fossils