5.3: Dating Rocks Using Fossils

Geologic Range and Faunal Succession

The oldest undisputed fossils on Earth are from rocks dated around 3.5 Ga. Although fossils this old are tiny, typically poorly preserved, and often not useful for dating rocks, they can still provide important information about conditions at the time they were buried. The oldest well-understood fossils are from rocks dating back to around 600 Ma, and the sedimentary record from that time forward is rich with fossil remains that provide a detailed record of the history and evolution of life on Earth. 

Fossils alone cannot provide us with numerical ages of rocks, but over the past century geologists have acquired enough isotopic dates 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 age limits on most fossils.

If we understand the sequence of evolution on Earth, we can apply that knowledge to determining the relative ages of rocks. This is what the Principle of Faunal (Fossil) Succession is built upon.

If we can identify a fossil to the species level, or at least to the genus level, and we know the time period in which 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. 

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.

A geoscientist can also date rocks using the geologic ranges of a fossil assemblage from a single sedimentary layer. A fossil assemblage (or faunal assemblage) is a group of organisms that are 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. Imagine that four species of fossils (A, B, C, D) were collected from one stratum. Imagine that fossil species A lived between 14-7 Ma, fossil species B lived between 12-4.5 Ma, fossil species C lived between 9.5-1 Ma, and fossil species D lived between 8.3-3 Ma.  If these geologic ranges for each species are plotted side by side in a geologic range chart where each range is depicted as a bar, the overlap of the species can be determined (Figure \(\PageIndex{1}\)). Comparison of these geologic ranges shows only a short time interval (from 8.3-7 Ma) when all four species overlap. Therefore, we can narrow down the likely age of the rock to just 700,000 years during which all four species coexisted. 

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. "Geologic Range Chart" by Steven Earle vs licensed under CC BY. 

Smilodon californicus: California's State Fossil

The "saber-toothed tiger," Smilodon, is the California State Fossil and the second most common fossil mammal found in the La Brea tar pits (see the Transverse Ranges Province for more information on the La Brea tar pits). Hundreds of thousands of Smilodon bones have been found at La Brea since they were first identified in 1932. These finds have permitted remarkably detailed reconstructions of how Smilodon lived. We now know Smilodon was about a foot shorter than living lions but was nearly twice as heavy. Also, unlike cheetahs and lions (which have long tails that help provide balance when the animals run) Smilodon had a bobtail. These suggest that Smilodon did not chase down prey animals over long distances as lions, leopards, and cheetahs do. Instead, it probably charged from ambush, waiting for its prey to come close before attacking.

Smilodon is a relatively recent sabertooth, from the Late Pleistocene. It went extinct about 10,000 years ago. Fossils have been found all over North America and Europe. Smilodon fossils from the La Brea tar pits include bones that show evidence of serious crushing or fracture injuries, or crippling arthritis and other degenerative diseases. Such problems would have been debilitating for the wounded animals. Yet many of these bones show extensive healing and regrowth indicating that even crippled animals survived for some time after their injuries. How did they survive? It seems most likely that they were cared for, or at least allowed to feed, by other saber-toothed cats. Solitary hunters with crippling injuries would not be expected to live long enough for the bones to heal. Smilodon appears to have lived in packs and had a social structure like modern lions. They were unlike tigers and all other living cats, which are solitary hunters. Occasional finds of sabertooth-sized holes in Smilodon bones suggest the social life of Smilodon was not always peaceful. The cats may have fought over food or mates as lions do today. Such fights were probably accompanied by loud roaring. From the structure of the hyoid bones in the throat of Smilodon, we know it could roar.

Figure \(\PageIndex{1}\): Paste Caption Here This work by lora_313 via Flickr is licensed under CC BY 2.0. 

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 a 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 and can be used to estimate the age of ammonite fossils, and thus the rocks in which they are found. Generally, the more complex the suture pattern, the younger the fossil. Figure \(\PageIndex{2}\) and the following table demonstrate how these features appear in different specimens and how they are correlated to periods of geologic time. This figure compares three different ammonites: Goniatites, Ceretites and the youngest one, Ammonites. These ammonites have distinctive septae patterns that can be used to identify them and they are known to have lived at distinctive time periods such that they can be used as indicators of time.  

Summary of Ammonite ages and suture patterns used to bracket ages as shown in Figure \(\PageIndex{2}\)

Fossil Name	Fossil patterns	Fossil age Range
Ammonites	Alternating saddles and lobes that are complex with sub-saddles and sub-lobes superimposed upon them.	Middle Triassic through Upper Cretaceous
Ceretites	Alternative smooth and rounded saddles and lobes that are more regular in shape than those of Goniatites.	Early Permian through Latest Triassic
Goniatites	Simple but irregularly shaped saddles separated by lower lobes.	Early Devonian through latest Permian

 

Figure \(\PageIndex{2}\): The main sutural patterns of Ammonoidea (ammonites), and relative distribution in time and within the principal taxonomic groups. Line drawings next to each image depict the suture patterns of that organism. This work by Martha House is licensed under public domain / a derivative from the original work.

Query \(\PageIndex{1}\)

References

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