HOW ARE STRATA ASSOCIATED WITH FOSSILS? - SCIENCE - 2025

The association of strata and fossils arises because the rocks or sediments that make up the soil are stacked in layers that contain fossils.

The oldest fossils are found in the deepest layers and the youngest or most recent ones, near the surface. It is as if the rock layers represent a vertical timeline.

In the background there are no fossils of modern animals or plants, but they are all already extinct. In contrast, closer to the surface there are fish, amphibians and reptiles, then mammals and birds, and finally modern mammals including humans.

Three concepts related to the study of strata and fossils

1- Fossils are the remains or traces of prehistoric organisms. They are more common in sedimentary rocks and in some pyroclastic materials such as ash.

They are extremely useful in determining the relative age of the strata. They also provide information on organic evolution.

2- Most of the fossils are the remains of extinct organisms, or species of which individuals are no longer alive.

3- The kinds of fossils found in rocks of different stages differ because life on earth has changed over time.

Law of horizontality

Science says that dust, mud, sand, and other sediments settle in horizontal layers. As they are stacked vertically they harden and form rocks.

If we start to examine the oldest rock layers we get to a level where there are no human fossils.

If we continue, at a certain level there are no fossils of flowering plants or birds, no mammals or vertebrates, no land plants, no oysters and no animals.

These concepts are summarized in the general principle called the law of fossil succession.

Law of Fossil Succession

William Smith (1769-1839), an English engineer, discovered the principle of fossil superposition. This principle states that the kinds of animals and plants that are found as fossils change over time.

When we find the same kinds of fossils in rocks from different places, we know that they are the same age.

The changes in life forms are recorded in the fossils of the rocks. Natural disasters or catastrophes destroy life periodically. Plant and animal species disappear and other forms of life are born.

In this way, fossils help geologists calculate the age of rocks. The groups of fossils follow each other in a regular and determined order.

Today the marine fauna and flora is very different from the terrestrial, and they also differ from one place to another. Likewise, fossilized animals and plants in different environments are different.

Just as animals help identify the environment, rocks help collect information about the environment in which that animal or fossil lived. Fossils in rocks follow the principle of superposition, they are consistent in different places.

References

1. John Watson. (1997). Fossil Succession. 10/08/2017, from USGS Website: pubs.usgs.gov
2. Prof. Sean Tvelia. (2017). Rocks, Fossils and Time. 10/08/2017, from Suffolk County Community College Website: thisoldearth.net
3. Editor. (2014). Rock Layers: Timeline of Life on Earth. 10/08/2017, from Prehistoric Planet Website: prehistoricplanet.com
4. Editor. (2017). Dating Fossils in the Rocks. 10/08/2017, by Nat Geo Website: nationalgeographic.org
5. Berthault, G. 2000. Experiments in Stratification. Acts & Facts. 29 (10).