Stage (faunal stage)
A faunal stage refers to a distinct layer of rock that contains fossilized remains of animals, which scientists use to identify specific time periods in Earth's geological history. These layers are formed over millions of years and typically contain similar types of fauna, allowing researchers to establish correlations between the layers and the time periods they represent. The study of faunal stages is integral to stratigraphy, the science of rock layers, and helps scientists date these layers through relative dating methods, relying on the presence of specific animal remains rather than precise numerical ages.
Throughout history, scientists have recognized nearly one hundred faunal stages, with each stage often characterized by one or two predominant index fossils that are unique to that layer. This framework helps paleontologists and other scientists deduce the environmental conditions of the time and the evolution of animal life. As scientific methods have advanced, especially in the twentieth century, the analysis of faunal layers has become more sophisticated, incorporating techniques like radioactive dating and seismography. While earlier classifications were heavily based on the fauna found within the layers, contemporary practices increasingly consider geological characteristics in defining these stages. Overall, understanding faunal stages enriches our knowledge of prehistoric life and the Earth's geological timeline.
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Stage (faunal stage)
Faunal stage refers to a layer of rock which includes fossilized animal remains scientists use to identify a time period in the distant past. Layers of rock formed during the same period of time include similar kinds of animal life. These periods encompass long stretches of time measured in millions of years. Scientists use these layers to set boundaries and give names to different time periods.
Background
Fauna is a term used to refer to all the animal life in a particular area or region during a set period of time. Fauna varies around the world; for example, polar bears are native to the Arctic, while elephants are part of the fauna of Africa. Fauna also changes over time, such as when animals go extinct.
As animals die, their remains can create a fossil record in the soil or sediment. The organic bones, teeth, and other parts of the animal are replaced by minerals and turn into a form of rock. Sometimes, this happens to animals individually or in small groups. Other times, it happens to larger groups during a catastrophic event, such as a flood or natural disaster. These fossilized remains help scientists identify what life existed, where it lived, and when it lived, especially in the times before humans began recording observations.
Overview
The fossilized remains of animal life build up in layers of rock over time. Paleontologists and other scientists who study the progression of animal life over time refer to these layers as faunal stages. Faunal stages can help uncover a great deal of information about an area in the prehistoric era.
Faunal stages are one of the smallest units used in stratigraphy, or the study of layers of rock and other materials. Scientists have identified almost one hundred different faunal stages throughout Earth’s history. The principles of stratigraphy are used by a number of scientific disciplines, such as geologists, archaeologists, paleobiologists, and paleontologists. The main principle in stratigraphy is superposition, or the assumption that these layers are deposited in sequence over time. This means that unless something such as an earthquake or human excavation disturbs the layers, the material in the layers at the bottom is older than what is in the layers above it.
Dating the rocks themselves is difficult. Scientists can use techniques such as the level of magnetism present in the rocks to get an idea, but it is much easier to use organic material produced by plants and animals—or flora and fauna—to date the layers. The principles of stratigraphy indicate that layers that contain the same types of faunal life were created at about the same time. This allows scientists to use relative dating, or the practice of dating a layer by what it contains. Scientists can then set date ranges and boundaries for time periods in the distant past without knowing the exact dates involved. Relative dating allows scientists to reasonably assume that all animal life within a period existed at the same time.
It was during the nineteenth and early part of the twentieth century that scientists began using faunal layers to establish date ranges for prehistoric history. At first, they studied the faunal layers in more limited geographic areas. For example, scientists used the layers to identify the ancient animals that lived in various regions of North America. Faunal stages are regional because animals are generally dependent on the environmental conditions in particular regions to live. This not only helps to define the layer, it helps give scientists a clue as to the environmental conditions at the time.
Later in the twentieth century, scientific techniques and equipment advanced to the point that scientists were able to analyze faunal layers with greater accuracy and at deeper levels. This included radioactive dating, which analyzes and compares the impurities in materials to help determine their ages, and seismography, which uses waves of energy sent through the Earth to help create an image of what is below the surface. This allowed scientists to define layers in broader areas and by other characteristics, so they are not always referred to as faunal layers in contemporary times.
Faunal layers allow scientists to create a chronology of layers based on the animal remains they contain. For example, the layers that contain the remains of most dinosaurs are below the layers that contain woolly mammoths and sabre tooth tigers, so under the principle of superposition, the dinosaur layers are older.
The stratigraphic definitions for the various layers are referred to as chronostratigraphic units. Chronostratigraphy refers to the process of drawing correlations between the layers of rock and time. The chronostratigraphic units include eonathem, erathem, system, series, subseries, stage, and substage. Scientists from several different disciplines use these units to help categorize different periods of time in Earth’s geological history. The International Commission on Stratigraphy (ICS) has designated specific and standardized boundaries for these units known as Global Boundary Stratotype Section and Points (GSSPs).
Faunal stages are subdivisions of these units and part of the overall process by which paleontologists determine the boundaries for them. Establishing the parameters for a faunal stage usually involves one or more index fossils. Faunal stages generally have fossils from one or two animals that are very common in that layer but found in few if any other layers. When these fossils are uncovered, they help scientists know that the areas where they are found are part of the same time period, even if the rocks or other material found in them are different.
Stages based on fauna are important for helping scientists learn details about the layers that make up Earth’s geological history. In the past, the time periods defining Earth’s history were given names based on the fauna they contained. However, scientific advances in more recent years have helped to more firmly establish the boundaries of the various layers based on the rocks and mineral materials they contain. As a result, the labels used for the boundaries more often reflect names based on geological determinations than on fauna.
Bibliography
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