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Scientists learn about extinctions by studying the fossil record and the chemical composition of rocks and chemicals around excavation sites. Looking at the relative abundance of fossil families in a rock layer indicates whether or not plant and animal families in the world at that time were thriving and diversifying or in decline.
A mass extinction will appear in the rock layers as a dead zone (containing few fossil remains) between layers with evidence of extensive life above and below it. The dead zone represents the time of a mass extinction and its aftermath.
Examining the chemical content of the rock strata gives clues about the causes of the extinction, including information about the climate, volcanic activity, extensive fires (soot would appear in the layers), flooding , cosmic collisions (meteorite impacts add rare elements and alter existing rocks), etc.
The study of extinctions is based on the fossil record, which is incomplete and skewed (for example, some organisms fossilize less readily than others and are therefore underrepresented in the fossil record). Dating the extinction (and also the first appearance) of a species is difficult, if not impossible. Dating known fossils can only give a range for the lives of that particular group organisms, not the entire species; the known fossils of an organism are only a minuscule subset of the original species.
Rarer fossils will yield even less accurate estimates than the more common fossils. Rare fossils may be rare because there were a small number of organisms to begin with, or because that organism is underrepresented in the fossil record. Since these fossils are less abundant, the chances of a fossil being found is low, causing the date estimates to be based on incomplete data.
There are other difficulties inherent in interpreting the fossil record. The
Signor-Lipps Effect explains how a fossil record that appears to be a gradual extinction could actually represent a sudden extinction. If many organisms go extinct at the same time, the fossil record wouldn't necessarily represent the rarer species and the more common equally. The rarer species might disappear from the fossil record long before the time of extinction, simply due to chance.
DATING ROCK LAYERS
First the rock layers must be dated, by using some of the following methods:
- Using radio-isotope analysis of igneous rocks - For example, when lava cools, it has no lead content and it captures radioactive Uranium (U-235). Over time, the unstable radioactive Uranium decays into its daughter, Lead-207, at a constant, known rate (its half-life). By comparing the relative proportion of Uranium-235 and Lead-207, the age of the igneous rock can be determined. There are other radioactive elements that can be useful in this type of analysis.
Radioisotope dating cannot be used directly on fossils since they don't contain the unstable radioactive isotopes used in the dating process. To determine a fossil's age, igneous layers (volcanic rock) beneath the fossil (predating the fossil) and above it (representing a time after the dinosaur's existence) are dated, resulting in a time-range for the dinosaur's life. Thus, dinosaurs are dated with respect to volcanic eruptions.
- Determining the magnetism of the rocks -The Earth's magnetic field has changed over geologic time, leaving different magnetic fields in rocks from different geological eras.
- Noting the position of rocks - Sedimentary rock layers (strata) are formed episodically as earth is deposited horizontally over time. Newer layers are formed on top of older layers, pressurizing them into rocks. Paleontologists can estimate the amount of time that has passed since the stratum containing the fossil was formed. Generally, deeper rocks and fossils are older than those found above them.
- Looking for index fossils - Certain common fossils are important in determining ancient biological history. These fossils are widely distributed around the Earth but are limited in time span. Examples of index fossils include Brachiopods (which appeared in the Cambrian), Trilobites (which originated in the Permian period and are common in the Paleozoic layer - about half of Paleozoic fossils are Trilobites), Ammonites (from the Triassic and Jurassic periods), many Nanofossils (microscopic fossils from various eras which are widely distributed, abundant, and time-specific), etc.
CHEMICAL ANALYSIS OF THE SITE
A chemical and mineral analysis of the dead zone reveals much about conditions at the time of the extinction.
- The existence of altered forms of quartz, such as shocked quartz, tektites (quartz that has been vaporized and re-formed into non-crystalline beads), stishovite (a form of quartz also formed under conditions of high heat and pressure), and others.
- The overabundance of Rare Earth Elements (Osmium, Gold, Platinum, Nickel, Cobalt, Palladium, and Iridium, the Siderophile Elements), can be indicative of collision with a chondritic meteor (stony meteors with chondrules, spherical blobs of silicates which pre-date planetary formation and which consist of elements also common in the Earth's core).
- Soot in a layer indicates huge fires.
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