The fossil record is famously incomplete, and though we still have much to learn about prehistoric life, we can at least, thanks to a particular type of fossil, begin to organize it. These unique relics, known as Index Fossils, serve one of the most significant purposes in the paleontological world.
Basics
There are many types of fossils---trace fossils, molds and casts---some of which may or may not fall into an even greater category of fossils that paleontologists use to determine and identify geological periods. This is done by looking for short-lived species. For example, if an organism existed for a very short amount of time, let's say during the Jurassic Period, then a paleontologist who finds a sample of sediment with those particular fossils inside can easily deduce when the sediment was created. These guide fossils, as they may be called in some circles, are crucial in identifying the chronology of certain sediment layers. But what are the most important characteristics to consider when identifying Index Fossils?
Abundance
To best allow paleontologists to correlate rocks with other rocks found at locations far apart, it's important that the index fossil be very widely distributed across a vast geographical region. Usually, these are marine organisms, which can easily be dispersed and often appear independent of rock type. What that means is that they can be found regardless of the type of rock, indicating that rocks containing the same index fossils are from a similar time period, and don't just carry those fossils because of their specific rock type.
Rapid Evolution and Extinction
A short lifespan of a given species is ideal for determining the exact age of certain geological strata. If an organism only existed for a narrow amount of time on the geological time line, then dating the rock from which its fossils are found is a much more precise and accurate measurement.
Physical Features
Ideally, index fossils are small. Large fossils are rarer and more complex, whereas smaller fossils are more abundant and hence easier to find and identify. Index fossils should have distinctive shapes and features, like spiraling shells. It's also helpful if the fossils are identifiable on the spot, without the use of a laboratory or special tools.
Examples
Some fossils have become so common and well-studied that they've become staple index fossils within the geological world. Such fossils include Trilobites, which look like a prehistoric horseshoe crab; Ammonites, predecessors of the modern nautilus; and calico scallops.
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