In this article we shall discuss what an index fossil is, why they are useful, and what qualifies a fossil to be an index fossil.
Why do we need index fossils?Edit
Consider the fauna living on the continental shelves of the east and west Atlantic ocean (the littoral fauna). Obviously these faunas will be different — they have had 130 million years to follow separate evolutionary trajectories, and since they require shallow water they aren't going to cross from one side of the ocean to the other.
So when we look at the successions in the fossil record of the littoral faunas on each side of the Atlantic, we will see two faunal successions, an eastern and a western. What's more, the sediments in which they are deposited will not particularly correlate, since they will have different points of origin.
However, mixed in with the littoral fauna there will also be fossils of free-floating or free-swimming surface organisms: pelagic fauna. Now such species will spread throughout the ocean, since there's nothing to stop them from doing so.
So we can and do find the same pelagic species on both sides of the Atlantic. We can use such species to correlate the other littoral species and the sediments in which they are deposited. For if we find a pelagic species (call it S) somewhere in the fossil record on both coasts, then this gives us a way to divide up both successions into species that were deposited before S, along with S, or after S.
Given enough such species, we can find many such correlations, and this would allow us to represent the eastern and western deposition on the same timeline.
Such species would be examples of index species, and their fossils would be examples of index fossils, which we may define generally as follows: an index fossil is a fossil of a species that was sufficiently widely distributed that its fossils can be used to correlate the deposition of fossils and sediments in widely separated locations.
What makes a good index species?Edit
Obviously, the first requirement for an index species is that it should indeed be widely distributed. Pollen makes a good index fossil, being wind-borne; so do foraminifera, which are pelagic species as in our example.
An index species should also be readily preserved in the fossil record. Birds, for example, would make bad index fossils, because although there are many species which have wide (indeed intercontinental) ranges, they fossilize very poorly: their skeletons come apart easily, and then their delicate honeycombed bones are highly susceptible to decay. Typically, only fragments will survive, and even if these were common, which they are not, it would still be hard to tell one species from another by studying them.
Finally, we would ideally like an index fossil to have a short time of deposition as a proportion of the fossil record, since we want to use it to identify a particular chapter in the history of deposition: it should represent a geographically broad but temporally narrow slice of the record.
Some examples of marine index fossils are shown in the table below. Note that they are not claimed to form an evolutionary sequence, any more than a list of Presidents does: they are a temporal sequence, and should be understood as such.