# Historical Geology/Principle of faunal succession

In this article we shall introduce the principle of faunal succession, and discuss how we know it is valid and why it should be so. In subsequent articles we shall discuss further how it can be applied to stratigraphy. The reader will find it useful to have read previous articles on Steno's principles, way-up structures, and fossils.

## The principle of faunal succession

By using the principle of superposition and by using way-up structures to determine the up and down directions at the time of deposition, we can find the order of deposition in an assemblage of strata. This means of course that we can also find the order of deposition of the fossils within the strata.
Now, looking at some particular assemblage of strata, we may find that the deposition of species A stopped before the deposition of species B started. (We can write this for short with the notation A < B.)
Then the principle of faunal succession says that if we look at a different location and find species A and B, then we will also find that A < B. This is not to say that we will always find A in the same assemblage of strata as B, or vice versa, but it says that if we do, then we will find that A was deposited before B; that the order is the same in different locations. While, as discussed later in this article, exceptions to this are certainly possible (which is why we do not call the principle a law) the principle is very generally applicable, and so it does deserve to be called a principle.
Note that the relation < is transitive: that is to say if A < B and B < C, then we will also find that A < C. (Again, this is not to say that we always will find A and C in the same location, it just tells us what order they will be in if we do.) This allows us to establish a linear order on the fossil record, as we shall discuss further in our article on the geological column.

## How do we know, and why is it so?

How do we know? Because we looked. The principle is a simple one which tells us directly what we should see if we look, so it is easy to verify that it is generally valid.
The question of why it is so is more interesting. It is also in a sense irrelevant, since it is possible to verify the principle and apply it without having the faintest idea why it should be true; and in fact this is what geologists did when the principle was first discovered.
However, we do now understand the reason why the principle holds good; it is an elementary consequence of the theory of evolution. It would take a textbook equal in length to this one to explain the theory of evolution and to sketch out the nature of the evidence for the fact of evolution. However, for the purposes of understanding the principle of faunal succession, the reader really needs to understand only one corollary of the theory: that any particular species will only evolve once.
So it is not possible for species A to evolve before species B in North America and for species B to evolve before species A in South America; each species must arise at one time in one place.
Nor is it possible for species A to arise and go extinct, for species B to arise and go extinct, and then for species A to arise a second time; again, species A can only evolve once, and extinction is final.
(Those readers, if any, who deny evolution will just have to look on the validity of the principle of faunal succession as one of life's little mysteries, or to be more accurate as one of life's huge, gigantic, preposterously enormous mysteries; but the practical validity of the principle is beyond question.)
Now, the underlying mechanism of the principle does allow for the principle to be violated in particular instances. It would for example be possible to have A < B in one location and B < A in another, through some such scenario such as the following.
* Species A arises at location X.
* Species A spreads to location Y.
* Species A goes extinct at location X.
* Species B arises at location X.
* Species B spreads to location Z.
* Species B goes extinct at location Z.
* Species A spreads to location Z.
It would then be the case that at location X we have A < B, but at location Z we have B < A. This sort of elaborate dance is quite unlikely to happen, and if it ever does, it can't happen very often, since the principle of faunal succession does in fact generally hold good.
In summary, the principle of faunal succession should work in theory and does work in practice.

## Note on vocabulary

The principle of faunal succession does not just apply to fauna (i.e. animals), so strictly speaking the principle should be called: "The principle of faunal, floral, fungal, and everything-else-al succession", but that would be inconvenient; and in any case, the name of the principle is so well established that it is too late to do anything about it now.
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