Signor-Lipps effect

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The Signor-Lipps effect is a paleontological principle proposed by Phil Signor and Jere Lipps which states that, since the fossil record of organisms is never complete, neither the first nor the last organism in a given taxon will be recorded as a fossil.

The most spectacular example is the coelacanth, which was thought to have become extinct in the very late Cretaceous - until a live specimen was caught in 1938.

But the Signor-Lipps effect is more important for the difficulties it raises in paleontology:

It makes it very difficult to be confident about the timing and speed of mass extinctions, and this makes it difficult to test theories about the causes of mass extinctions. For example the extinction of the dinosaurs was long thought to be a gradual process, but evidence collected since the late 1980s suggests it was abrupt, which is consistent with the idea that an asteroid impact caused it.
The uncertainty about when a taxon first appeared makes it difficult to be confident about the ancestry of specific genera. For example if the earliest known fossil of genus X is much earlier than the earliest known fossil of genus Y and genus Y has all the features of genus X plus a few of its own, it is natural to suppose that X is an ancestor of Y. But this hypotheses might be made doubtful at any time by the finding of a fossil of Y that is earlier than any known fossil of X - unless an even older fossil of genus X is found, and so on.