Fossils that show intermediate characteristics are called transitional fossils â€" they have characteristics that are intermediate in nature to organisms that existed both prior to it and after it. Transitional fossils are strongly suggestive of evolution because they indicate the progression of from just as evolutionary theory predicts. Transitional fossils are frequently misunderstood, and like macroevolution, creationists tend to redefine the term to suit their purposes.
There are many examples of transitional fossils in the fossil record, including large-scale transitions such as from reptiles to birds (like the controversial archaeopteryx) and from reptiles to mammals, as well as more detailed transitions, such as those among the many hominids or the development of horses. The fact that, despite the rarity of fossilization, we have a wealth of transitional fossil data and that the fossil data generally conforms to the phylogenetic tree is strongly supportive of the idea of evolution.
Creationists vs. Transition Fossils
Creationists will critique transitional fossils in a variety of ways. They might claim that a transitional fossil is not proof of an evolutionary relationship since you can't prove that it is, in fact, an ancestor of any later organism. It's true that we can't prove this in the strictest sense, but transitional fossils are suggestive of an evolutionary relationship rather than proof of it. As is so often the case, this is an example of creationists demanding proof when science deals rather with supporting evidence then claiming that the absence of absolute proof demonstrates that evolution isn't science at all.
Without actually going back in time and watching the birth/hatching/etc. of each successive organism in an evolutionary chain, we can not "prove" that an evolutionary relationship exists. Even if you accept evolution, you can't be sure some organism is actually an ancestor of existing species â€" it, for example, might be a side-branch on the evolutionary tree that died out.
However, even if a transitional fossil is a side-branch, it still shows that creatures with intermediate characteristics existed, and this indicates the strong possibility that a similar organism could exist that is an ancestor of an existing species. When you consider that such transitional fossils fall into the phylogenetic tree well within the area you would expect them to, it is a nicely verified prediction of the general theory of evolution and further support for the theory.
Evolution Denial & Denying Transitions
Creationists will also sometimes state that a transitional fossil is not, in fact, a transitional. For example, with archaeopteryx, some have claimed that it is not a transitional between reptiles and birds and instead assert that it is a true bird. Unfortunately, this is another example of a creationist lie or distortion. If you look at the evidence it is clear that archaeopteryx has characteristics in common with reptiles that modern birds do not posses.
Archaeopteryx is a transitional fossil as the concept "transitional fossil" is defined in science: it possesses intermediate characteristics of entirely different species of animals. We can't say for sure it is actually an ancestor of modern birds rather than a side-branch that ultimately died out, but as explained that is not a genuine problem.
Creationist complaints that transitional fossils are not real transitional fossils are based on their ignorance of what a transitional fossil is or simply on outright distortions of fact. It is not that there isn't room for debate on the nature or categorization of various fossils, because there is always room for debate. However, creationist debates are almost never informed debate and as such do not accomplish much.
Creationists of the Gaps
Finally, creationists will sometimes belabor the fact that there are gaps in the fossil record. Even if we have a transitional fossil between two groups of organisms that is suggestive of an evolutionary relationship, creationists will demand intermediaries between the intermediaries. And, if those are found, creationists will want intermediaries between the new organisms. It's a no-win situation. Since creationists try to put forth the strawman that you need "absolute proof" of an evolutionary relationship to accept it, they insist that if we do not have a record of every single organism in the chain we can't say some organism is an ancestor of another.
This is a useless and spurious criticism. We cannot say for certain that any particular fossilized organism was definitively in the evolutionary history of any other organism, but that isn't absolutely necessary. The fossil record still provides massive inferential evidence of evolution in general and specific fossils are suggestive of evolutionary relationships between specific organisms. This allows us to make strong, informed conclusions (this is science) about the evolutionary history of many organisms and these conclusions are supported by the evidence both by fossil and nonfossil evidence.
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