|Themes > Science > Paleontology / Paleozoology > Fossils And Fossilisation > Transitional Fossils|
Fossils that show intermediate characteristics are generally called transitional fossils. Transitional fossils are fossils that have characteristics that are intermediate in nature to organisms that existed both prior to it and after it. As such, transitional fossils are strongly suggestive of evolution.
There are many examples of transitional fossils in the fossil record. Examples include 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.
Mention transitional fossils to a creationist and you will most likely get a dirty look. Transitional fossils are frequently misunderstood, and like macroevolution, creationists tend to redefine the term to suit their purposes. As explained above, transitional fossils are fossils that have characteristics that are intermediate between other organisms. If the transitional fossil can be dated to a time between the organisms it is an intermediate to, it is strongly suggestive of an evolutionary relationship between the organisms.
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. They are right. We can't prove that. As has been explained, transitional fossils are suggestive of an evolutionary relationship - they are not proof of it. Once again we run into problems with creationists looking for proof when science deals rather with supporting evidence.
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 - it might be a side-branch on the evolutionary tree that died out.
However, transitional fossils are just one more piece of evidence that is suggestive and supportive of evolution. 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 transitionals 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.
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. We can't say for sure it is actually an ancestor of modern birds, but as explained, that is not a significant issue.
In general, creationist arguments that transitionals are not real transitionals 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.
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. I have already shown how we cannot say for certain that any particular fossilized organism was definitively in the evolutionary history of any other organism. But that doesn't matter. The fossil record is still extraodinarily suggestive of evolution in general, and specific fossils are suggestive of evolutionary relationships between specific organisms. We can make very well informed, provisional conclusions (this is science) as to the evolutionary history of many organisms. And these conclusions are supported by the evidence; in many cases by both fossil and nonfossil evidence.
Summary of Fossil Evidence
While the fossil record is certainly not "complete" (fossilization is a rare event, so this is to be expected), there is still a wealth of fossil information to be considered. If you examine the fossil evidence, you see that the fossil record uniformly supports the idea of common descent. The general order of the fossil record, the correspondence of the order of organisms found in the fossil record with the order suggested by examining living organisms, the correspondence of the fossil record to the phylogenetic tree (including transitional organisms), and the biogeography of the fossil record all support the idea of common descent. Since the fossil record is a record of historical organisms, it strongly suggests a process of evolution occurring throughout history.