Fossils are the remains of organisms from the distant past embedded and preserved in the earth's crust. Fossils may be any sort of organism, from a single-celled bacteria to a tree leaf to a dinosaur, and may be anywhere from ten thousand to several billion years old. They usually consist of parts of the organism which were mineralized during the organism's lifetime, such as bones and exoskeletons.
A transitional fossil is one that bridges the evolutionary gap between two organisms. A fossil which is transitional between species A and species B will typically have some features of both. For instance, Archaeopteryx has both reptilian teeth and avian feathers.
Creationists also tend to misunderstand the nature of transitionals, and ask questions like "what good is half a wing?"
Gaps in the Fossil Record
The argument that there are gaps in the fossil record is used extremely often by apologists and is basically the same as saying that there are no transitional fossils. There are several large problems with this argument, however:
- When Darwin's The Origin of Species was first published the fossil record was very sparse , and thus the size of the gaps it contained (i.e., between older species and their supposed descendants) was a problem for the theory of descent from a common ancestor. However, as the years passed scientists found more and more fossils, some of which "filled in" the gaps, thus decreasing the "jumps" required to move from one species to another. Nowadays gaps do still exist, and even though there are large gaps remaining, in many places the gaps are comparatively very small. It's not logical to assume that the gaps that exist nowadays will never be filled.
- In reality, there's almost no way to convince a creationist that transitional fossils exist. For example, suppose scientists don't know how one animal evolved into another. Creationists then tout this gap (the gap between fossil A and fossil B) as proof that there are no transitional fossils. However, when a transition (fossil C) is discovered between the two, the creationist will simply say that there are now two gaps, one between fossil A and fossil C and one between fossil C and fossil B. In other words, the more transitions are discovered, the more gaps creationists posit.