What is a transitional fossil?
Transitional fossils can be thought of as the remains of organisms that represent an intermediate state of morphology (body form) relative to another organism. Technically, other characteristics, such as behavior or a DNA sequence, could also be transitional, but the fossil record rarely preserves these types of traits.
Many people think that a transitional fossil must be a direct ancestor to qualify as such. This is not true. Rather, the fossil could be a lineage that split off from the ancestral stock and simply preserves an intermediate character from a common ancestor. We likely will never know if a fossil ever truly represents a literal “ancestor”, but the information preserved in these fossils is nonetheless important.
The most convincing transitional fossils
Transitional fossils that I think are the most convincing evidence of evolution and shared ancestry possess two characteristics:
1) The fossil is found in rocks that correlate with a date (e.g., estimated via Potassium-Argon dating) that is intermediate to the date of a putative “ancestral” fossil (which should be older) and the date of a putative “descendant” fossil (which should be younger).
2) One or more physical characteristics of the fossil should be intermediate in form to the putative “ancestral” fossil and the form of the putative “descendant” fossil.
For example, let’s say we have fossils A, B and C. A is found in rock deposits that date to 65 million years ago, B dates to 55 million years ago and C dates to 45 million years ago. All three fossils are clearly similar, all resembling a horse, but A has 20 teeth, B has 16 teeth and C has 12 teeth. B in this case would be a transitional fossil between A and C.
Let it be clear that characteristic 1 need not be necessary for something to be described as a transitional fossil (see above), but I think that fossils satisfying both characteristics 1 and 2 are more convincing, and as such, I plan on only focusing on these types of fossils.
Why aren’t there more?
I have been asked “Why, if fossils can truly be transitional, are there not dozens or hundreds of transitions preserved?” In other words, shouldn’t there be more out there?
One possible answer is that the fossil record is simply incomplete. We don’t expect the fossil record to ever be complete due to something called taphonomic bias, i.e., biases in the way things are preserved. For example, things that live in water are more likely to be buried in a way that prevents decomposition than organisms that live on land.
Another possibility is that the evolution of certain traits happens at a tempo that simply is not preserved in the fossil record. Typically we refer to fossils that are, say, 25 million or 33 million years old. We can’t necessarily pinpoint fossils that are 25.01 million, 25.02 million or 25.03 million years old, so if a transition happened during that period, we would not be able to detect all of its stages.
Also worth considering is the fact that typically only hard body parts are preserved, which prevents scientists from determining if there are, say, intermediate livers or eyes. Perhaps one lineage of birds transitioned from blue to green to yellow, but we may never know if that information isn’t preserved in the fossil record.
Finally, some transitions are likely quite sudden, so-called “saltations”. For example, scientists think that certain mammals may have lost their teeth and/or enamel roughly in one step. Certain mutations in specific genes can bring about a sudden change in a characteristic rather than occurring through a smooth, gradual process.
In my own field of research, we think that the eyes of certain mammals went from being ultraviolet sensitive to blue sensitive through a single mutation in an underlying gene. Many traits perhaps evolve via these saltations and simply are not yet appreciated.
Even though transitional fossils are not necessarily as common as some skeptics may like, I’d argue that they’re more common than skeptics may realize.
Questions for creationists
What are some different explanations for so-called transitional fossils? Is it a coincidence more ‘modern’ looking species are found in what appear to be younger rocks than more ‘primitive’ looking species? Why might have God made some organisms appear transitional but not others?