This is a rather in depth discussion going on elsewhere all starting with my blog about Shubin’s Your Inner Fish.
However, there seems to a major misunderstanding and I thought I might bring it up here to correct any misunderstandings people have.
Fossils or organisms that show the intermediate states between an ancestral form and that of its descendants are referred to as transitional forms.
Now, let’s parse this out, so we all know what we’re talking about.
Fossils of organisms – so a transitional can be any living or once living thing. (In fact, every living or once living thing is a transitional, but I’ll get to that in a minute.)
that show the intermediate states – So we are looking for something with an intermediate character. Now, that doesn’t mean ‘half a wing’ or ‘half a lung’. What it means is a trait that gives us a clue about the change over from one thing to the other. It is not “No lung, half a lung, complete lung.” It is “no lung, proto-lung, modern lung”. The difference is subtle and creationists will (and are) confuse the issue to make it seem implausible.
between an ancestral form and that of its descendants – note that it doesn’t say “ancestor”. It says “ancestral form”. Kublai Khan is an ancestral form of me, even though he is not my ancestor. He is a human, who lived 800 years ago. However, I have no Asian traits in my genome (that I’m aware of, if anyone wants to spring for my genetic history, that would be awesome (I’ll publish the results here)). “Its” is unfortunately ambiguous here. But I assure that other factors result in there being no requirement that the descendant be directly related (distant cousins are fine).
So, that’s the definition. Note that there is no requirement of time. The transitional fossil or organism doesn’t have to exist in any particular point in the timeline of any species, ancestral or descendant.
You are a transitional form between your parent and your child. If you parent dies, you are still transitional. If you die, you are still transitional. If your child dies… guess what… you are still transitional.
Further a transitional can exist between three current species (or whatever). For example, many genetic traits in humans are polygenic. That means (for our purposes today) that there is a broad range of available values. For example, height. Let’s say that you have three children… fraternal triplets (to remove the time issue). One child is very tall. One child is very short. The other child is just right (meaning you can actually find jeans that fit).
The average height child is transitional between the tall and short child. If you measured the length of bones that are responsible for height (femur, tibia and fibula, veterbrae, etc), you would probably find that the bones of the average child were all intermediate between the tall and short children’s bones. It’s transitional.
Now, let’s look at some examples. Of course, mention of Shubin brings up Tiktaalik. Why is it transitional?
Because the wrist bones have characteristics of both fish fins AND tetrapod wrists.
What about Archeopteryx? The classic feathered dinosaur. Even if you find true birds older than it is and true dinosaurs younger than it is, Archeopteryx is still transitional between birds and dinosaurs. Out of some 23 defining characters between birds and dinos, Archeopteryx has 4 that are definitely bird and 17 that are definitely dinosaur (meaning that these 17 characters are different in birds and dinosaurs, if the thing has teeth it’s not a bird) and the other 3 are variable depending on the species of bird or dino we’re talking about.
It’s intermediate between bird and dinosaur. If conditions and luck are on our side, we might find a fossil that has 5 definite bird like traits and another that has 6 and another that has 7.
Unfortunately, the conditions for fossilization are rare and as rock is recycled, it is difficult to find the right kind of rock, of an appropriate age, with appropriate conditions for fossilization as it became rock, in an area with the species we hope to find.
So this is all about transitional fossils. And, finally, a list of transitional fossils.