To be clear, a transitional species is B in the series A->B->C.
Actually, in paleontology, that isn't quite true. It would actually be a very rare event to find species B in any lineage -- and even if B were found, it would be difficult to determine whether it was really in direct line between A and C or on a side branch.
A transitional species is actually a species that is another descendent of B but close enough in time so that it retains many of B's characteristics that C has lost, and so can say something about what B must of looked like.
Consider the diagram:
A
|
------------B-----------
| |
------C------ D
| |
E F
Now E and F are modern species that help difine a taxon, like a modern mammal and a modern salamander are tetrapods. The had a common ancestor, C, and further back they had a more fish-like ancestor, B. B is unlikely ever to be found. However, B had other descendent species, like D which would represent, for example, Tiktaalik. Now D (and Tiktaalik) is not an ancestor to E and F; however, it is close to the ancestor B in time, and would still have many of the same physical characteristics. So D gives a lot of insight into what the Actual ancestor, B, was like.
Having found D and recognizing its significance, we would label D as a transitional fossil.
Edited by Chiroptera, : Oops. Hit submit instead of preview.
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