Surely every fossil and every organism alive is a transitional life form to something else. Although fossil records are often patchy due to the nature of fossilisation itself -- e.g. the period of 5-15MYA in the African jungles that might show the evolution of
homo sapiens from a common gorilla- or chimp-like ancestor is particularly bad for fossils -- things just don't fossilise much in jungle environments, dead animals are quickly eaten by scavengers, the soil isn't right, fossil-forming sedimentary events just haven't happened, etc.
Anyhow, there are examples in the fossil record of little proto-Tyrannosaurus rex predator species many millions of years later becoming the much larger Tyrannosaurus rex -- the animals grew larger and evolved into new species.
Further, there is now evidence that genes can re-program themselves and adapt in the living animal as circumstances change, and then pass on these changes to their descendants -- that species are far more adaptable than the 'random mutations' and 'accidents' suggested in early Darwinian theory. This is actually closer to Lamarck's theories of continual and dynamic adaption that were discredited, but now have new currency (epigenetics). Further, some of Mendeleev's results concerning his theory of genetic transmission were falsified to create a better fit of the data to his theory, meaning that biology textbooks contain errors of fact -- genetic transmission is far more complex than originally postulated.
http://en.wikipedia.org/wiki/Epigenetics