"In 1859, when Charles Darwin’s On the Origin of Species was first published, the fossil record was poorly known, and Darwin described the lack of transitional fossils as “the most obvious and gravest objection which can be urged against my theory”, but explained it by the extreme imperfection of the geological record.[1] He noted the limited collections available at that time, but described the available information as showing patterns which followed from his theory of descent with modification through natural selection.[2][3] Indeed, Archaeopteryx was discovered just two years later, in 1861, and represents a classic transitional form between dinosaurs and birds.[3]
Many more transitional fossils have been discovered since then and it is now considered that there is abundant evidence of how all the major groups of animals are related, much of it in the form of transitional fossils.[3]"
"It is commonly claimed by critics of evolution that there are no transitional fossils.[5][3][6] Such claims may be based on a misunderstanding of the nature of what represents a transitional feature[5], or may be an active tactic employed by creationists, seeking to distort or to discredit evolution theory.[3] The claim has been called a “favourite lie” of creationists by Donald Prothero which is “manifestly untrue”.[3]
A common creationist argument is that no fossils are found with partially functional features. However, there is evidence that a complex feature with one function can adapt to a wholly different function through evolution in a process known as exaptation. The precursor to, for example, a wing, might originally have only been used for gliding, trapping flying prey, and/or mating display. Nowadays, wings may still have all of these functions, while also being used for active flight.
Although transitional fossils elucidate the evolutionary transition of one life-form to another, they only exemplify snapshots of this process. Due to the special circumstances required for preservation of remains, only a very small percentage of all life-forms that ever have existed can be expected to be represented in discoveries. Thus, the transition itself can only be illustrated and corroborated by transitional fossils, but it will never be “caught in the act” as it were. Critics of evolution often cite this argument as being a convenient way to explain the lack of ‘snapshot’ fossils that show crucial steps between species. However, progressing research and discovery are managing to fill in gaps.
The theory of punctuated equilibrium developed by Stephen Jay Gould and Niles Eldredge and first presented in 1972[7] is often mistakenly drawn into the discussion of transitional fossils. This theory, however, pertains only to well-documented transitions within taxa or between closely related taxa over a geologically short period of time. These transitions, usually traceable in the same geological outcrop, often show small jumps in morphology between extended periods of morphological stability. To explain these jumps, Gould and Eldredge envisaged comparatively long periods of genetic stability separated by periods of rapid evolution."
