From Wikipedia: A transitional fossil or transitional form is the fossilized remains of a life form that illustrates an evolutionary transition. It can be identified by having certain primitive (plesiomorphic) traits in comparison with its more derived relatives, such as defined in the study of cladistics. "The Missing Link" is a popular term used for transitional forms.
A popular term to designate transitional forms with is "the missing link". The term is especially used in the regular media, but inaccurate and confusing. This is partly because it implies that there was a single link missing to complete the picture, which now has been discovered. In reality, the continuing discovery of more and more transitional fossils is further adding to our knowledge of evolutionary transitions. The term probably arose in the 19th century where the awaited discovery of a "missing link" between humans and so-called "lower" animals was considered to be the final proof of evolution. The Australopithecus afarensis fossil (more commonly known as "Lucy") is seen as a key transitional fossil.
This is a very tentative list of vertebrate transitional fossils (fossil remains of a creature that exhibits primitive traits in comparison with more derived life-forms to which it is related). An ideal list would only recursively include 'true' transitionals, i.e. those forms morphologically similar to the ancestors of the monophyletic group containing the derived relative, and not intermediate forms. See the article on transitional fossils for an explanation of the difference with intermediate forms. Since all species are supposed to be in transition due to natural selection, the very term "transitional fossil" is essentially a misconception. But the fossils listed represent significant steps in the evolution of major features in various vertebrate lines, and therefore fit the common usage of the phrase.
- Fish to Amphibians
- Amphibians to Amniotes (early reptiles)
- Synapsid (mammal-like "reptiles") to mammals
- Diapsid reptiles to birds
- Evolution of whales
- Non-human apes to modern humans