A transitional fossil is the fossil remains of a creature that exhibits primitive (or "conservative") traits in comparison with the more derived life-forms to which it is related. The "missing link" is a popular term used for transitional forms. According to evolutionary theory, in which all populations of organisms are in transition, a "transitional form" is a human construct that vividly represents a particular evolutionary stage, as recognized in hindsight.

When Charles Darwin's The Origin of Species was first published, the fossil record was poorly known, and the claim that there was a lack of transitional fossils was perfectly reasonable; indeed, Darwin stressed in his work that this was the most formidable obstacle to his theory. However, the discovery of Archaeopteryx only two years later was seen as a stunning triumph for Darwin's theory of common descent. Similarly, Othniel Charles Marsh assembled corroborative evidence of transitional fossils in his reconstruction of the evolution of horses as a single, consistently developing lineage. However, some argue that the many gaps in the fossil record that do exist are actually evidence rather than mere lack of evidence, and that the gaps require explanation. The reasons why gaps exist in the fossil record are explained in fossil.

Specimens claimed to be transitional forms include the whale ancestors called pakicetids and various hominids assessed in a linear way as proto-humans. In addition, the fossil record of the ancestors of the modern horse and zebra as mentioned above is considered one of the best examples of a clear transition between species.

Transitional forms and cladistics

Before the general acceptance of 'cladistics' or 'phylogenetic systematics' in paleontology, evolutionary trees were often drawn as the emerging of one group from another. The transitional forms were placed at the borders of these. With the establishment of cladistic methods, relationships are now strictly expressed in so-called cladograms, illustrating the actual branching of the evolutionary lineages.

The different so-called 'natural' or 'monophyletic' groups form nested units that do not overlap. Within cladistics there is thus no longer a transition between groups, but a differentiation within groups. In this context, transitionals can be defined as the different branches of a cladogram between one particular branch and the crown-group, i.e. the group that is placed at the end of a cladogram.

One example of this is the frequent occurrence in traditional taxonomy (evolutionary systematics) of the assignment of the same rank to a certain group as the group that contains it phylogenetically speaking. Birds e.g. are due to their ancestry part of the reptiles, but Aves (Birds) and Reptilia (Reptiles) both had the rank of Class. Thanks to cladistics, it is much clearer to see that the transition of reptilian forms to avian forms is a smooth transition and not a sudden one.

Transitional forms vs. intermediate forms

The terms 'transitional' and 'intermediate' are for the most part used as synonyms to each other. However, a distinction between the two can be made:

• Transitional can be used for those forms that do not have a significant amount of unique derived traits that the derived relative does not possess as well. In other words: a transitional is morphologically close (if not identical) to the actual common ancestor of itself and the derived relative.
• Intermediate can be used for those forms that do have a large number of uniquely derived traits not connected to its derived relative.

According to this definition, e.g. Archaeopteryx, that does not show any derived traits that more derived birds do not possess as well, is transitional. In contrast, e.g. the Duck-billed Platypus is intermediate because it retains certain reptilian traits no longer found in modern mammals and at the same time possesses a lot of derived traits of its own, as a highly specialized aquatic animal.

Following this definition, all living organisms are in fact to be regarded as intermediate forms to some other related life-form. Indeed there are many species alive today that can be considered to be transitional between two or more groups.

Misconceptions

It is commonly stated by anti-evolutionists that there are no known transitional fossils. This position is based on a misunderstanding of the nature of what represents a transitional feature. A common creationist argument is that no fossils are found with partially functional features. It is entirely plausible, however, that a complex feature with one function can adapt a wholly different function through evolution. The precursor to e.g. a wing, might originally have only been meant for gliding, trapping flying prey, and/or mating display. Nowadays, wings can still have all of these functions, but they are also used in 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 living beings, only a small percentage of all life-forms that ever have existed can be expected to be discovered. Thus, the transition itself can only be illustrated and corroborated by transitional fossils, but it will never be known in detail. However, progressing research and discovery managed to fill in several gaps and continues to do so.

The theory of punctuated equilibrium developed by Stephen J. Gould and Niles Eldredge is often mistakenly drawn into the discussion of transitional fossils. This theory, however, only pertains to well-documented transitions within species or between closely related species over a geologically short period of time. These transitions, usually traceable in the same geological outcrop, often show small jumps in morphology between periods of morphological stability. To explain these jumps, Gould and Eldredge envisaged comparatively long periods of genetic stability separated by periods of rapid evolution.