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A transitional fossil is the fossil remains of a creature that exhibits certain primitive (or basal) traits in comparison with its more derived descendants. "Missing link" is a popular term used for transitional forms. According to modern evolutionary theory, all populations of organisms are in transition. Therefore, 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 lack was the most formidable obstacle to his theory. However, the discovery of Archaeopteryx only two years later was seen as a triumph for Darwin's theory of common descent. Gaps remain in the fossil record, however; and while some argue that this is a problem for evolutionary theory, most scientists accept that the rarity of fossils means that many extinct animals will always remain unknown. (See fossil.) Though the evolution of the horse and its relatives, as Othniel Charles Marsh assembled surviving fossils in his reconstruction of the evolution of horses in the form of a single, consistently developing lineage with many "transitional" types, is often cited as a family tree with a number of clear transitional fossils, modern cladistics gives a different, multi-stemmed shrublike picture, with many dead ends: see evolution of the horse. Other specimens cited as transitional forms include the "walking whale" Ambulocetus, the recently-discovered lobe-finned fish Tiktaalik, with many tetrapod characteristics, and various hominids considered to be proto-humans.
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: According to this definition, Archaeopteryx, which does not show any derived traits that more derived birds do not possess as well, is transitional. In contrast, 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, for example, 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 very 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, pertains only 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. The Missing Link 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. See also | ||||||||
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