Editing Transitional fossil (section)

==Evolutionary and phylogenetic taxonomy==
===Transitions in phylogenetic nomenclature===
{{Main|Phylogenetic nomenclature}}
{{See also|Evolutionary taxonomy}}
[[File:Spindle diagram.jpg|thumb|left|Traditional spindle diagram showing the [[vertebrate]]s classes "budding" off from each other. Transitional fossils typically represent animals from near the branching points.]]
In evolutionary taxonomy, the prevailing form of [[Taxonomy (biology)|taxonomy]] during much of the 20th century and still used in non-specialist textbooks, [[Taxon|taxa]] based on morphological similarity are often drawn as "bubbles" or "spindles" branching off from each other, forming evolutionary trees.<ref>For example, see {{harvnb|Benton|1997}}</ref> Transitional forms are seen as falling between the various groups in terms of anatomy, having a mixture of characteristics from inside and outside the newly branched [[clade]].{{sfn|Prothero|2007|p=[https://archive.org/details/evolutionwhatfos00prot_0/page/84 84]}}

With the establishment of [[cladistics]] in the 1990s, relationships commonly came to be expressed in [[cladogram]]s that illustrate the branching of the evolutionary lineages in stick-like figures. The different so-called "natural" or "[[Monophyly|monophyletic]]" groups form nested units, and only these are given [[phylogenetic nomenclature|phylogenetic names]]. While in traditional classification tetrapods and fish are seen as two different groups, phylogenetically tetrapods are considered a branch of fish. Thus, with cladistics there is no longer a transition between established groups, and the term "transitional fossils" is a [[misnomer]]. Differentiation occurs within groups, represented as branches in the cladogram.<ref name="Palaeos">{{cite web |url=http://palaeos.com/vertebrates/tetrapoda/amphibians.html |title=Amphibians, Systematics, and Cladistics |last=Kazlev |first=M. Alan |website=[[Palaeos]] |access-date=2012-05-09}}</ref>

In a cladistic context, transitional organisms can be seen as representing early examples of a branch, where not all of the traits typical of the previously known descendants on that branch have yet evolved.{{sfn|Prothero|2007|p=[https://archive.org/details/evolutionwhatfos00prot_0/page/127 <!-- quote=fossil "transitional form". --> 127]}} Such early representatives of a group are usually termed "[[Basal (phylogenetics)|basal taxa]]" or "[[Sister group|sister taxa]],"{{sfn|Prothero|2007|p=[https://archive.org/details/evolutionwhatfos00prot_0/page/263 <!-- quote=fossil "transitional form". --> 263]}} depending on whether the fossil organism belongs to the daughter clade or not.<ref name="Palaeos" />

===Transitional versus ancestral===
A source of confusion is the notion that a transitional form between two different taxonomic groups must be a direct ancestor of one or both groups. The difficulty is exacerbated by the fact that one of the goals of evolutionary taxonomy is to identify taxa that were ancestors of other taxa. However, because evolution is a branching process that produces a complex bush pattern of related [[species]] rather than a linear process producing a ladder-like progression, and because of the incompleteness of the fossil record, it is unlikely that any particular form represented in the fossil record is a direct ancestor of any other. Cladistics deemphasizes the concept of one taxonomic group being an ancestor of another, and instead emphasizes the identification of sister taxa that share a more recent common ancestor with one another than they do with other groups. There are a few exceptional cases, such as some marine [[plankton]] [[Micropaleontology|microfossil]]s, where the fossil record is complete enough to suggest with confidence that certain fossils represent a population that was actually ancestral to a later population of a different species.<ref>{{cite journal |last1=Prothero |first1=Donald R. |last2=Lazarus |first2=David B. |date=June 1980 |title=Planktonic Microfossils and the Recognition of Ancestors |journal=[[Systematic Biology]] |volume=29 |issue=2 |pages=119–129 |doi=10.1093/sysbio/29.2.119 |issn=1063-5157 }}</ref> But, in general, transitional fossils are considered to have features that illustrate the transitional anatomical features of actual common ancestors of different taxa, rather than to ''be'' actual ancestors.<ref name=Prothero>{{harvnb|Prothero|2007|pp=133–135}}</ref>