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This is a partial list of [[transitional fossils]] (fossil remains of groups that exhibit both "primitive" and derived traits). The fossils are listed in series, showing the transition from one group to another, representing significant steps in the evolution of major features in various lineages. These changes often represent major changes in morphology and anatomy, related to mode of life, like the acquisition of feathered wings for an aerial lifestyle in [[birds]], or limbs in the [[fish]]/[[tetrapod]] transition onto land.
[[Charles Darwin|Darwin]] noted that transitional forms could be considered [[common ancestor]]s, direct ancestors or collateral ancestors of living or extinct groups, but believed that finding actual common or direct ancestors linking different groups was unlikely.<ref>Stauffer, RC (1975) Charles Darwin's Natural Selection; being the second part of his big species book written from 1856 to 1858. Cambridge: Cambridge University Press. p. 236.</ref><ref>Darwin, C. R. 1859. ''On the origin of species by means of natural selection, or the preservation of favoured races in the struggle for life''. London: John Murray. p. 187.</ref> Collateral ancestors are relatives
ousins in genealogies in which they are not in your direct line of descent but do share a common ancestor (in this case it is a grandparent). This kind of thinking can be extended to groups of life. For instance, the well-known ''[[Archaeopteryx]]'' is a transitional form between non-avian dinosaurs and birds, but it is not the most recent common ancestor of all birds nor is it a direct ancestor of any species of bird alive today. Rather, it is considered an extinct close evolutionary "cousin" to the direct ancestors. This may not always be the case, though, as some fossil species are proposed to be directly ancestral to others, like how ''[[Australopithecus anamensis]]'' is most likely to be ancestral to ''[[Australopithecus afarensis]]''.<ref>{{cite journal | last1 = Delezene | first1 = LK | last2 = Kimbel | first2 = WH | year = 2011 | title = Evolution of the mandibular third premolar crown in early Australopithecus | doi = 10.1016/j.jhevol.2011.01.006 | pmid = 21481921 | journal = Journal of Human Evolution | volume = 60 | issue = 6| pages = 711–730 }}</ref> ==Nautiloids to ammonoids==
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