Editing Transitional fossil (section)

==History==
{{see also|History of paleontology|Timeline of paleontology}}

===Post-Darwin===
[[File:Rhynia reconstruction.svg|thumb|upright|Reconstruction of ''[[Rhynia]]'']]
The idea that animal and plant species were not constant, but changed over time, was suggested as far back as the 18th century.<ref>{{cite journal |last1=Archibald |first1=J. David |date=August 2009 |title=Edward Hitchcock's Pre-Darwinian (1840) 'Tree of Life' |url=http://www.bio.sdsu.edu/faculty/archibald.html/Archibald09JHB42p561.pdf |journal=[[Journal of the History of Biology]] |volume=42 |issue=3 |pages=561–592 |doi=10.1007/s10739-008-9163-y |issn=0022-5010 |pmid=20027787 |citeseerx=10.1.1.688.7842 |s2cid=16634677 }}</ref> Darwin's ''On the Origin of Species'', published in 1859, gave it a firm scientific basis. A weakness of Darwin's work, however, was the lack of palaeontological evidence, as pointed out by Darwin himself. While it is easy to imagine [[natural selection]] producing the variation seen within [[genus|genera]] and families, the transmutation between the higher categories was harder to imagine. The dramatic find of the [[Archaeopteryx#History of discovery|London specimen]] of ''[[Archaeopteryx]]'' in 1861, only two years after the publication of Darwin's work, offered for the first time a link between the [[class (biology)|class]] of the highly derived birds, and that of the more basal [[reptile]]s.{{sfn|Darwin|1859|loc=[http://www.readprint.com/chapter-2217/Charles-Darwin Chapter 10]}} In a letter to Darwin, the palaeontologist [[Hugh Falconer]] wrote:

<blockquote>Had the Solnhofen quarries been commissioned—by august command—to turn out a strange being à la Darwin—it could not have executed the behest more handsomely—than in the ''Archaeopteryx''.<ref>{{cite journal |last=Williams |first=David B. |date=September 2011 |title=Benchmarks: September 30, 1861: Archaeopteryx is discovered and described |url=http://www.earthmagazine.org/article/benchmarks-september-30-1861-archaeopteryx-discovered-and-described |journal=EARTH |issn=1943-345X |access-date=2012-02-24 |archive-url=https://web.archive.org/web/20120507072544/http://www.earthmagazine.org/article/benchmarks-september-30-1861-archaeopteryx-discovered-and-described |archive-date=7 May 2012  }}</ref></blockquote>

Thus, transitional fossils like ''Archaeopteryx'' came to be seen as not only corroborating Darwin's theory, but as icons of evolution in their own right.<ref>{{harvnb|Wellnhofer|2009}}</ref> For example, the Swedish [[encyclopedic dictionary]] ''Nordisk familjebok'' of 1904 showed an inaccurate ''Archaeopteryx'' reconstruction (see illustration) of the fossil, "ett af de betydelsefullaste paleontologiska fynd, som någonsin gjorts" ("one of the most significant paleontological discoveries ever made").<ref>{{harvnb|Leche|1904|pp=[https://runeberg.org/nfba/0758.html 1379–1380]}}</ref>

===The rise of plants===
Transitional fossils are not only those of animals. With the increasing mapping of the [[Phylum#Land plant phyla (divisions)|divisions]] of plants at the beginning of the 20th century, the search began for the ancestor of the [[vascular plant]]s. In 1917, [[Robert Kidston]] and [[William Henry Lang]] found the remains of an extremely primitive plant in the [[Rhynie chert]] in [[Aberdeenshire]], Scotland, and named it ''[[Rhynia]]''.<ref name="Kidston1917">{{cite journal |last1=Kidston |first1=Robert |author-link1=Robert Kidston |last2=Lang |first2=William Henry |author-link2=William Henry Lang |date=27 February 1917 |title=XXIV.—On Old Red Sandstone Plants showing Structure, from the Rhynie Chert Bed, Aberdeenshire. Part I. Rhynia Gwynne-Vaughanii, Kidston and Lang |url=https://www.biodiversitylibrary.org/item/130747#page/961/mode/1up |journal=Transactions of the Royal Society of Edinburgh |volume=51 |issue=3 |pages=761–784 |doi=10.1017/S0263593300006805 |s2cid=251580286 |issn=0080-4568 |oclc=704166643 |access-date=2015-05-18 }}</ref>

The ''Rhynia'' plant was small and stick-like, with simple [[Dichotomy|dichotomously]] branching stems without leaves, each tipped by a [[sporangium]]. The simple form echoes that of the [[sporophyte]] of [[mosses]], and it has been shown that ''Rhynia'' had an [[alternation of generations]], with a corresponding [[gametophyte]] in the form of crowded tufts of diminutive stems only a few millimetres in height.<ref name="Kerpetal2004">{{cite journal |last1=Kerp |first1=Hans |last2=Trewin |first2=Nigel H. |last3=Hass |first3=Hagen |year=2003 |title=New gametophytes from the Early Devonian Rhynie chert |journal=Transactions of the Royal Society of Edinburgh: Earth Sciences |volume=94 |issue=4 |pages=411–428 |doi=10.1017/S026359330000078X |s2cid=128629425 |issn=0080-4568 }}</ref> ''Rhynia'' thus falls midway between mosses and early vascular plants like [[fern]]s and [[Lycopodiopsida|clubmoss]]es. From a carpet of moss-like gametophytes, the larger ''Rhynia'' sporophytes grew much like simple clubmosses, spreading by means of horizontal growing stems growing [[rhizoid]]s that anchored the plant to the substrate. The unusual mix of moss-like and vascular traits and the extreme structural simplicity of the plant had huge implications for botanical understanding.<ref>{{harvnb|Andrews|1967|p=32}}</ref>