Transitional fossil: Difference between revisions

Revision as of 01:26, 10 May 2021

Revision as of 18:40, 22 September 2021

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''[[Archaeopteryx]]'' is a [[genus]] of [[Theropoda|theropod]] dinosaur closely related to the birds. Since the late 19th century, it has been accepted by palaeontologists, and celebrated in lay reference works, as being the oldest known bird, though a study in 2011 has cast doubt on this assessment, suggesting instead that it is a non-[[Avialae|avialan]] dinosaur closely related to the origin of birds.<ref name="Xiaotingia">{{cite journal |author1=Xing Xu |author-link1=Xu Xing (paleontologist) |author2=Hailu You |author3=Kai Du |author4=Fenglu Han |date=28 July 2011 |title=An ''Archaeopteryx''-like theropod from China and the origin of Avialae |journal=[[Nature (journal)|Nature]] |volume=475 |issue=7357 |pages=465–470 |doi=10.1038/nature10288 |issn=0028-0836 |pmid=21796204 |s2cid=205225790 }}</ref>

It lived in what is now southern Germany in the [[Late Jurassic]] [[Period (geology)|period]] around 150&nbsp;million years ago, when Europe was an [[archipelago]] in a shallow warm tropical sea, much closer to the equator than it is now. Similar in shape to a [[European magpie]], with the largest individuals possibly attaining the size of a [[raven]],<ref name="Erickson etal 2009">{{cite journal |last1=Erickson |first1=Gregory M. |author-link1=Gregory M. Erickson |last2=Rauhut |first2=Oliver W. M. |author3=Zhonghe Zhou |author-link3=Zhou Zhonghe |last4=Turner |first4=Alan H. |last5=Inouye |first5=Brian D. |author6=Dongyu Hu |last7=Norell |first7=Mark A. |date=9 October 2009 |title=Was Dinosaurian Physiology Inherited by Birds? Reconciling Slow Growth in ''Archaeopteryx'' |journal=[[PLOS One]] |volume=4 |issue=10 |page=e7390 |bibcode=2009PLoSO...4.7390E |doi=10.1371/journal.pone.0007390 |issn=1545-7885 |pmc=2756958 |pmid=19816582 |display-authors=3 |doi-access=free }}</ref> ''Archaeopteryx'' could grow to about 0.5&nbsp;metres (1.6&nbsp;ft) in length. Despite its small size, broad wings, and inferred ability to fly or glide, ''Archaeopteryx'' has more in common with other small [[Mesozoic]] dinosaurs than it does with modern birds. In particular, it shares the following features with the [[Dromaeosauridae#Deinonychosauria|deinonychosaur]]s ([[Dromaeosauridae|dromaeosaur]]s and [[Troodontidae|troodontid]]s): jaws with sharp teeth, three fingers with claws, a long bony tail, hyperextensible second toes ("killing claw"), [[feather]]s (which suggest [[homeothermy]]), and various skeletal features.<ref name="Yalden 1">{{cite journal |last=Yalden |first=Derek W. |author-link=Derek Yalden |date=September 1984 |title=What size was ''Archaeopteryx''? |journal=[[Zoological Journal of the Linnean Society]] |volume=82 |issue=1–2 |pages=177–188 |doi=10.1111/j.1096-3642.1984.tb00541.x |issn=0024-4082 }}</ref> These features make ''Archaeopteryx'' a clear candidate for a transitional fossil between dinosaurs and birds,<ref name="UCal MoP">{{cite web |url=http://www.ucmp.berkeley.edu/diapsids/birds/archaeopteryx.html |title=''Archaeopteryx'': An Early Bird |author=<!--Staff writer(s); no by-line.--> |website=[[University of California Museum of Paleontology]] |publisher=[[University of California, Berkeley]] |___location=Berkeley, CA |access-date=2006-10-18}}</ref> making it important in the study both of dinosaurs and of the origin of birds.

The first complete specimen was announced in 1861, and ten more ''Archaeopteryx'' fossils have been found since then. Most of the eleven known fossils include impressions of feathers—among the oldest direct evidence of such structures. Moreover, because these feathers take the advanced form of [[flight feather]]s, ''Archaeopteryx'' fossils are evidence that feathers began to evolve before the Late Jurassic.<ref>{{harvnb|Wellnhofer|2004|pp=282–300}}</ref>

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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 |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>

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