What is Transitional fossil?
[[File:Archaeopteryx lithographica paris.JPG|thumb|200px|The London specimen of ''[[Archaeopteryx]]'', discovered only two years after the publication of ''[[On the Origin of Species]]'']] '''Transitional fossils''' (or '''missing links''') are the [[fossil]]ised remains of lifeforms that show features typical of two distinct groups. It may be a rare fossil, such as ''[[Archaeopteryx]]'', which shows early features of a group which later becomes widespread. The rarity of transitional forms is interesting. It suggests the new forms are not yet dominant in the [[ecology]] of their time, and their populations are relatively small. It follows that their fossils would be scarce. In the [[Solnhofen limestone]] where ''Archaeopteryx'' was found, there are more examples of [[pterosaur]]s of several [[genera]] than there are of the proto-bird.<ref>up to 29 species and nine genera of pterosaur have been found there. Bartell K.W., Swinburne N.H.M. and Conway-Morris S. 1990. ''Solnhofen: a study in Mesozoic palaeontology''. Cambridge University Press. Transl. and revised from Bartel K.W. 1978. ''Ein Blick in die Erdgeschichte''. Ott.</ref> In the [[Jurassic]], the pterosaurs were dominant in the air. Large numbers of different bird species are found in the famous Chinese [[lagerstatte]]n deposits such as the [[Jehol biota]]. The Lower Cretaceous birds are clearly more bird-like, and the features they developed enabled them to invade [[habitat]] [[niche]]s which had been the preserve of pterosaurs, or maybe were not previously exploited at all. Pterosaur decline (if present) seems unrelated to bird diversity.<ref>{{cite journal |title=Estimating the effects of sampling biases on pterosaur diversity patterns: implications for hypotheses of bird/pterosaur competitive replacement|journal=Paleobiology|issue=3|pages=432–46|year= 2009|author1=Butler, Richard J. |author2=Barrett, Paul M. |author3=Nowbath, Stephen |author4=Upchurch, Paul |name-list-style=amp |volume=35 |doi=10.1666/0094-8373-35.3.432|s2cid=84324007}}</ref> In fa