Polydactyly in stem-tetrapods: Difference between revisions - Wikipedia

m

m

Line 4:

[[Tetrapod]]s evolved from animals with fins such as found in [[lobe-finned fish]]es. From this condition a new pattern of limb formation evolved, where the development axis of the limb rotated to sprout secondary axes along the lower margin, giving rise to a variable number of very stout skeletal supports for a paddle-like foot.<ref>Coates, M.I. and Clack, J.A. (1990): Polydactyly in the earliest known tetrapod limbs. ''[[Nature (journal)|Nature]]'', 347, pp.66-69.</ref> The condition is thought to have arisen from the loss of the [[Fish anatomy#Spines and rays|fin ray]]-forming proteins [[actinodin 1]] and [[actinodin 2]].<ref name="zhang2010">{{cite doi|10.1038/nature09137}}</ref>

Early groups like ''[[Acanthostega]]'' had eight digits, while the more derived ''[[Ichthyostega]]'' had seven digits, the yet-more derived ''[[Tulerpeton]]'' had six toes.<ref name=hall/> ''[[Crassigyrinus]]'' from the fossil-poor [[Romer's gap]] in early [[Carboniferous]] is usually thought to have had five digits to each foot. The [[Anthracosauria|Anthracosaurs]], which may be stem-tetrapods <ref>Laurin, M. 1998. The importance of global parsimony and historical bias in understanding tetrapod evolution. Part I. Systematics, middle ear evolution, and jaw suspension. Annales des Sciences Naturelles, Zoologie, Paris, 13e Série 19:1-42</ref><ref>Marjanović, D. and M. Laurin. 2009. [http://www.springerlink.com/content/f661861786335660/?MUD=MP The origin(s) of modern amphibians: a commentary. Evolutionary Biology 36:336–338].</ref> or reptiliomorphs,<ref>Gauthier, J., A. G. Kluge, and T. Rowe. 1988. The early evolution of the Amniota; pp. 103-155 in M. J. Benton (ed.), The phylogeny and classification of the tetrapods, Volume 1: amphibians, reptiles, birds. Clarendon Press, Oxford</ref> retained the five-toe pattern still found in [[Amniotes]], while further reduction had taken place on other [[Labyrinthodont]] lines, leaving the forefoot with four toes and the hind foot with five, a pattern still found in [[Lissamphibia|modern amphibians]].<ref>[[Michael Benton|Benton, M.]] (2005): [[Vertebrate Palaeontology (Benton)|Vertebrate Palaeontology]] 3rd edition. Blackwell Publishing</ref> The increasing knowledge of Labyrinthodonts from Romer's gap has led to the challenging of the hypothesis that [[pentadactyly]], as displayed by most modern tetrapods, is [[plesiomorphic]]. The number of digits was once thought to have been reduced in [[amphibians]] and [[reptiles]] independently,<ref name=hall/><ref>Coates, M. I. 1991. New palaeontological contributions to limb ontogeny and phylogeny; pp. 325-337 in J. R. Hincliffe, J. M. Hurle, and D. Summerbell (ed.), Developmental Patterning of the Vertebrate Limb. Plenum Press in cooperation with NASO Scientific Affairs Division, New York</ref> but more recent studies suggest that a single reduction occurred, along the tetrapod stem, in the Late Devonian or Early Carboniferous.<ref>Laurin, M. 1998. [http://www.jstor.org/stable/2411316 A reevaluation of the origin of pentadactyly. Evolution 52:1476-1482]</ref><ref>Ruta, M. and M. I. Coates. 2007. Dates, nodes and character conflict: addressing the lissamphibian origin problem. Journal of Systematic Palaeontology 5:69-122</ref> However, even the early [[Ichthyostegalia]]ns like ''AcantostegaAcanthostega'' and ''Ichthyostega'' appear to have had the forward ossified bony toes combined in a single stout digit, making them effectively five-toed.

==References==