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Year : 2004
Tome : 155
Volume : 4
Pages : 187-196
Title : The origin of birds and the question of homology of the fingers of their manus : disagreements between phylogenetic systematics and developmental biology data.
Authors : J.P. MAGNOL, C. MOURER-CHAUVIRE et F. BERNEX
Summary : Since the discovery of Archaeopteryx lithographica in 1861, scientists had widely accepted the reptilian origin of birds. During the second half of the 20th century, especially following the discovery by Ostrom of Deinonychus, a dromaeosaurid (theropod=carnivorous dinosaur), theropods were closely related to birds on the light of a global similarity (phenetics).Moreover, according to Gauthier, the first results of cladistic methodology (analysis of multiple synapomorphies), subsequently confirmed this assumption.Nowadays, the thesis of a filiation between theropods and birds is nested within the most severe orthodoxy, especially on account of the recent discoveries in China (Liaoning) of «feathered» dinosaurs. Nevertheless a few non-conformist voices rose up. Feduccia and his school have particularly targeted some persistent contradictions and long-standing disagreements, for instance the numeration of the fingers of the tridactyl manus (II, III, IV for living birds, according to embryology/ I, II, III for theropods according to paleontological data). No consensus has emerged until 1999.At that date, a developmental biologist (Wagner) and a pioneer of cladistic methodology (Gauthier) accepted, for the first time, the II, III IV numeration of the fingers of living birds and proposed an answer allowing to preserve the filiation between maniraptoran theropods and birds, namely the dissociation between the developmental origin of a repeated element (prechondrogenic condensations of fingers and its subsequent individualization into a fully functional character.This homeotic transformation may have occurred during the evolution of theropods :the prechondrogenic condensations CII, CIII, CIV may have conformed to the Morse’s law (loss of fingers I and V) but the identity of the definitive fingers may have changed -reflecting a frame shift- in later ontogeny (CII —> DI, CIII —> DII, CIV —> DIII). Later on, several independent works have demonstrated that it is possible (under experimental conditions) to obtain an «anterior» homeotic transformation of the condensations II, III, IV into definitive fingers DI, DII, DIII by genetic manipulation (Hox, Shh, Gli3...). So, Wagner and Gauthier’’s hypothesis, elegant and intellectually attractive as it appears, is up to future cladists to prove materially, which seems particularly uneasy given the complete lack of data about fossil hand’s ontogeny.
Keywords : dinosaurs - birds - fingers - manus - homeotic transformation.
Correspondence : J.P. MAGNOL
Adress : Service d’Histologie, Anatomie-Pathologique, École Nationale Vétérinaire de Lyon, 1 av. Bourgelat, 69280 Marcy l’Etoile, France.
Link : pdf

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