Oryctos
|
Accès privé :
| volume 7, 2008 |
|
Richard Hinchliffe, Bird wing digits & their homologies: reassessment of developmental evidence for a 2,3,4 identity. Oryctos 7, 7-12.
The theory of descent of birds from theropod dinosaurs demands that their fore-limb digit identities (1,2,3) are the same as those of birds and thus the conventional embryological identification of these as 2,3,4 remains a major problem for acceptance of this theory. Are the 2,3,4 identities of bird wing digits correct? The paper analyses the developing bird wing as a specialisation of the general developmental ‘bauplan’ for the pentadactyl skeleton. Evidence from the chick embryonic skeletogenic pattern supports interpretation of the main digits as 2,3,4 on the basis of timing, position & connections, using comparative methods eg comparison with other amniote patterns of limb skeletogenesis. Fresh support for 2,3,4 identity comes from evidence of i) a temporary embryonic digit 1 in the ostrich and of ii) a condensation-specific Sox9 molecular domain in a digit 1 position in the chick wingbud. In contrast, the recent ‘frame shift’ hypothesis of Wagner proposes molecular identity transformation by which theropod identities for 1,2,3 have become shifted to avian digit condensations 2,3,4. Support for ‘frame shift’ is claimed from evidence (Vargas & Fallon 2004) that the expression domain for Hox d 13 alone characterises digit 1, but a domain for Hox d 12 and 13 characterises digits 2-5. Here it is argued that this evidence for ‘frame shift’ is speculative and insufficiently convincing to support reinterpretation of the wing digits as 1,2,3.Evidence that wing digits have been correctly identified as 2,3,4 continues to provoke doubts about the ‘dinosaur-bird’ theory. The presence of well-defined feathers in the bird-like hands of dromaeosaurs (from Liaoning, China) may be due to their possibly being secondarily flightless birds, thus suggesting that birds may derive from a lineage separate from that of theropods.
|
Retour à la page d'accueil d'Oryctos
|
|
