On a few embryonic structures forming evolutionary connections between the two Sauropsida classes (morphogenesis of the nasal capsule, the epithelial nasal tube and the organ of Jacobson in Sauropsida XVI)
Slabý, O.
Folia Morphologica 33(2): 192-200
1985
ISSN/ISBN: 0015-5640 PMID: 4029830 Document Number: 255812
In a study of the morphogenesis of the nasal apparatus of various Lacertilia species and several birds, the author found that mutually similar structures, which could be assumed to be homologous and to be evolutionally connected with each other, were recapitulated in representatives of both classes in the early stages of morphogenesis. For instance, a trace of the anlagen of the part of the main nasal tube cavity known as the aulax is to be found in Gecko verticillatus Laur. and in Varanus, but in Iguana iguana L. it is more pronounced. Anlagen of a structure resembling the anlagen of the avian vestibular turbinal (the atrioturbinal) can also be found in some Lacertilia, especially Chamaeleon embryos. Lastly, a prenasal process is present in Iguana, Tejus and Varanus embryos. On the basis of these findings and studies and of various theories on the origin of birds, the author concludes that the origin of the birds must be sought among ancient Permian or Triassic common ancestors of the groups Eosuchia, Crocodilia and Dinosauria (including Pseudosuchia) and birds, since precisely during the Permain and the Triassic environmental changes were such that a far more frequent incidence of mutations (including the development of plumage) can be presumed. When given characteristic structures which appear during morphogenesis in birds are also found during the morphogenesis of members of Lacertilia, they can be regarded as a kind of "preadaptation". All the structures discussed here were considered to be "new acquisitions" of birds. They are thus not "true" preadaptations like those so much discussed in evolutionary biology.