Another impassable gulf between birds and reptiles is feathers, which are peculiar to birds. Reptile bodies are covered with scales, and those of birds with feathers. The hypothesis that bird feathers evolved from reptile scales is completely unfounded, and is indeed disproved by the fossil record, as the evolutionary paleontologist Barbara Stahl admits:
How [feathers] arose initially, presumably from reptiles scales, defies analysis... It seems, from the complex construction of feathers, that their evolution from reptilian scales would have required an immense period of time and involved a series of intermediate structures. So far, the fossil record does not bear out that supposition.116
A. H. Brush, a professor of physiology and neurobiology at the University of Connecticut, accepts this reality, although he is himself an evolutionist: "Every feature from gene structure and organization, to development, morphogenesis and tissue organization is different [in feathers and scales]."117 Moreover, Professor Brush examines the protein structure of bird feathers and argues that it is "unique among vertebrates."118
There is no fossil evidence to prove that bird feathers evolved from reptile scales. On the contrary, feathers appear suddenly in the fossil record, Professor Brush observes, as an "undeniably unique" character distinguishing birds.119 Besides, in reptiles, no epidermal tissue has yet been detected that provides a starting point for bird feathers.120
Many fossils have so far been the subject of "feathered dinosaur" speculation, but detailed study has always disproved it. The prominent ornithologist Alan Feduccia writes the following in an article called "On Why Dinosaurs Lacked Feathers":
Feathers are features unique to birds, and there are no known intermediate structures between reptilian scales and feathers. Notwithstanding speculations on the nature of the elongated scales found on such forms as Longisquama ... as being featherlike structures, there is simply no demonstrable evidence that they in fact are.121
116 Barbara J. Stahl, Vertebrate History: Problems in Evolution, Dover, 1985, pp. 349-350. (emphasis added)
117 A. H. Brush, "On the Origin of Feathers," Journal of Evolutionary Biology, vol. 9, 1996, p.132.
118 A. H. Brush, "On the Origin of Feathers," Journal of Evolutionary Biology, vol. 9, 1996, p.131.
119 A. H. Brush, "On the Origin of Feathers," Journal of Evolutionary Biology, vol. 9, 1996, p.133.
120 A. H. Brush, "On the Origin of Feathers," Journal of Evolutionary Biology, vol. 9, 1996, p.131.
121 Alan Feduccia, "On Why Dinosaurs Lacked Feathers," The Beginning of Birds, Eichstatt, West Germany: Jura Museum, 1985, p. 76. (emphasis added)
"Both fossil and developmental evidence suggests that feathers evolved through a series of transitional stages, each the result of a developmental evolutionary novelty or, in other words, a new mechanism of growth (Prum 1999, Prum and Brush 2002, 2003). The first feathers, like those of Beipiaosaurus , were unbranched, hollow cylinders that developed from the tubular elongation (the feather germ) of a placode (Figure 9 below). The advantage of a tubular feather germ is that growth of a structure (in this case, a feather) can occur without an increase in the size of the skin itself (in contrast to, for example, scales; Prum 2005). An important step in the evolution of the first feathers was a change in characteristics of the placode. Both scales and feathers begin development from placodes, but feather development, in contrast to scale development, requires generation of suprabasal cell populations (dermal condensations) to form the follicle (see Figure 8 above). The development of placodes where dermal condensations occur, an evolutionary novelty, required changes in gene expression and timing. However, such changes are known to be an important mechanism in the origin of morphological innovations in many other organisms (True and Carroll 2002, Prum 2005). ased on Prum’s (1999) model of feather evolution, the next step after the origin of the feather follicle was the differentiation of the follicle collar into barb ridges to generate barbs (Stage II; Figure 10 below). The resulting feather would consist of a tuft of barbs extending from the calamus (Figure 10 below). Such a feather is hypothesized to have evolved before the origin of the rachis (Stage IIIA) because the rachis is initially formed by the fusion of barb ridges. In addition, barbs are hypothesized to evolve before barbules because barbules develop within layers of pre-existing barb ridges (Prum 1999). Feathers comparable in structure to hypothesized Stage II feathers have been reported from fossils of non-avian theropods, such as Sinornithosaurus mellenii (Figures 11 and 12 below; Xu et al. 2001, Norell and Xu 2005)." University of Kentucky
ReplyDelete"Nevertheless, Archaeopteryx does seem to have some features that place it pretty much exactly between the non-avian dinosaurs and the birds and one of these is the size of its brain." Evolutionary origins of the avian brain Nature, 501, 93–96(05 September 2013)
ReplyDeletenice info thanks for this articel
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