Birds are dinosaurs: Simple answer to a complex problem

Auk, The, Oct 2002 by Feduccia, Alan

There is substantial evidence to support the alternative hypothesis, that is, the classical scale-to-feather model, which conforms nicely with what we know about (1) feather embryology, (2) the fact that avian foot scutes can be transformed into feathers using either bone morphogenetic protein or retinoic acid, and (3) the fact that the primitive early Cretaceous birds Confuciusornis and Protopteryx have two central tail feathers that are scalelike, without branching (Zhang and Zhou 2000). However, in addition it is particularly interesting to note that in half developed ostrich embryos there is a zone of more or less delimited scales near the dorsal border of the lateral apterium. The border of each scale is a feather rudiment already sunk into a short feather-sac, near the posterior end (Holmgren 1955; A. Feduccia pers. obs.). At the margin of the scale-covered area of the pectoral callosity, particularly in Rhea, the scales appear to be combined with feathers, at the top end of each scale. This close developmental association of scale and feather would be remarkable if feathers evolved through a filament stage.

In a recent paper, Harris et al. (2002) attempt a complex model of feather evolution based on function and regulation of two intercellular signaling molecules, Shh and bone morphogenetic protein 2 (Mnp2). Despite a very laudable scientific effort and some truly remarkable developmental biology, their entire model is based on the "fact" that "Recent paleontological discoveries have documented that feathers evolved in coelurosaurian theropod dinosaurs before the origin of birds." (Harris et al. 2002: 17 74). Those specimens preserve either dino-fuzz or feathers, and the association between the two has simply not been demonstrated. There is no credible evidence to justify the portrayal on the January 2002 Auk of Microraptor with a thick, white downy coating of putative protofeathers (A. Feduccia pers. obs.).

The morphology of Confuciusornis and Longisquama integumentary appendages suggests that modern feathers evolved through the stages involving elongated scales that became broken up into barbs and barbules. The latter discovery lends credibility to the hypothesis that the elongated integumentary appendages of the late Triassic Longisquama (Jones et al. 2000b) are in fact "parafeathers." Those structures, contra Prum (2002), have a hollow, central shaft, which is compartmentalized, attach to the body by papillae, and were individually molted, as evidence by the many separate Longisquama parafeathers discovered at the fossil site. Interestingly, the scales on the posterior forearm are elongated and must have been used in steering. Minimally, this specimen proves that the most featherlike structures in a nonavian Mesozoic vertebrate are found in a late Triassic basal archosaur, a thecodont. A hypothetical arboreal, Triassic basal archosaur illustrating the scale to feather model is shown in Figure 6.

The Longisquama specimens were examined over a period of some 3-4 days in April 1999, by a team of eight scientists, including two highly respected Russian paleontologists and a developmental biologist specializing in reptile integument (see authors, Jones et al. 2000a). Prum, who proclaimed the study to be "quackery" in print, says that he "examined Longisquama with the authors in April 1999," (Prum 2001: 1899), but his examination of that exceptional, complex Triassic reptile consisted of 5 to 10 min, at most, of a cursory perusal of the main slab, and he denied the presence of an antorbital fenestra, seen by all of the authors, and established by the careful Russian paleontologist Sharov in the 1970s. Interestingly, in addition to having a birdlike skull, Longisquama has a well developed furcula.