The quest for a plausible story about the evolution of feathers from scales has exercised many minds. A major problem has been to find evidence for primitive feathers - for these are said to be "desperately missing in the fossil record". The feathers of Archaeopteryx, located in the Jurassic, were not primitive but modern in all aspects. There are (later) Cretaceous feathers preserved in amber, but these are also modern, "featuring barbs and barbules organised in asymmetrical vanes". Evolutionary biologists have therefore been relatively unconstrained by data, and this is recognised to be an unsatisfactory situation. Previously, one particular stage was highlighted by the theoreticians, and a new research paper claims to have documented this transition: "We recently discovered several isolated feathers fossilized in an Early Cretaceous amber of France, which display a primitive structure that illustrates the early formation of this critical stage."
To give an idea of the thinking of the authors, here are comments from Didier Neraudeau:
"What is very important in our discovery is that we have found a new clear example of the gradual trend of the evolution in general and in particular in the case of the transition between a primitive filamentous down and a modern feather. Moreover, it shows that in many cases, when an evolutionary stage is predicted by the theory, it can often be found in the fossil record. It is a question of time. Thus, it does not really change our picture of evolution but it gives for the first time a proof of the gradual evolution of feathers from the primitive filaments of some theropod dinosaurs to the modern feathers of Archaeopteryx and Cretaceous birds."
Here is a part of the description. Note how small these feathers are. This description is followed by a discussion comprising three points.
"Seven identical feathers are lying side by side in the amber piece and very probably originate from a single individual. [. . .] The tangle of these feathers, their three-dimensional disposition in amber, and numerous dust grains hinder the observation of some of them. The following description is mainly based on the three best visible. These are 2.3, 1.6 and 1.1 mm long as measured along the rachis. Filamentous, long and free barbs lacking barbules are inserted opposite to each other on each side of a rather flattened rachis and form two vanes." [. . .] "These feathers are morphologically close to the down, ornamental or afterfeathers, and not to the contour, remiges or rectrices. However, they have a thick and long rachis, unlike classical down feathers whose barbs generally diverge from the very short apex of the rachis."
1. The authors are over-quick in declaring that these fossils are primitive. They are not utilising a methodology of multiple hypotheses. There are at least two other hypotheses worth considering. The first is that the fossils are of down from a chick. This must be considered because the fossils are "morphologically close" to down. Since the fossils are so small, the comparison needed is with down feathers from a newly hatched chick, not a mature bird. There is no indication that the work has been done to make any valid comparisons. The second alternative is that the fossils are degenerate feathers that have lost functionality and taken on a simplified structure. The present lack of suitable derived bird fossils should not prevent this hypothesis being explored. The approach adopted by the authors illustrates Kuhnian "normal science", where data are fitted into the paradigm. This is theory-led science.
2. The Late Albian age assigned to these fossil feathers post-dates the appearance of modern feathers in the Jurassic. Furthermore, they are contemporaneous with other fossil feathers that are modern in appearance. This is an example of the "temporal paradox" that has been also recognised with theories of 'theropod to bird' evolution. The supposed transitional forms are far too late to be transitional!
3. Theoretical discussions of the evolution of any characters generally point out some selective advantage of a developing trait. This discussion is absent from this paper. These feathers have barbs but no barbules. Further exploration of this takes us back to the two alternative hypotheses above and reminds us that "normal science" is not about following the evidence wherever it leads but fitting it into a predetermined theoretical framework.
The early evolution of feathers: fossil evidence from Cretaceous amber of France
Vincent Perrichot, Loic Marion, Didier Neraudeau, Romain Vullo, Paul Tafforeau.
Proceedings of the Royal Society, Biological Sciences, 275, May 22 2008, 1197-1202 | doi 10.1098/rspb.2008.0003
Abstract: The developmental stages of feathers are of major importance in the evolution of body covering and the origin of avian flight. Until now, there were significant gaps in knowledge of early morphologies in theoretical stages of feathers as well as in palaeontological material. Here we report fossil evidence of an intermediate and critical stage in the incremental evolution of feathers which has been predicted by developmental theories but hitherto undocumented by evidence from both the recent and the fossil records. Seven feathers have been found in an Early Cretaceous (Late Albian, ca 100Myr) amber of western France, which display a flattened shaft composed by the still distinct and incompletely fused bases of the barbs forming two irregular vanes. Considering their remarkably primitive features, and since recent discoveries have yielded feathers of modern type in some derived theropod dinosaurs, the Albian feathers from France might have been derived either from an early bird or from a non-avian dinosaur.
Highfield, R. Missing link feather fossils found in France, The Daily Telegraph, 20/02/2008
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