Prum vrs Maderson. Feathers are flying!

In Richard Prum's 1999 paper 'Development and Evolutionary Origin of Feathers', Richard Prum creates a theory to explain how feathers could have evolved from keratinous hairs, like those found on some theropods. He says that we should not base our theories on adaptive and functional explanations, these he says, are a hindrance. Rather they should be based on a transition from the simplest follicle, to more advanced ones.

Problematic is that his theories are not based on ontogeny, the examination of events in embryos. Prum states: 'the stages of the model (his model) are inferred from the hierarchical nature of development mechanisms of the follicle, rather than from an analysis of the ontogenetic progression of plumages grown within the follicles of birds.' When he does refer to an embryonic study, he says: 'During the development of the first feather papillae in the embryo (before day 12 in the chick Gallus gallus), the barb ridge primordia appear as longitudinal condensations within the feather papillae before the follicle and collar are fully formed.' He then says that this must be derived because the follicle must have came first. He admits that extant downs 'indicate that they were derived from closed pennaceous feather morphologies', but in his model, downs must have come first. You may see why this theory began to bother me.

In contrast, the model of Paul F. A. Maderson and Lorenzo Alibardi (2000) is solidly based on embryonic data and function. They ask, what would be involved for a transition from scales to feathers? They present many embryonic skin studies, and consider, what steps would be required for a transition from the skin of a basal amniote, to a squamate, to that of a crocodilian embryo (considered to be basal archosaur skin), to that of a feathered chicken embryo. They source very recent studies of embryogenesis integumentary development. They seriously consider gene expressions and known laws. They put their findings in a model, a few areas are logically deducted as not all stages are yet know in TEM studies. Read it for the model:

Paul F. A. Maderson and Lorenzo Alibardi (2000). 'The Development of the Sauropsid Integument: A Contribution to the Problem of the Origin of Feathers'. Amer Zool., 40: 513-529

They logically theorize that a keratagenin protofeather was first manipulated by muscle, which thus helped form the first collar. 'A protofollicle and protofeather possess the faculty of controlled maneuverability for later selection.' They conclude: 'Regal's thesis that movable, elongated scales, initially facilitating behavioral thermoregulation-not insulation per se, had a form proadapted for later selection in both aerodynamic and insulatory contexts.'

Feathers evolved in non-endothermic birds. They would be ineffective heat shields because they evolved in tracts. While Maderson's model allows feathers to have evolved for manipulated thermoregulation and flight, it does not require a covering of hair on an ectothermic animal. When insulation was later advantageous on endothermic birds, feathers were easily able to adapt to that function. Finally, a great feather paper and conclusions that makes since! I think I'm going to cry.

P.S. Maderson, an author on feathers since at least 1972, believes that the plumages on Longisquama are feathers.

Thanks,

Evan Robinson

The problem of course is that these hairlike structures have not been proven to be keratinous.

That claim reminds me of what Lazell said in 1994 (Herpetol. Rev.):

'Frost and Etheridge (1993) state that 'leading systematists' cleave to the cladistic party line. Maybe so, but that begs many questions: leading in what sense? Number of new names coined for old taxa? Number of pages published divided by number of new facts presented (ah, but division by zero is impossible). Whom do they lead? Where do the followers get led to in terms of understanding evolution?'

Cladism has led its followers into believing that they should not seek an adaptive or functional explanation for the evolution of morphological characters! Some morphologists must think that organisms evolved in a vacuum.

That is simply Wagnerian ground plan divergence articulated: evolution proceeds from the simple to the complex in a stepwise progression. The problem of course is that real evolutionary history is not so orderly or progressive. Organisms cannot anticipate the future function of a morphological feature nor can they build it up gradually in anticipation of the form of the final end product, as Prum's model of feather evolution would suggest. Every intermediate step in the evolution of a complex character must be adaptive and functional, as evolutionary biologists know. And there is simply no adaptive or functional explanation for some of the intermediate stages of feather evolution in Prum's model (these proposed intermediates are neither aerodynamic nor insulatory), unless one invokes the ad hoc hypothesis of 'display.' And as S. J. Gould has pointed out, the Burgess Shale organisms are just as specialized and complex as are living organisms, because they too are adapted to their environments. The Burgess Shale organisms are just as much a product of natural selection as are living organisms.

As Feduccia (1996:130) points out, '...if feathers evolved initially as thermoregulatory structures, then the contour feathers of modern birds should be essentially similar to the original feather, that is flight feathers should be secondary.' Instead, contour feathers are '...miniature flight feathers and are thought to derive from them (Parkes 1966, 83).' That would suggest flight feathers are close to the form of the original feather. And indeed the fossil record has confirmed this theory, since Longisquama's feathers appear to be adapted for flight, as Sharov noticed, but not for thermoregulation.

Indeed, there is no reason for feathers to be arranged in tracts if they originally evolved for the purpose of insulation. Yet the contour feathers of most birds are arranged in tracts. In fact, in ostriches, the feathers are hairlike and they are arranged more uniformly on the body, but the ostrich embryo retains the feather tracts, 'proving that continuous feathering is secondary (Beddard 1898, personal observation.)' (Feduccia 1996:128). This is further evidence that refutes the hypothesis that feathers originated as theromoregulatory structures.

Longisquama's feathers show that feathers evolved originally for the purpose of gliding flight. The oldest known feathers are virtually useless as insulation. No wonder the enantiornithine and sauiurine birds show growth rings in their bones.