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epidermal structures.



The current thread on archosaur origins has developed a problem. In
discussing the history of various epidermal structures,we have become
sloppy with some important definitions. Consider:

PELAGE is the hairy covering of mammals. Only mammals have hair and hence
only mammals have pelage.

PLUMAGE is the complete feather covering in birds. In terms of
morphological components, plumage is more complex that pelage

Both birds and mammals have scales, as do many other vertebrate. They are
all produced in the epidermis, and can be collectively referred to as
epidermal derivatives. The simple epidermis consist of a number of cells
layers which produces keratins as a specialized protein. On significant
difference between feathers and scales is that feathers are tubular and
scales are planer. The surfaces are probably not homologous. There is no
evidence that scales are transformed into feathers or that feathers are
transformed into scales. They share some common STEPS in development, but
consist of different structural proteins. However, scales on any
vertebrate (sharks and scales included) are not considered pelage. 

Interestingly, while there are hypotheses as to the evolution of hair, no
one talks of primitive or derived morphology of hair. Presumably the
earliest mammal had hair, but there is no fossil evidence for this. By
the same token, until recently we had no clear hypothesis for the
morphology of the a primitive feather. There are numerous hypothesis as
to WHY feathers evolved, none of them testable. The large number of
different functional roles of extant feathers is not helpful in trying to
reconstruct the morphology of a primitive one. Much less infer the
function of the earliest feathers. This  approach to reconstruct the
evolutionary history of feathers based on specific function has failed to
identify an unequivocal plesiomorphic function of feathers or to predict
a transitional series of ancestral feather morphologies.

Please be cautious in using these terms. They can only introduce
confusion in our communication and thinking about these most interesting
problems.

Cheers,

Alan


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