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epidermal structures


Tom Hepp pointed out that some birds have on their legs "bird scales that
have feathery tips'. And that "It becomes obvious that somehow the two
must share a common ancestral structure".

This is indeed true. The common structure is the placode. This stage is
the first identifiable in ontogeny and common to all epidermal
structures. This concentration of cells has the potential to produce any,
and  all, the cells of the epidermal derivatives. Biochemical
differentiation begins very early with the presence of feather keratin
mRNA before any visible cytological differentiation. A placode is common
to the development of all epidermal structures. One mechanism that is
important is the nature and sequence of gene activity in the cells of the
placode. Again, this may precede any visible cell changes.

The appearance of feathers on the tips of scales is not the result of a
mature scale adding a feathered fringe. The process is that the feathers
are produced first, early in the development of the nascent scale. The
feathers are produced early (or the cells that will produce the feather
undergo differentiation and then are replaced, in sequence by cells that
are specialized to produce scales), the genetic activity of the tissue
changes, and a scale forms in the remainder of the growth sequence. This
all happens during the growth period of the molt. The process involves
changes in gene activity or regulation in both the presumptive
ramogenetic zones and the intervening cells. Differences in the timing of
gene controlling amitosis, for example, would change the subsequent
morphology. A similar process, involving the same genes (eg BMP etc)
happens as digits form. By the same token, changes in the activity of the
keratin gene families (beta keratins)  provide the appropriate structural
proteins. Remember the scale and feather beta keratins differ from one
another and are transcribed from separate gene families.

The feathering on the legs is a well regulated phenomena in several
species. Owls are ptarmigan come to mind and accompany a regular
(prebasic) molt. Feathered legs also occur as a variant on pigeons and
chickens. No systematic implications here, it's simply that these species
have been the subjects of breed experiments for probably 200 years.

I also hold that filaments, or fibers, would probably be an adequate term
to describe the fringing structures that are not plumage (specific to
birds) or pelage (specific to mammals). It assumes nothing about
composition (ie keratin, chitin, other biopolymer), geometry (solid or
tubular), or function. In any case current function probably reflects
some degree of convergence or is variable. "Hair-like" structures occur
on many different animals and plants (and even inanimate objects such as
crystals). Filaments describe the morphology. Anything further
complicates the issue as it may imply function, or another level or
organization, both of which may exceed existing data. The current example
being pelage. Pelage  defines  the combined existence of all the hairs on
a mammal regardless of individual morphology or function. It is a level
of organization as well as a morphological unit.

A neologism  such as PTELAGE, is unnecessary. The structures, often
reconstructed on pterosaurs are filaments. A more complicated term may be
fringing elements. They are indeed "hair-like". But they are not hair (as
produced in mammals) and hence not a pelage.


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