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Re: Tawa hallae: everything you know about basal saurischians is wrong...
Erik Boehm <email@example.com> wrote:
> If we assume the integumentary structures (dinofuzz), arose
> as modification of scales, then they were homologous.
But I don't think we can assume that feathers arose from scales. In fact, the
most recent work has argued that feathers are evolutionary novelties.
Over the last half of the 20th century, neo-Darwinian approaches
to the origin of feathers, exemplified by Bock (1965), have
hypothesized a microevolutionary and functional continuum between
feathers and a hypothesized antecedent structure (usually an
elongate scale). Feathers, however, are hierarchically complex
assemblages of numerous evolutionary noveltiesâthe feather
follicle, tubular feather germ, feather branched structure,
interacting differentiated barbulesâthat have no homolog in any
antecedent structures (Brush 1993, 1996, 2000; Prum 1999).
(From Prum and Brush, 2002; Q. Rev. Biol. 77: 261-295)
The concept of feathers as evolutionary novelties builds upon the work of Alan
Brush in the 1990's, who emphasized that feathers were distinct from reptilian
scales, aside from the presence of beta-keratin.
If the respective filamentous/fuzzy structures of certain theropods (feathers),
ornithischians, and pterosaurs are each novel (de novo) structures, then they
are not homologous.
> Bird wings and bat wings are homologous, in that they both
> come from an ancestral amniote forelimb.
The limbs of all tetrapods are homologous - as well as being homologous to the
pectoral and pelvic fins of fishes.
By contrast, the limbs of vertebrates are not homologous to the limbs of
arthropods (e.g., insects, spiders). However, there are shared developmental
genetic similarities behind the origins of vertebrate and arthropod appendages.
So similar developmental genetic mechanisms could be behind the origin of (say)
theropod feathers and pterosaur 'hair' (pycnofibres). But unless the same
precursor structure was inherited
om a common ancestor, feathers and pycnofibres are not homologous.
> Did amniotes have an integumentary structure that synapsids
> made into fur, and diapsids into feathers?
Unlikely. The two (fur and feathers) appear to be separate and independent