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Re: Tawa hallae: everything you know about basal saurischians is wrong...



Erik Boehm <erikboehm07@yahoo.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.  

For example:

       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 inh
 from 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 
evolutionary novelties.


Cheers

Tim