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Re: New feather-like fossil from the Jurassic of Kazakhstan, Dzik et al 2010

 While some parts of the original hypothesis have merit, how would
 "Stage IIIa" and "Stage IIIb" have originated?

 Independently? How then would "Stage IIIa+b" have originated?

 Sequentially? Then "Stage IIIa+b" and the later one of the two others
 would be coincident.

 It does not compute.

The way I understood it, both of these stages were hypothetical, and Prum & Brush couldn't decide if the evolutionary pathway ran through IIIa _or_ through IIIb. Only one of these stages would ever have existed then.

 It is the lack of "Stage II-but-nothing-higher" taxa or even periods
 in the fossil record that is puzzling.

I thought *Sinosauropteryx* has stage I and probably II feathers and lacks all higher stages?

As a compsognathid, it fits very nicely in phylogenetic terms. In stratigraphic terms, I don't see your problem, what with *Anchiornis* being apparently Oxfordian in age or so.

 The point where I differ is that Prum invoked a convoluted and
 partially genetically implausible, partially evolutionarily
 nonsensical two-step mechanism for first- and second-order branching
 and another mechanism for the rachis, while I'd rather see a
 branching "algorithm" superimposed on the proto-rachis (which needs
 to be nothing more than a protofuzz fiber) to get first-order
 branching and a proper rachis, and superimposed on the first-order
 branching to get second-order branching.

Ontogenetically, AFAIK, the barbs _are_ rachides that move (literally move) onto the biggest rachis. (Or onto the second-biggest one, the aftershaft.) They do not originate from branching, and they're arranged in a circle before the "conveyor-belt mechanism" starts. In Prum's model, I think, the entire circle is supposed to be homologous to a stage I feather.

 Out of curiosity: did mammalian guard hair evolve from the undercoat

Completely unknown, but the opposite seems likely; it has even been proposed that all hairs evolved from whiskers, and various old books say there's evidence for whiskers on the snout of *Dimetrodon*. Of course, neurovascular foramina can mean anything, can't they. :-/