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RE: AW: New feather-like fossil from the Jurassic of Kazakhstan, Dzik et al 2010
> I wasn't aware that some of the stages were currently
> disputed.Â Do you have a reference for that?
The stages as fossil entities are (for the most part) not disputed. But the
phylogenetic hypothesis squares not that well with BAD.
Start with Kellner in Chappe & Witmer 2002: "A Review of Avian Mesozoic Fossil
Google Scholar might help you locate primary literature on fossil feathers. It
is a bit outside the usual paleontological scope, but there is quite a lot of
it (gscholar: 23,700 hits for *fossil feather*).
Map that on Fig. 6 in Prum & Brash 2002. The pattern anticipated there - either
you have branching feathers or you haven't, but if you have you may have *any
kind of them* - is not weakened. On the contrary.
What we have initially is a sort of coarse fur = protofuzz. Generally
unbranching. Quite old. Macroscopically probably not really different from
pycnofibers or mammalian hair. Does not fossilize very well (it probably shed
much like mammalian hair); associated with skeletons it's everywhere, but even
in amber it seems to be rare (in rock it is simply unidentifiable I'd presume)
Then there are actual feathers. There is very little really good old material,
but by the very start of the K, theropods were already molting all over the
place. And it is simply not possible to consistently assign unassociated
material to a major theropod lineage. Even flight feathers may come from at
least two major lineages. Note also that the absence of barbules in fossils
does not confirm the absence of barbules in life ("unbarbuled" flight feather
fossils exist). And for vaned plumage to be monophyletic, it must have evolved
far earlier than it's good for BAD, and it also raises the question of the lack
of vaned feathers in many taxa.
One feather type is ubiquitious, both today as in the fossil record - "body
feathers"; essentially, a grade from weaker and decomposed semiplumes
(sometimes lacking barbules wholly or partly) to stronger ans wholly or partly
vaned contour feathers. If anything, the "weak" extreme (semiplumes) was more
common 130 Ma ago than today. But fully vaned (but otherwise underived, i.e.
not flight-adapted) contour feathers were also numerous, and some more derived
feathers *than that* are known (mostly in association) from the early K and
became widespread outside crown Aves eventually.
(The following uses the terminology from Prum & Brash 2002
http://people.eku.edu/ritchisong/FeatherEvolution2.jpg - which incidentially
established a evolutionary genetical scenario, Fig. 4, which is nice but
totally unfounded. It may be correct, but only by chance; it is not based on
significant amounts of actual genetic data, cf. p.290-291. This is all the more
unfortunate since they were partly right, but assumed perhaps far too complex a
mechanism; within two years, things like
http://www.eeb.yale.edu/prum/pdf/Harris_etal_2002.pdf and http://bit.ly/9AqnsT
- note "Table 1" - and http://bit.ly/97XXdI and
But all the known fossil diversity - except "protofuzz" - can be found in crown
Aves (A visit to your local bird collection is essential at some time or
another. The diverse feather derivations in Aves need to be appreciated up
close for the problem to be fully understood.) Contour plumage in most
individual birds runs the entire range from "Stage IIIa" to "Stage IV". And if
you have read the pattern-generation paper I linked (you may also want to check
out "The algorithmic beauty of plants"), you might get a hint what happens
there. Self-similar structures in nature are usually produced by iterating a
genetic pathway, not by utilizing different pathways.
In short, the mechanism making the most basic feathers found in crown Aves
(semiplumes with vestigial barbules) can also produce ANY type of feather-like
structure to date recorded from other theropods - except "protofuzz".
Thus, it is *probably* far more parsimonious to assume that "Stage II" was a
derivation of "Stage IIIa" (parallel to modern down, "Stage IIIb"), and that
"Stage IIIa" to "Stage IV" - possibly even "Va" - was technically really one
step (acquisition of the mechanism - probably a (bio)chemical oscillator, think
BelousovâZhabotinsky reaction or see the hair paper above - that causes
periodically branching beta-keratin deposits to form smaller branches of
beta-keratin deposit periodically; the frayed ends of the distal barbules may
simply something that would have to be actively prevented at that scale).
Whether to loop the program ("Stage IV") or not ("Stage IIIa") is less
significant; as soon as the "program" is there it *can* be looped (and such
evo-devo loops are ubiquitious in nature).
The fossil record is likewise not in favor of an extended period of "Stage
IIIa", let alone "Stage II/IIIb". It becomes quite dense when the evolution of
vaned feathers ("Stage IV") was in full swing, but even though "protofuzz"
survived en masse, there are few if any or "Stage IIIa-only" taxa in the early
K, and I am not aware of an unequivocal "Stage II/IIIb-only" taxon.
>From another aspect, too, "Stage II/IIIb" is the weakest point (apart from the
>rather implausible "Stage IIIa+b"). Except in altricial nestlings, without
>covering feathers "Stage II" and "Stage IIIb" are definite "fitness: 0"
>candidates; even semiplumes might be significantly detrimental without at
>least partically or incipiently vaned feathers to cover them. But again, there
>is no indication that the transition between semiplumes and vaned feathers
>requires a major genetic innovation.
So, I'd like to see this tested:
"Stage I" -> "Stage IIIa-IV" grade -> "Stage II", "Stage IIIb", "Stage Va",
"Stage Vb" (all independently derived).
One step to a semiplume, half a step to a vaned feather, everything else is
adaptive radiation from there.
Applying this to the phylogeny in Prum & Brash's Fig. 6, the avian-type feather
seems to have evolved coincident with the origin of the Maniraptoriformes,
possibly a bit earlier. From thereon, the question of how much the barbule
system was actually realized in an individual taxon's individual feather was
governed ontogenetically and by necessity. Most flightless theropods - if not
nude - could do with strong semiplumes ("open pennaceous feathers" of Prum &
Brash 2002), barbules so weak they would not usually survive fossilization, but
in combination with the physical properties of the material strong enough to
maintain feather shape under the normal operating parameters.
Things like this, essentially:
http://thumbs.dreamstime.com/thumblarge_354/1232062798Z2dUBn.jpg - not really
vaned, but not really semiplumes either. These are Green River fossils and a
Recent specimen, but the bulk of the Mesozoic record does not look much
In short, there is quite little indication that anything that could produce
branching feathers could not also produce something that came pretty close to a
vaned feather. Prum's scenario (originally from 1999, mind you!) requires the
opposite, and more (a fairly implausible transition from tufted to branching
feathers, without any solution for the maintenance problem posed by tufted
feathers in the absence of a protective covering of branching feathers and/or a