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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. 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 
Feathers".

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 
http://onlinelibrary.wiley.com/doi/10.1111/j.1432-0436.2004.07209004.x/abstract 
were published.)

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://www.fossilmall.com/Western-Fossils/vert2/VE16D.jpg, 
http://www.fossilmall.com/Western-Fossils/vert5/Aves-L.jpg, 
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 
different.

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 
preen gland).


Regards,

Eike