[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]
Re: New feather-like fossil from the Jurassic of Kazakhstan, Dzik et al 2010
> I take the completely opposite view. The current
> evo-devo hypothesis
> spearheaded by Prum and Brush appears well supported, with
> marshalled from both the fossil record and molecular
... as far as were published back in 1999. Prum & Brush 2002 are certainly not
so optimistic on the value of the molecular data available even then. But that
all has changed.
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
It does not compute.
> > Undisputable evidence of stages III and IV has not, I
> think, been found yet
See Kellner in Chiappe & Witmer 2002. Actually, the bulk of the fossil record
seems to be semiplumes and symmetrical vaned feathers and stuff that is
intermediate between these (i.e. anything from "Stage IIIa" to "Stage IV").
Probably 8 in 10 isolated feathers belong in that group. In the absence of any
good case for BAND, we have to assume that these feathers are all from
theropods, or at least dinosaurs, as long as this is reasonable. (It is
It is the lack of "Stage II-but-nothing-higher" taxa or even periods in the
fossil record that is puzzling. Pratt's hypothesis does not square well with
this, and to invoke absence of evidence will only take one so far. Especially
as feathers do not fossilize that badly, and the node which separates "Stage
I-and-perhaps-Stage II" taxa from "anything goes" taxa can be quite well
pinpointed by now.
> As for the contention that the evo-devo hypothesis is
I didn't make that, and indeed anyone who'd make that would be the teleologist.
The only way to consider Prum's hypothesis teleological is to approach it with
a preconception of feather evolution as a process of straightly increasing
complexity. Neither Prum says so, nor would I.
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.
In other words, Prum takes a protofuzz fiber, shortens it, then lengthens it
again as soon as barbs have been evolved; in how far would this be
parsimonious? The ontogeny of rachis and barbs appears to be linked; why should
such a mechanism evolve in two steps?
(Possible hint: the barbs of tinamou feathers do not fully separate. They
remain permanently joined a the tips. Perhaps all speculation on feather
evolution is completely off track in assuming an additive - growing keratin
patterns - rather than a subtractive mechanism - removing keratin in patterns.)
> with certain stages being inherently maladaptive; again,
> where is the
> evidence for this?
The fitness exhibited by crown Aves lacking contour plumage and exposed to the
elements, even if kept protected from predators and well furnished with food in
a tropical climate, is not especially high. Take away the protective covering -
contour feathers or brooding parents - and rachisless feathers do in fact seem
to be fatally maladaptive.
Though earlier theropods would have differed in some thermodynamic aspects, a
lack of presence of "Stage II" in the absence of at least "Stage IIIa+b" *must
be falsified* before "Stage II" can be accepted at that place where the
hypothesis puts it, because:
> hypothetical morphological stages can be found among real feathers of
> modern birds. This directly refutes the contention
> that certain
> stages are selectively disadvantageous, and are therefore
> as intermediate morphologies.
How so? If they *co-occur* in modern birds (and they are also contemporary the
fossil record), no information on the sequence in which they evolved is given,
nor is any information given on each type's viability on its own.
And if the fitness of traits (a,b) is 0 ~ w(a) < w(b) << w(a,b), a,b would get
exceptionally common eventually, but this would still be entirely
noninformative. What would be informative is the acquisition of b; the
acquisition of a is also marginally informative at beast, because it has to
precede acquisition of b.
At least in crown Aves, "Stage II/IIIb" are only reliably functional because
"Stage IIIa/IIIa+b" are also present.
A plesiomorphic state that depends on an apomorphic state derived from it for
viability is impossible.
Then, one-step evolution of flat branching structures from three-dimensional
tufts - you know any cases where that occurred?
The necessary intermediate would seem to be a three-dimensional branching
structure; we don't have these in feathers (except perhaps in
_Sinornithosaurus_), but filoplumes might match. Wouldn't make the hypothesis
any more parsimonious though; and as regards _Sinornithosaurus_, it was too
advanced and removed from Aves to have any real bearing on the issue. Prum's
hypothesis virtually requires Jurassic feathers of down-like or
_Sinornithosaurus_ "branching filament" type preceding the first incipiently
*A lot* of "protodown" or "branching filaments".
How many could be expected given the overall density of the feather fossil
record? How many have been found, except those associated with
_Sinornithosaurus_? What is their age?
Evolution the other way round is ubiquitious. By shortening the main axis of a
branching structure, you'll eventually have to end up with a three-dimensional
tuft (particularly if your structures are closely spaced), even if the initial
structure was flat.
Another thing to check:
Back/upper belly plumage of Thamnophilidae. Definitely underappreciated.
_Dysithamnus mentalis_ for example is a species where it can be observed well
and which is usually easily procured.
and http://www.flickr.com/photos/10345314@N08/3363232267 give an impression,
but skins are better (it helps if the feathers are not in top shape).
Compare to, say, _Sturnus vulgaris_.
Why are "Stage IIIa", "Stage IIIa+b" and "Stage IV" considered different
feathers? In living birds, they are a grade within the very same sort of
Out of curiosity: did mammalian guard hair evolve from the undercoat hair? Did
the human-type gastrin pathway evolve before the secretin pathway?