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Re: kellner's ankle abstract
David Peters (firstname.lastname@example.org) wrote:
<Interesting assumption. IMHO this is the same reasoning that Feduccia et
al. use when they say that there's a bird ancestor out there, we just
haven't found it yet. In other words, you might be inventing an
explanation for the lateral toe when no invention is necessary. I remember
when some of the head honchos were saying that pedal digit V was somehow
genetically linked to manual digit IV and when one elongated, the other
did too. Yes, I scratched my head at that one too.>
I didn't, since it makes sense from a developmental point of view.
Development of the toes and fingers use largely the same control regions
and Hox expressions, both being dominated by sonic hedgehog, etc. In
matters of elongation, for example when TWO phalanges can equal or exceed
the length of a four to five-phalanx digit, the issue based on the
ancestral Saurian condition is one of elongation of the elements. No
evidence has arisen from fusion, nor do I think one can prove this without
an ontogeny (which Peters (2003, 2004) has refuted any other attempt to
elucidate aside from his own -- the *Pterodaustro* bonebed has been known
for several years, actually).
<Another point: phalanx reduction can come about two ways: via loss or via
fusion. The "loss" of one wing phalanx in Nyctosaurus comes about via
fusion of m4.2+m4.3 for instance. M4.4 remains pretty much the same in
morphology. When I mentioned that to Greg Brown, he said, funny, he had
never thought of that possibility. IMHO>
Phalanx reduction (or increase) or loss both occur in a developmental
way, and both provide means of testing:
1) development of a digit can be accellerated by sustained expression of
shh in the limb bud, through the active condensation zones. In bats, this
causes extremely developed metacarpals and phalanges of the manus.
2) development of a digit can, conversely, be retarded by limiting the
expression of shh in the limb bud, through the active condensation zones.
This may or may not be affected by the Hox genes in which one condensation
can be lost, for failure to express the genes nccessary to start a finger,
and the identity of digits may actually shift over to compensate.
3) limited, but well-expressed development in a digit can cause distal
condensation to cease, in which the more proximal elements take up the
role of the distal most elements, usually unguals. This occurs in mammal
digits, and why even carnivorans have claws despite thier 2-3-3-3-3
4) exaggerated development of digital condensation can possibly cause
multiple phalanges to be expressed instead of the "norm", as happens
during identity shifting, and may be why whales and ichthyosaurs, among
others, have so many phalanges.
5) duplication events can cause the identity of a bone to be repeated in
series, say for an interruption then later re-expression of the control
genes on the growth plate, but only in series, hence the polydactyly of
some amphibians, ichthyosaurs, etc.
Apply any of these developmental pathways to pterosaurs, then test.
<the same thing happens in ptero feet. Two short phalanges make one long
one on digit V.>
Based on what evidence?
<Why not go with the more parsimonious answer that the bird=dinosaur crowd
is pushing? i.e. Cladistics doesn't find ancestors exactly. Cladistics
only tells us, from all of the possibilities we input, which makes the
best sister taxa.>
No, this is not particularly correct. _Paleontology_ doesn't pretend to
know ancestors, because an ancestral form in the fossil record could just
as easily be a descendant of a lineage arising from that same ancestor,
and not BE the ancestor, or member of the ancestral population. Cladistics
finds that with a branching tree, which is expressed by means of grouping
groups of data (OTU's) together, even a 0.001% variable between sister
groups can still BE sister groups, mutually exclusive populations, as
exists between several South American barbets which nonetheless show
distinct plumage for all that they are genetically virtually
indistinguishable. Yet natural selection has developed one population,
apart from is parent population, and by the nature of taxonomy, they
become subspecies of the same species, mutual, even if one was ancestral
to the other. Cladistics finds only the "common ground," and cannot tell
you which species arose from which, only biogeography and observation of
evolutionary speciation events (in this case, sympatry).
<And there are better matches in the protorosaurs than anywhere in the
archosaurs -- at present. They just haven't been input that often.>
This is based on a biased opinion no one else has yet supported in print
since 2000, either the interpretation of pterosaurs or the relationship
with "prolacertiforms" or "protorosaurs."
<Of course you can always wait for a better archosaur. Lots of people are
doing just that.>
No one is waiting for the "better archosaur." They are using what data
they have that they can agree on, and sometimes don't, but can verify by
methods available elsewhere. So far, the latest trees from Peters,
available freely on the web, have no accompanying support, and in fact,
cannot be tested at this point. Thus ... heh ... it's not a scientifically
viable hypothesis -- yet.
Jaime A. Headden
Little steps are often the hardest to take. We are too used to making leaps
in the face of adversity, that a simple skip is so hard to do. We should all
learn to walk soft, walk small, see the world around us rather than zoom by it.
"Innocent, unbiased observation is a myth." --- P.B. Medawar (1969)
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