[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: Forthcoming theropod paper of note on the manus



Stephan Pickering (stefanpickering2002@yahoo.com) wrote:

<The numbering of theropod hands -- the best paper, thus far, on some
aspects of the question, remains that by G.P. Wagner and Jacques Gauthier
-- is problematic, given the poor fossil record of basal taxa. It is
possible that avian dinoosaurs exhibit embryogenetic reduction of manus
digit I earlier than some have thought vs. the idea of (as Frietson et al.
write in their mss.) "a repatterning of the initial embryonal Anlage
followed by the development of the digits that are present later on". Much
further work is needed, indeed. But, unless I am misreading the
literature, it is presumed theropod hands consist of I, II, III. The
recent embryological research may revise this, as avian embryos show five
digits, with I and V lost. Could it be that theropod hands are not I-III,
but II-IV? Feduccia & Nowicki, earlier this year, beat the mantra drums of
avians being anything but Dinosauria on the basis of embryology -- but
they are out-of-tune.  The subject of the manus is, actually, quite
interesting. The Wagner/Gauthier Frame Shift Hypothesis proposes that,
from theropod I-III, there transpired the shift to II-IV among immediate
clades ancestral to extant avian theropods.>

Larsson and Wagner (2002: _Journal of Experimental Zoology_ 294), oddly
enough, contradict the idea of a frame shift occuring between lacertilians
and birds and destroy the idea of frame-shift entirely; they employ how
the development of the digits II-IV really are II-IV in birds, not a
transformed set of I-III, and it is not likely for non-maniraptoriform
theropods to have II-IV. Wagner and Gauthier are faulted by the fact that
their work has a lot of speculation in it, aside from embryological
observation, and that linking one to the other required a few steps not
encounted by science yet. Larsson and Wagner, on the other hand [eh,
heh...] argue that a frame shift between lacertilians and birds did not
occur, and that birds just arrested the development of digit I, improved
digit IV, and the "signature" of each medial digit was transfered
laterally, so that digit II had two phalanges, one of which was lost, and
digit III has three, and so forth; this contradicts the normal pattern of
a 2-3-4-1-0 pattern seen in *Herrerasaurus* and no fourth digit in all
other known theropods. Whatever your phylogeny, theropod finger
transformation is not see easily solved without early stage _in ovo_
embryos, which we lack for the moment beyond any identification.

  Cheers,

=====
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.

__________________________________________________
Do you Yahoo!?
U2 on LAUNCH - Exclusive greatest hits videos
http://launch.yahoo.com/u2