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New in TREE
Several new articles on the topic of bird digit identity and homology:
First, Frietson Galis and others write up the first time I have seen in
print the idea that theropods early in their development incorporated
Wagner and Gauthier's Homeotic Frame Shift Hypothesis and never actually
had digit I in tetradactyl or tridactyl hands. They support HFS by the
understanding of bird feet undergoeing a change in identity in the second
or fourth digit that becomes essentially digit I. Discussion involves
things that were covered earlier on this list:
"The transition from five to four digits is impossible to deduce from
the currently known fossil records. It is uasually assumed that bird
ancestors are descended from either theropods or saurischians with hands
with reduction of digits IV and V, because this is the situation in the
five-fingered *Eoraptor* and *Herrerasaurus*." (pg. 8)
They also point to work on the presence of identifying the first
chondrified digit I in birds, by Larsson and Wagner, whereas before
Hinchliffe had stained the manus of an ostrich, but had stained for
cartilage and thus did not see the mesenchymal first Anlage. Even Feduccia
and Nowicki suffered from the lack of ability to identify a first Anlage.
Frietson, G.; Kundrát, M.; and Sinervo, B. 2003. An old controversy
solved: Bird embryos have five fingers. _Trends in Ecology and Evolution_
18 (1): 7-9.
Abstract: "New studies by Larsson and Wager [sic], and by Feduccia and
Nowicki of the embryogenesis of birds undisputedly show Anlagen for five
fingers. This has important implications. First, the early presence of
digir I, and its later disappearance, indicate that the evolutionsayr
reduction of digits occurred via developmental arrest followed by
degeneration. Second, it showes that the digits in the wings of birds
develop from Anlagen II-IV. This suggests that the hypothesized descent
of birds from theropods might be problematic, because theropods are
assumed to have digits I-III."
Feduccia replies, stating that Galis et al. are wrong, as their
conclusions are "unsupported by evidence." "Homeotic frame shifts ignore
highly constrained evolutionary change in limb development, negative
genetic pleiotropic effects of deleterious mutations, and the nagging
question of why a dramatic, unparsimonious, 'expensive' evolutionary shift
would occur when the avian hand was primitively pentadactyl." (pg. 9)
Expensive? You'd think loss of Anlagen and shift of digit identity would
be beneficial and remove a drag on developmental resources, withy loss of
Anlagen resulting in a neccessary lateralization of digit identity (both
Feduccia & Nowicki and Larsson & Wagner have commented on this). Galis et
al. cited earlier correspondence in _TREE_ between themselves and Feduccia
(Galis et al., 2002, _TREE_ 17: 256; Feduccia, 2002, _ibidem_) owing to
past conflict on the issue of HFS in the feet of some birds (such as the
heterodactyl configuration, where digits I and II face caudally, and pdI
on the external foot, rather than the medial).
Feduccia, A. 2003. Bird origins: Problem solved, but the debate
continues ... _Trends in Ecology and Evolution_ 18 (1): 9-10.
[Note the title ... bird _origins_ solved. Feduccia, like many other
opponents to the dinosaur origin of birds, has failed to find an ancestor
to birds to the exclusion of dinosaurs, and this must be done before one
can "solve" the issue.]
And Larsson and Wagner also reply (not to Feduccia), but largely through
some statements that Galis et al. made involving the pattern of digit
reduction in *Herrerasaurus*, which has mcIV-V ratios similar to
ceratosaurs, though Galis et al. attempted to show this condition as too
derived in *Herrerasaurus* to be ancestral to the condition of early
Larsson, H.C.E. and Wagner, G. 2003. Old morphologies mininterpreted.
_Trends in Ecology and Evolution_ 18 (1): 10.
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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