BURKE et FEDUCCIA: A QUERY CONCERNING THE EMBRYONIC THEROPOD MANUS

[Tetanurae <Tetanurae@aol.com>]

I have just recieved a copy of the paper by Burke and Feduccia concerning
digit homologies in birds:

Burke, A C, and Feduccia, A.  1997.  Developmental patterns and the
identification of homologies in the avian hand.  Science 278:666-668.

Though the plates are quite compelling, I would have to ask the authors if
perhaps their logic is somewhat circular.  With all due respoect, it is well
known that Alan Feduccia is a strong opponent to the idea that birds are
dinosaurs, so his ideas must be taken with some amount of skepticism.

To review, Burke and Feduccia followed the development of finger buds in
turtles, crocodiles and birds.  They concluded that the primary axis (ie, the
first digit to be produced) is digit IV and that the 3 digits medial to the
primary axis (which is also in line with the ulna) budded off after IV in a
cascade effect.  This is true in the crocodiles and turtles since the primary
axis can be definitively shown to be digit IV because there are three
additional digits medial to the primary axis and one digit lateral to it [for
the purposes of this post, digit I is medial, digit V is lateral].  

In birds however, only two digits form medial to the primary axis.  A small
splint forms lateral to the primary axis which Burke and Feduccia presume to
be digit V even though a) it appears to end up as a bone in the wrist and b)
does not resemble the digit V in the other embryos.

All in all, the evidence for the avian manus being homologous to digits II-
III-IV rather than I-II-III is rather compelling from these plates.  I would
ask the reader however, to imagine a hypothetical avetheropod, and that
hypothetical avetheropod's embryonic manus.  Remember that the avetheropod
manus is made up of digits I-II-III, because a clear "transition" of digital
loss can be demonstrated from say _Herrerasaurus_ [I-II-III-IV-V] ->
_Coelophysis_ [I-II-III-IV] -> _Allosaurus_ [I-II-III].

Since, avetheropods had lost all vestiges of digit IV, it seems most likely
that in an embryonic avetheropod manus, that the primary axis moved from digit
IV to digit III rather than retaining digit IV as the primary axis in line
with the ulna, budding digits I-II-III off of the primary axis, then having
the primary axis wither away to nothing and have the whole hand move
latterally so that digit III would then be in line with the ulna.

What do we have then in our hypothetical embryonic avetheropod manus?  We have
digit III as the primary axis where digits I and II bud off of.  Digit III is
also in line with the ulna.  This hypothetical embryonic avetheropod manus
would look remarkably similar to the embryonic manus of a bird.

This however doesn't falsify Burke and Feduccia's data, it simply makes the
point moot.  They have not "proven" anything with their study, just shown that
normally vertebrates have the primary axis as digit IV.  I have demonstrated
however that in vertebrates that demonstratably lack digit IV (avetheropods),
digit IV cannot be used as the primary axis and the resulting hypothetical
embryonic manus would look quite similar to the embryonic manus of a bird.

Something that would be far more convincing from Burke and Feduccia would
perhaps be a photograph that demonstrated that there were indeed three finger
buds medial to the primary axis rather than just two, as would be expected in
the avetheropod's embryonic manus.

The numerous synapomorphies found in adult avetheropod and bird hands lends
support however, to the idea that in birds, the primary axis has indeed
shifted to digit III and that birds are theropod dinosaurs.

Peter Buchholz
Tetanurae@aol.com

We take Loonies