Re: No more paedomorphosis

[Dinogeorge <Dinogeorge@aol.com>]

In a message dated 98-04-15 22:15:49 EDT, m_troutman@hotmail.com writes:

<< Norell and Makovicky have discussed this problem in their recent 
 dromaeosaur paper. The perfectly articulated pes shows the hallu 
 non-reversed in both feet. And the arrangement that you propose is not 
 seen in any birds where cursorial birds just lose the hallux ( the 
 exception is the roadrunner, where the zygodactyl foot has been turned 
 into an extraordinary running adaptation ). >>

If the hallux is mobile within the foot, it will often come to rest in
alignment with the other digits, particularly if it is directed laterally
rather than posteriorly in the living animal. Also, the position of the hallux
in cursorial, flightless dromaeosaurids does not necessarily reflect its
position in arboreal, probably ornithopting and perching (a la _Rahonavis_)
dromaeosaurid ancestors. BCF notes that the hallux is secondarily reduced and
displaced from a previously retroverted position, a cursorial adaptation.

I haven't had a chance to read this paper yet, but since the authors are
proponents of the BADD idea, they will present their findings to favor their
paradigm. Just as, for example, many people on this list are pretty sure in
advance that they will disagree with Feduccia and Martin on the subject of
bird-dinosaur relationships, and I myself place a BCF spin on everything. I've
seen and handled enough theropod foot specimens and footprints myself to be
quite convinced of my interpretation of theropod foot anatomy.

The frequent loss of the hallux in cursorial birds immediately raises the
question of why the hallux was not lost in theropods, if they had supposedly
always been cursorial and lacking an opposable, retroverted hallux. Certainly
the >fifth< digit was lost; why not the first, too, since theropod feet were
otherwise almost perfectly mesaxonic? BADD theorists must explain why,
although the hallux in theropods did not vanish, its >proximal< portion did,
leaving the distal portion with an articulated digit intact. So far, I've not
even seen an attempt at an explanation, besides some handwaving that amounts
to little more than "it did because it did."