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Re: Pterosaur arm supination
Okay, moving on but staying with your scenario, rotate your humeri
laterally as far as they will go. Now your hands are in the correct
positions for matching ichnites, which is where we want to be -- but
your elbows are now in front of your shoulder glenoid, as on David
Unwin's first preferred illustration of an Anhanguera walking. No
tetrapod since Ichthyostega walks like this. This configuration
depends on humeral rotation to move the body forward, not flexion and
extension of the humerus + antebrachium, as in all other erect
tetrapods. The saddle-shaped shoulder glenoid is designed to prevent
this. It likes up and down and back and forth instead.
The fact that the walking gait is strange is not a counter-argument.
As Jim already detailed, humeral rotation is actually quite possible,
as the glenoid is not a true saddle joint. In addition, the humerus is
highly resistant to torsion, as I discussed in Munich. The morphology
is consistent with a gait that includes significant amounts of
rotation. Rotation of the humerus was almost certainly important in
flight (just like all other flying vertebrates), and it wouldn't be
shocking if it was important in walking, as well.
Mike Habib mentioned that Chris's scenario does not depend on
phylogeny: it could have happened just as easily with an iguana-like
lizard or a proterosuchid-like archosaur. This is a magic trick
designed to seduce you into thinking phylogenetic analysis is
unimportant when it comes to certain taxa.
Please don't put words in my mouth; that is not at all what I getting
at. I consider phylogenetic analysis to be important for any group.
However, phylogenetic brackets are not always informative for every
problem. The limb condition in pterosaurs is clearly apomorphic (at
least compared to currently known taxa). The forelimb structure and
joint orientation is greatly derived no matter where you root
pterosaurs. The evidence for the tendon and bone arrangement, and the
tendon homologies, is based on direct anatomical and mechanical
analysis. Phylogenetic bracketing just doesn't help in this case.
That does not mean that building pterosaur trees is a waste of time;
quite the contrary.
To use analogy: bats are also highly derived. Their wing morphology is
apomorphic, no matter where you root them. The position of bats in
mammalian phylogeny can show us what the basal state likely was, and
from which morphotype the bat condition was derived. However, to
understand how the flight apparatus works we have to look at bats
directly. We observe, for example, that many bats utilize hefty
amounts of passive, distal phalangeal bending. Other mammals don't
generally do this, and it is clearly apomorphic in chiropterans. Thus,
using ancestor-state reconstruction we get limited information. Bats
are not forbidden from having strange, derived fingers just because
their sister taxon doesn't have them. Indeed, if we use a pure
phylogenetic bracket approach, we would conclude that bats cannot fly!
Chris's scenario also asks us to believe that a lizard-like tetrapod
with short limbs (that's the picture he used) would somehow evolve a
suppinated forelimb, one incapable of grasping medial objects while
still nonvolant and having posteriorly-pointed hands.
Why is this impossible? If that's what the evidence says, then so be
it. I'd caution against trying to do on-the-fly scenario likelihood
That pterosaurs evolved as leapers (does anyone know any proterosuchid
or lizard leapers?)
There are plenty of lizard leapers. Many of them are vertical
clingers. I have a lizard highly adapted to leaping, in a vivarium
right behind me, in fact. He is currently pouncing around happily.
that turned into gliders (does anyone know any gliders that turned
into flappers without a bipedal phase?).
Bats could have. The only group for which we have very good data on
the transition to flight is birds, which happen to have bipedal
ancestry. We don't know what the deal is with bats, nor pterosaurs, in
that regard. Insect wings are not derived from limbs, so they're a
separate case altogether.
Okay. In order to maintain the anterior orientation of the three
medial fingers, they have to rotate back 90 degrees to their original
untorsioned state. Isn't that alot of unnecessary voodoo? Metacarpals
I-III are famous for coming lose during taphonomy. Being pushed up
against the big metacarpal IV and becoming cemented there seems to be
a more parsimonious explanation than all that other hoopla.
It would be a bit more parsimonious, but the anatomy suggests that
pterosaurs are just weird with regards to finger morphology.
Most damaging of all, Chris's and John's scenario asks us to believe
that a fully-functioning digit IV stopped being able to flex and
started being able to hyper-hyperextend. The transition is never
explained, nor the transitional, pre-flight motive. Chris's
explanation is based on 'how do we get from here to there?' rather
than letting the fossils guide us.
It's weird, sure, but the morphology supports his conclusion regarding
the motions and homologies of digit IV and its musculature. The fact
that we don't know how the transition worked is not really
counter-evidence (though it does leave room for some interesting
research). I don't see the lack of known, specific scenario as a major
If Chris and John's hypotheses depend on muscle scars, perhaps they
have misidentified a few of them. After all, there are no modern
analogs. Certainly, their solution is not the most parsimonious.
I'm not prepared to assume that they made mistakes in anatomical
analysis merely because the result is not parsimonious. Just because
we tend towards parsimony doesn't mean that organisms can't be
Michael Habib, M.S.
Center for Functional Anatomy and Evolution
Johns Hopkins School of Medicine
1830 E. Monument Street
Baltimore, MD 21205
(443) 280 0181