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

Re: Pterosaur arm supination



Comments inserted...

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 estimates.


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 issue.


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 apomorphic.


Cheers,

--Mike


Michael Habib, M.S. PhD. Candidate Center for Functional Anatomy and Evolution Johns Hopkins School of Medicine 1830 E. Monument Street Baltimore, MD 21205 (443) 280 0181 habib@jhmi.edu