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Re: Lizard study asks: Why go bipedal?
Surprisingly, endurance and speed both were found to be
inconsequential. Those were the two forces that I figured would have
driven the push towards bipedalism. Apparently this is not the case.
The only correlate found was that bipedalism tends to go with a faster
acceleration. Other than that, the authours are viewing bipedalism as
more of a side effect of speedy locomotion, rather than anything else.
To quote one of the authours: "The lizards were pulling a wheelie."
Also keep in mind that the fastest living lizards are all quadrupedal
runners. So, speedy locomotion in lizards does not, in and of itself,
produce bipedalism. At least, it does not do so in all clades.
Ctenosauria don't "pull wheelies" despite having a faster running speed
than the species in the paper.
What were theropods using their forelimbs for? If one believes the
majority of paleo-art out there, the answer is apparently nothing
(seriously guys, have them grab something with those claws).
Actually, most theropods wouldn't be able to grab anything with the
claws - at least, not long after the jaws had already passed it. To be
honest, it really isn't clear what those forelimbs were for.
Furthermore, what the heck do kangaroos get out of having their
forelimbs freed up? I've seen wallabies use their forearms to help
hold food, but kangaroos in general just seem to use them for > standing.
Well, 'roos do engage in male-male combat using the forelimbs. My
guess, though, would be that the freeing of the forelimbs occurred
simultaneously with the shift to saltation. The saltatory gait is
important for endurance, having massive elastic storage capacity.
One advantage alluded to, but never really elaborated on, was the
faster acceleration noted in bipeds. Though maximum speed was no
different than in a quadruped, this speed was obtained faster.
Ecologically I could see this being very advantageous.
I can, as well. The question is whether or not this hold across more
species - it would be good to know the accelerations in the fastest
running lizard species (which are quad runners), for example.
Bringing this around to archosaurs, how does one figure this affects
views on archosaur bipedality? If there is no speed benefit (barring
the acceleration component), and no increase in endurance by removing
the "Carrier's constraint," then what other reasons would cause a
quadruped to go the bipedal route?
Good question. Of course, the bipedal gain in lizards, given that they
are sprawlers, may not be a good model for understanding bipedal gaits
in archosaurs. Note, as well, that Carrier's Constraint wouldn't apply
to most archosaurs, anyway (except perhaps to crown-group
crocodilians). Apparently it doesn't mean as much as expected for
lizards, either, which I admit is a surprise.
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