RE: bipedal crocodylomorphs

["Thomas R. Holtz, Jr." <tholtz@geol.umd.edu>]

> From: owner-dinosaur@usc.edu [mailto:owner-dinosaur@usc.edu]On Behalf Of
> David Peters
>
> >>>>
>
> The notion of bipedalism among various diapsids is over 100 years old
> and I'm not sure who first coined the idea ? but if you're looking for
> analogs and evidence, IMHO, the worst way to answer this question is to
> go searching among synapsids (sorry Jaime). They're different in every
> way.
>
> Rather, seek those answers among the diapsid taxa that you know are
> bipedal and then work backward phylogenetically  to the quads. Between
> them try you'll be able to determine what marks them as different
> morphologically.
>
[snip]

> A modern look at the subject would be refreshing, since it is my
> impression that the earliest notions were just that, notions, without
> much substance other than relative limb length and the authority of the
> author. I mean, really, what did they have to work with back then?

Part of the problem here is the consideration of bipedality and quadrupedality 
as mutually exclusive conditions. Instead, they are
part of a continuum. MANY lizard species, for instance, are bipedal during 
their rapid phase of locomotion. This is not just true of
big charismatic ones (basilisks, frilled lizards, etc.): this is true of a vast 
range of forms. Indeed, this may be the ancestral
condition for Diapsida: sprawling quadrupedality at low speeds, sprawling 
bipedality at high speeds.

See pretty much any lizard locomotion paper for the last decade or so for 
details. Examples include:

http://www.biology.uc.edu/faculty/jayne/videos.htm (video footage)

Bauwens, D., Garland, T. J., Castilla, A. M. and Van Damme, R (1995). Evolution 
of sprint speed in lacertid lizards, morphological,
physiological and behavioral covariation. Evolution 49, 848-.

Fieler, C. L. and Jayne, B. C (1998). Effects of speed on the hindlimb 
kinematics of the lizard Dipsosaurusdorsalis. J. Exp. Biol
201, 609-.
Irschick DJ, Jayne BC.  1999.  Comparative three-dimensional kinematics of the 
hindlimb for high-speed bipedal and quadrupedal
locomotion of lizards.  Journal of Experimental Biology.  202:1047-1065.

Irschick DJ, Jayne BC.  1999.  A field study of effects of incline on the 
escape locomotion of a bipedal lizard, Callisaurus
draconoides  Physiological and Biochemical Zoology.  72:44-56

Snyder, R. C (1949). Bipedal locomotion of the lizard Basiliscus basiliscus. 
Copeia 1949, 129-.

Snyder, R. C (1952). Quadrupedal and bipedal locomotion of lizards. Copeia 
1952, 64-.

Snyder, R. C (1962). Adaptations for bipedal locomotion of lizards. Am. Zool 2, 
191-.

                Thomas R. Holtz, Jr.
                Vertebrate Paleontologist
Department of Geology           Director, Earth, Life & Time Program
University of Maryland          College Park Scholars
        Mailing Address:
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