RE: Brachiosaur Defense

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

> From: owner-dinosaur@usc.edu [mailto:owner-dinosaur@usc.edu]On Behalf Of
> Larry Febo
>
> >  I'm sure a proper clarification of the functional
> >locomotor possibilities suggested by arctometatarsaly
> >in the pes of some maniraptoriform theropods is best
> >described in Holtz (1994a) only briefly recapped by
> >Hutchinson and Padian (1997).
> >
> >  Holtz, T.R., Jr. 1994a. The arctometatarsalian pes,
> >an unusual structure of the metatarsus of Cretaceous
> >Theropoda (Dinosauria: Saurishcia). _JVP_ 14: 480-519.
>
> Thanks for the ref. Jaime. On reading it a third time, (things take a bit
> longer to sink in than they used to!),I see that Dr. Holtz interprets it
> more as a force transducing structure.....(a shock absorber?). But, before
> risking further misinterpretation, perhaps I`d better wait to get the
> details straight from the horse`s mouth....(hopefully when he
> trots back to
> the office Mon morning ).

"Slide in" is more like it: the University is closed due to an ice storm,
but I had a few things to take care of.

Previous interpretations of the arctometatarsus as a kind of energy storage
and release device (either by fore-and-aft rotation or as a pistoning
motion) have been proposed... er, previously.  I suggest that these might be
in operation in some of the forms, but as the amount of motion is limited,
the amount of energy stored would not be too considerable.  Furthermore,
some of these motions would be prohibited in certain forms (i.e.,
fore-and-aft rotation in tyrannosaurids, where the crescent-shape of the
proximal mtIII would lock that bone and keep it from rotating).  I offer the
further possibility of a method of redistributing the forces from mtIII to
mtII and IV.  Further analyses await to test this...

In any case, if the energy storage system was in affect it would not allow
theropods with arctometatarsi to jump about like kung fu fighters.  The
analogy would be to the snap-ligaments in, for example, horses: it is a way
of changing kinetic energy to potential when the foot touches down and the
ligaments stretch; when the foot kicks off again the energy is released
giving an extra boost (and thereby reducing the cost of locomotion for that
step).

> PS, do any modern birds have this condition? Or are the
> metatarsals totally
> fused?

The latter, at least after hatching.

                Thomas R. Holtz, Jr.
                Vertebrate Paleontologist
Department of Geology           Director, Earth, Life & Time Program
University of Maryland          College Park Scholars
                College Park, MD  20742
http://www.geol.umd.edu/~tholtz/tholtz.htm
http://www.geol.umd.edu/~jmerck/eltsite
Phone:  301-405-4084    Email:  tholtz@geol.umd.edu
Fax (Geol):  301-314-9661       Fax (CPS-ELT): 301-314-7843