Kangaroos Ricochet, Don't They?

["Jaime A. Headden" <qilongia@yahoo.com>]

Stanley Friesen wrote:

<<How does an animal with a cylindrical head adapt
itself to scaling trees, which would require more of a
spherical femoral head?>>

  This is true. Pterosaurs and birds (including
Archie) both possess spherical femoral heads with a
distinct cervix femoralis, as do dinosaurs (sans the
sphericum). Dinosaurs, being primarily terrestrial by
most definitions and analyses (and I could just go
on..., but for the sake of ref checkers, we'll try
Holtz, 1994 and Gatesy et al. (various dates) for
starters), possess an inverted cervix femoralis, which
as I do understand is one of Novas' (1992?)
synapomorphies of Dinosauromorpha (or is it
Ornithodira [Gauthier, 1986] == { *Marasuchus* +
Dinosauria }?

<...What is the situation with _Marasuchus_ and it
kin? These may be closer to the ancestral state in
dinosaurs. Certainly it has been suggested that
_Marasuchus_ was a leaper, which may mean an
indri-like vertical leaper.>

  Kinda, but not as an arboreator [sp.?]. Almost
historically from Romer's original description,
*Marasuchus* (for use of the more complete specimen
that Sereno and Arucci (1996) removed from
*Lagosuchus* based upon a general lack of
autapomorphies in the holotype of the latter name) has
been seen as a jumper, or a leaper, but almost always
in a terrestrial context. In this sense, the idea of a
"hopper" has developed, at which point I do not know,
have to research the articles in question (pre Sereno
and Arucci, basically) with Bakker's famous "Lago
after a Lizard" drawing in _Dino. Heresies_ being
reproduced finely (and it is a good and possibly
accurate portrayal of the animal -- it even has
"dinofuzz"). However, the idea of a hopper follows the
following concept: every known hopper uses all four
limbs to locate itself, including rabbits, agoutis,
maras (heh, heh, had to throw it in), etc. Kangaroos
and wallabies (macropodids) as well as kangaroo mice
and jerboas, locate themselves differently, though,
and do not use the forelimbs at all in their
distinctive _modus locatus_, which Muybridge applied
the term "ricochet" to, also using the tail as a
propulsive element; they use the forelimbs only when
"walking."

  Given the relative shortness of the forelimbs
relative to the hind and the ridiculously long distal
limb elements in the latter (including the pes) the
likelyhood of independant fore to hind limb motion is
great to me. Hopping rodents do tend to have long
enough forelimbs, nowhere near the ratio of kangaroos.
Now, I'm going to have to draw up a log of ratios to
figure out the proper ratio, but strictly speaking,
the spine of *Marasuchus* would have to be bent out of
joint to acheive a functional quadrupedal locomotion
-- thus ricochetting seems to me to be the more likely
of the various forms of fast motion in lagosuchids
(theoretically).


=====
Jaime "James" A. Headden

"Come the path that leads us to our fortune."

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