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re: T. rex/scavenger



Since I brought up the subject of T. rex scavenging, let me
respond to a number of interesting points and queries in summary
below.  I apologize for exerpting items.  Also, please remember,
I am citing and defending Alexander's paper, but not necessarily
championing the work. It seems like a solid study, however.

Citations:

 Alexander, R. McNeill, 1991, How dinosaurs ran. Scientific
American, April, p. 130-136 (sorry, no volume # on my copy).

 Alexander, R. M. 1989. Dynamics of dinosaurs and other extinct
giants. Columbia University Press.


Questions & Comments:

    """1) Can a cat such as a felis domesticus or a felis leo
gallop by this same measurement?"""

---yes.

   """2) Do we have a good estimate of the actual mass of a rex -
i.e. have we included the weight-saving features that rex's avian
characteristics would imply was present? """

---qualified yes. Thomas Holtz informed us that the weight
estimates are overblown by Alexander.  I defer to that opinion.

3) Does the rex bone structure imply it is of the same strength
level as hadrosaur or ceratopsian bone?"""

--- No, theropod shaft bone has extremely dense cortical tissue
and hollow medullary cavities: strong in proportion to dimension.
Probably much stronger in the shaft than an equal limb of a
ceratopsian, but the condyles have similar internal structure and
were probably equivalent.  However, these factors were
incorporated into the analysis that led to Alexander's
conclusions

     """I don't believe that the study showed that T. rex could
not sustain running speed (to do that, one would have to show
that the stresses in the bones would exceed the strength of the
bones..."""

 ---I believe that bone strength was analyzed in terms of simple
engineering modulus stiffness (expressed as Froude #'s ). Thus,
if the data are presented correctly, Alexander showed that T. rex
has leg bone strength comparable to elephants, which as we well
know are plantigrade and can only trot.

   """For example, horses have surprisingly small radii bones for
the speeds that they can move...."

---Yes, but still, the proportion of bone to weight is much
greater than in T. rex.

    """I envision rex as stalking its prey like a lion does. It
was probably colored with camouflage (green?), sneaking through
trees and bushes until it got close enough to make a short(!)
sprinting lunge."""

---Problem is, Alexander's model militates against any sprinting,
and a broken leg would not help survival.  Also, damn few
Cretaceous forest trees in Montana would be likely to shelter T.
rex sized animals (perhaps a paleobotanist can add data here?)

   """Actually, I don't see any advantage in "armlessness" to a
scavanger."""

---Perhaps, not, but the use-it-or-lose-it argument suggests that
if there is no advantage to having forelimbs, they will go.

   """Actually, Alexander's studied was seriously flawed.  He
used an 8 tonne mass for T. rex, based on an incorrect scaling (a
hip height of 3.6 m)....However, Farlow et al. have a
paper in review which might change some of this..."""

---I'll defer to your opinion on this, but that still puts the
bone-strength estimate on the low side for running.  In which way
might Farlow's paper change estimates?

   """The evidence for running tyrannosaurids is much stronger
than for running ceratopsians."""

---I agree that the arguments for parasagital posture in
ceratopsians is weak.  I recall an impressive exhibit at a past
SVP with life-size casts of a Torosaurus pectoral girdle and
limbs showing that one has to open the joints absurdly to achieve
the Bakker-style gait.  The bones simply would not accommodate
it. And if the front legs were splayed, its hard to envision true
galloping.

      """There is, however, a major plausibilty problem with
large theropods being pure scavengers, namely, that there is no
large vertebrate known to follow this strategy in the modern
world...."""   (other arguments similar to this)

---True, but there's nothing like a large dinosaur in the modern
world. Strict uniformitarianism might hold up this argument, but
an actualistic approach wouldn't.  Basically, we now commonly
accept that dinos weren't mammalian or avian-type  homeotherms,
but, rather, inertial homeotherms. (there are valid
counterarguments, by the way). This recognizes the uniqueness of
dinosaurs. Same with behavior. It is irrelevant to model
slavishly the biology of modern faunas using extinct,
unparalleled forms. If we had a living, gigantic, terrestrial,
non-mammalian tetrapod population with mixed behaviors and
habitats to sample, and none were giant scavengers, then this
point would be well taken. But we don't, and that's the
limitation of models as data surrogates.


I've enjoyed the discussion thus far.


David Schwimmer, Columbus College
schwimm@uscn.cc.uga.edu