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Re: cause of Gigantism in sauropods
On Feb 9, 2011, at 11:07 AM, Don Ohmes wrote:
> The notion that the measurement of average prey/predator size ratio in
> today's world delineates as a matter of scientific principle a null
> hypothesis about the behavioral interaction between giant sauropods and
> giant theropods is just that -- a notion, otherwise known as an opinion.
> The laws of physics directly constrain certain activities (e.g.,
> locomotion, bite force), and data from extant animals can indeed form a
> null, but this principle does not extend to behavior.
Behavior is indeed more difficult, but behavior is, itself, also constrained by
physical parameters, and some behavioral constraints are extremely widespread,
applying to a wide range of clades and morphologies. While using modern
ecologies as a framework is not without its drawbacks, we often have very few
other options. To say that the use of average predator:prey size ratios in
modern systems is nothing more than a "notion" or "opinion" is an unfair
assessment - it is a reasonable (albeit far from ideal) hypothesis that can be
falsified in a number of ways. One way it cannot be reasonably falsified,
however, is by simply constructing potential scenarios.
> Alligators are tactically capable of eating people, but statistically
> rarely do -- taking that data to form a null hypothesis about the
> behavior of the similarly capable Nile Crocodile, or even individual
> alligators, is unlikely to prove useful.
True, but if you take crocodilians on the whole, and look at the average
predator size and compare it to the average size of an adult human, you will
find that those crocodilians which are as large or larger than a human, on
average, are far more likely to eat them than those that average a smaller body
mass than a human. In fact, your example above works *against* your own
argument below - you argue that giant theropods were tactically capable of
eating adult sauropods. They may have been capable, but I suggest that this
behavior was rare, given the extreme differences in body masses that would
exist in most of those cases (there are, of course, cases where one could put a
very large theropod up against one of the smaller-bodied sauropods, in which
case the mass ratios suggest that a different circumstance altogether).
> The data, both extant and fossil, tell us that -- 1) giant theropod jaws
> could encompass the necks of even very large sauropods
If they could reach them. Even then, in actuality, I'm skeptical that the jaws
of giant theropods probably could wrap effectively around the necks of the very
largest sauropods. Furthermore, if one supposes that the jaws opened extremely
wide, the mechanical advantage at that most open position is very weak - so
even if you assume the greatest possible amount of soft tissue mobility and
joint flexibility for some of the larger theropods, it is questionable whether
the bite force at the most open position would be sufficient. Note, as well,
that there is a general tradeoff between the mechanical resistance of the jaws
and the mobility of the skull; tyrannosaurids, for example, have the strongest
skulls but the least kinesis.
> 2) bite force was such that one full-on bite to any portion of the neck and
> head was
> likely to be mortal,
Which analysis is this based upon? It involves not only analysis of predator
bite force and penetration, but also reconstruction of sauropod soft tissue.
The necks of sauropods have rarely been reconstructed in detail, especially in
cross-section. I believe Mike Taylor sent out a link to one such
reconstruction, but it was focused on muscular anatomy, and the vessels were
therefore placed somewhat arbitrarily. I don't know of any others that have
done a more complete job of this, but perhaps I've missed one (sauropods are
not my focus, after all).
> 3) the mobility of the giant theropods on hard,
> flat ground was (barring anomalous and unexpected giant sauropod
> locomotive capability) overwhelmingly superior, and 4) the likely
> relative nutritional requirements of the respective morphologies meant
> that time was very much on the theropod's side in any protracted engagement.
This is true in the case of many extant examples, as well, and yet very rarely
do highly mobile predators attack less mobile prey 8x their own size (or
larger). Just as you have produced a scenario in which a theropod can dispatch
a sauropod, I can create a scenario in which the sauropod easily murders the
theropod - it quickly becomes a "pirates versus ninjas" story (to borrow a term
used on the DML previously for this subject) unless we can show, for example,
that giant theropods possessed anatomical features that were not easily
explained in the context of feeding on prey their own size or smaller, but only
easily explained in the context of hunting giant prey. Alternatively, we might
find evidence of direct attacks on large sauropods, such as healed bite wounds,
that suggest predation attempts. Keep in mind that mutually assured
destruction does not do a predator much good - even if a giant theropod could
bite the neck of a sauropod and kill it, the predator might still be mortally
wounded or outright killed - it would be remarkably easy for a giant theropod
to get its neck broken trying to gnaw the carotid of a 40+ ton sauropod.
We just don't have the kind of information we need to reconstruct these sorts
of battles with confidence in a "blow by blow" sort of way, and therefore we
are obligated to use general trends in ecology to constrain our suppositions
until such a time that we do have that information. The mass ratio phenomenon
is not hard and fast, by any means, but it is a very widespread pattern, and it
crosses both phylogeny and morphology, which suggests it is broadly applicable
to fossil forms. Personally, I don't understand the insistence that theropods
must have been attacking prey many times their own size, risking death in the
process, when evidence suggests that there was a huge biomass of smaller
available prey (especially juveniles) available for consumption.
> Ignoring tactical capacity on grounds of conjectured behavioral
> limitations when evaluating the possible evolutionary paths by which
> this coeval morphological couplet came into full flower is neither
> useful nor "scientific".
Interesting. I would argue that making a supposition about "tactical capacity"
at a level of detail we cannot currently reconstruct, while ignoring widespread
patterns of body mass impacts on predation, is rather less useful. Unless
giant theropods possess some morphological suite of features that is
specifically associated with feeding on giant prey, rather than more "typical"
prey, then I fail to see how we can build such detailed scenarios with any
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