Re: cause of Gigantism in sauropods

["Habib, Michael" <MHabib@Chatham.edu>]

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 
confidence.

Cheers,

--Mike


Michael Habib
Assistant Professor of Biology
Chatham University
Woodland Road, Pittsburgh PA  15232
Buhl Hall, Room 226A
mhabib@chatham.edu
(443) 280-0181