Re: Lack of Running Giant Theropod Tracks

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

On Nov 28, 2010, at 3:07 PM, don ohmes wrote:

>>> 
> 
>> While a thought experiment such as this may seem intuitive, 
> 
> It does not "seem intuitive". It is simple, relevant and extremely 
> convincing. 
> Care to run through a few trials? Take it out o' the thought category?

It is conceptually simple - which makes it intuitive.  Whether or not it is 
relevant is questionable; the relevance of a scenario is difficult to validate. 
 Being convincing is a matter of opinion.  All told, I would consider the idea 
to fall into the category of intuitively intriguing but difficult to test or 
validate.  Not sure how a relevant trial would be run.

> 
>> the reality is that large vertebrate predators in modern systems rarely 
>> attack 
>> prey many times larger than themselves, 
>> 
> 
> There is no extant analogue to the vulnerabilties of the sauropod and the 
> strengths of the theropod. That they evolved together is in itself 
> convincing. 

A lot of potential correlative pairs exist if we go looking for them; they are 
not convincing in and of themselves.  Now, if you can actually show a 
statistical correlation consistent with co-evolution, you'd have something more 
to work with.  Right now the "evolved together" observation is a rough and 
qualitative assessment - that doesn't make it wrong, but it isn't a formal 
test, either.

> And exceptions abound. How are macro-statistical analyses relevant?

Macro-statistical analyses are relevant because they provide information on 
constraints and general patterns.  If a given pattern holds the vast majority 
of the time, with only scattered exceptions, then it provides a good null model 
for data-poor systems in which the general patterns are unknown.

>> the observation is that size matters a great deal.  
> 
> The idea that a theropod sitting under a tree would not chomp down on a 
> browsing  
> sauropod head because there was this huge edible bulk *10 meters  distant* is 
> weak. No other way to put it.

The argument that sauropod size would not be relevant to predation resistance 
is weaker still, however, because living macrovertebrate predators rarely 
attack animals many times their size, *regardless of morphology* - to make a 
convincing argument that theropods regularly predated giant sauropods many 
times their size, you must therefore demonstrate, with more than a thought 
experiment, that the morphology of sauropods was sufficiently unique to 
overwhelm this general trend.  Your behavioral scenario seems reasonable enough 
- a theropod attacks a sauropod at the neck and kills it - but your assertion 
that this was a common event that drove the evolution of giant theropods is 
currently weak as it appears to rely entirely upon such behavioral speculation. 
 The argument would be stronger if, for example, you could demonstrate that the 
largest theropods nearly always lived alongside very large sauropods (in a very 
specific sense, not a broad timescale sense), or that theropods had some 
specific mechanical adaptations that are better explained by predation on 
sauropods than as adaptations to predation on alternative prey.

To place the issue in context, consider that we can easily create circumstances 
or scenarios in which living macropredators should be able to kill animals much 
better than themselves, but observe that these cases rarely occur.  
Furthermore, given the current data on life histories in sauropods, it appears 
that juvenile mortality was very high (many eggs, few adults), but adult 
mortality appears low (old individuals relatively common among adults).

> 
>> In terms of surprise, I find myself personally surprised at the continued 
>> focus 
>> on predation of adults, given that our
>> current knowledge of dinosaur reproduction strongly implies that there was a 
>> vast biomass of juveniles present for
>> many species.
> 
> Right. The mega-theropods were huge because they ate small agile prey. 
> Ho-kay. 
> That said, the biomass was there, apparently.

Most terrestrial vertebrate predators feed on prey much smaller and relatively 
more agile than themselves.  Because both features are relative, there is 
nothing strange about mega-theropods feeding on small agile prey; it's just 
that "small" might be 500-1000 kg, which would be "large" in a modern 
ecosystem.  Size intervals that are made up of rarely-predated adults in the 
modern world would have been heavily composed of more commonly-predated 
juveniles in a Late Jurassic to Cretaceous ecosystem.

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