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Re: Lack of Running Giant Theropod Tracks
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
> 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
>> 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
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
> 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
> 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
>> 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.
> 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.
Assistant Professor of Biology
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