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Re: Re: astroblemes and extinctions



>> Actually, this is probably true, but doesn't require an impact.  Large
>> animals have, of course, larger requirements for food, water, and space,
>> whereas smaller animals can get by with less.
>
>"Less" is a relative term.  The cube-square law has an effect here, as
>does the nature of endothermy, and it seems likely to me, as a rough
>guess, that a breadbox-sized predatory mammal might need nearly as much
>food as an elephant-sized mass-homeothermic herbivorous archosaur; and
>the mammal's food requirements are much higher on the food chain. 
>
>(This is pure speculation on my part; I don't have the depth of
>knowledge to work out the real relationships here.  Can you shed some
>light, Tom Holtz?)

(I'll try...)

You're very correct in addressing several parts of the problem:
  the trophic level of the animal (herbivore, first order carnivore, second
order, etc.); and (very important) the metabolism-style of the animal.  As a
rough guess, a bread-box sized mammalian predaotr (a cat or small dog) still
requires less _mass_ of food than an elephant-sized gigantothermic herbiovre
(assuming the generalized relationship of ectotherms requiring 1/10th the
fodder as endotherms, one tenth of the massize amount of elephant's daily
meal is still a lot).

So, my statement is a simplification (maybe an OVER simplification), but
still represents a basic problem of being big:  You might need less
food/water/living space PER UNIT MASS than a smaller animal, but you still
have a lot of mass...  Put another way, five tons of mice need more food
than a five ton elephant, but the five ton elephant needs more food total
than a sustainable population of mice.

In any case, it is likely this sort of relationship which is responsible for
the general pattern of terrestrial mass extinctions: the bigger they are,
the harder they fall...  Not to say that nothing small died - many lineages
of small vertebrates died out at the K/T boundary as well.

(One final point - this pattern doesn't hold as well for the marine
environment, where many groups of small organisms, including single-celled
forms, became extinct along with the bigger rudists, ammonites, marine
reptiles, etc.)

Thomas R. HOLTZ
Vertebrate Paleontologist, Dept. of Geology
Email:Thomas_R_HOLTZ@umail.umd.edu (th81)
Phone:301-405-4084