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Re: Freshwater



Chris Lavers rises to the challenge of explaining the _pattern_
of extinctions.  I will attempt a rebuttal.  

On Tue, 11 Aug 1998, Dr. Chris Lavers wrote:
> 1) Under the impact scenario, photosynthesis stopped and plants died. This
> devastated terrestrial food chains and hit large animals with big appetites
> arranged in simple feeding heirarchies hardest (lots of greenstuff for
> large herbivores, lots of herbivore for large carnivores).

This works inasmuch as non-avian dinosaurs were big and part of "simple
feeding hierarchies".   But some (many?) were not.  Some were smaller and
doubtless preyed on small things like mammals, birds, and other smaller
reptiles (lizards and such) that were still present.  They were still the
fastest, baddest, open-field predators known.  Even if some of their
smaller members had survived, this should have allowed subsequent
radiations.  In an extinction "event" that allowed mammals and birds to
survive on trophic grounds (i.e., they were not dependent on mass plant
production), a few non-avians should have made it through as well.  Having
survived, they should have been preeminently situated to kick Cenozoic
butt! 

> 2) Small animals with large populations, high fecundity and low absolute
> food requirements were less at risk. Enough food could be found in the
> nooks and crannies of the environment. Insectivores in particular subsisted
> on a virtually indestructible food resource largely grounded in detritus
> feeding.

At any given point in time there
were millions of juvenile non-avian dinosaurs present (many more than
adult forms!). Many were doubtless insectivorous!  Croc babies, for
example, get by on a very different diet than adults--including insects.
At what time of the year must a bolide have landed such that mammalian
insectivores could survive but non-avian insectivores could not?  In which
continents was the dust cover sufficient to allow full placental survival
but complete non-avian juvenile elimination?  How long must such a dust
cloud be supported in the atmosphere that non-avian juveniles pass into
adulthood and perish because adult food is wanting?  I realize they grew
quickly, though.    
In addition, non-avian dinosaurs had, or at least had the potential to
have, big clutches.  They had the potential to rebounnd quickly from major
setbacks.

> 3) Rivers receive organic detritus from the land and many food chains are
> partly built on this material (detritus - invertebrate - small fish - big
> fish - turtle/croc). The flow of detritus persisted long after edible
> material firther out on the watershed had been exhausted so the food chain
> persisted also.

OK.  But plenty of other explanations for turtle and croc survival.  Why,
for example, do they survive today?

> 4) Animals at the top of freshwater food chains were ectotherms with low
> food requirements and the ability to fast for long periods.

Same as #3.