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Re: Freshwater



At 11:42 14/08/98 -0400, you wrote:

>
>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.  

THANKS FOR REPLYING. I'D BE INTERESTED IN SOME CLARIFICATION ON A NUMBER OF
POINTS (MOSTLY MATTERS OF FACT).

I AM NO EXPERT ON THIS BUT PREVIOUS LIST CONTRIBUTORS HAVE SAID THAT THERE
IS LITTLE EVIDENCE OF SMALL DINO SPECIES IN THE LATEST MAASTRICHTIAN. IS
THIS RIGHT?

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.  

AND THUS PRECARIOUSLY PERCHED AT THE TOP OF A DEVASTATED TERRESTRIAL FOOD
CHAIN. THE POPULATIONS OF MOST TERRESTRIAL VERTEBRATES WOULD HAVE CRASHED
AFTER THE IMPACT. UNDER THIS  SCENARIO, PREDATORY SPECIES WOULD PROBABLY
HAVE COME OFF WORST. (IF BAD LUCK HADN'T DONE THEM IN ALREADY)   

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! 

BUT THEY DIDN'T SURVIVE. EVEN IF BAD LUCK WERE THE CAUSE, THEY DIDN'T
SURVIVE. I AGREE THAT IF THEY HAD THEY WOULD PROBABLY HAVE KICKED 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?

ANY TIME IF YOU DOUBT THE PREMISE OF YOUR ARGUMENT.

  In which
>continents was the dust cover sufficient to allow full placental survival
>but complete non-avian juvenile elimination?  

AS ABOVE

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 DON'T KNOW. LONG ENOUGH.

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.

IS THERE ANY EVIDENCE THAT JUNENILES WERE INSECTIVOROUS? (CROCODILES ARE
NOT GOOD DINOSAUR ANALOGUES). THE "JUVENILE" ARGUMENT SEEMS VERY WEAK TO
ME. THE LITTLE DINO *SPECIES* ARGUMENT IS FINE BUT JUVENILES ARE *NOT* THE
SAME AS SMALL SPECIES. JUVENILES GROW INTO ADULTS FAST (IF THEY MANAGED IT
AT ALL) AND WOULD HAVE HAD TO FIND SOME INTACT, OPERATING ECOLOGICAL
REMNANT TO FULFIL THEIR FEEDING REQUIREMENTS (SUFFICIENT TO SUPPORT
NUMEROUS BREEDING CYCLES TO HAVE ANY HOPE OF REPOPULATING). I DON'T KNOW
WHETHER THERE WERE ANY INTACT, RELATIVELY UNALTERED REMNANTS AVAILABLE IN
TIME (I KNOW THIS BEGS THE QUESTION, BUT THE EVENTUAL EXTINCTION OF ALL
ADULTS AND JUVENILES SUGGESTS THAT DOUBT IS REASONABLE). INSECTIVOROUS
MAMMALS AND BIRDS WOULD HAVE AVOIDED THIS PROBLEM ALTOGETHER.
>
>> 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.  

I'D BE INTERESTED TO KNOW OF ANY THAT ARE AS PLAUSIBLE. 

<Why, for example, do they survive today?

??I DON'T UNDERSTAND THIS QUESTION. EXPLAIN? THEY SURVIVE TODAY FOR THE
SAME REASON(S) THAT THEY ALWAYS HAVE DONE (PROBABLY).
>
>> 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.

SAME RESPONSE AS #3