[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: Freshwater




On Mon, 17 Aug 1998, Chris Lavers wrote:

> 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?

Around 6% of Campanian+Maastrichtian non-avian dinosaurs had adult forms 2
meters or less long. (From Graham Worth's Dinosaur Encyclopedia.) But
there could be some preservational bias--smaller fossils don't preserve as well 
as big.
 
> 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)   

I'm not sure I see a mechanism for population crashes vs. extinction.
Insectivorous mammals are, I believe, usually highly territorial.  As
such, the insects are either present or not in a particular territory.
I'm not sure they have the ability to compensate by ranging further.  Just
one more level of complexity.

> >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.

Yes. But there is enough doubt to go around, don't you think?  

> IS THERE ANY EVIDENCE THAT JUNENILES WERE INSECTIVOROUS? (CROCODILES ARE
> NOT GOOD DINOSAUR ANALOGUES).

I don't know of any evidence, but it _is_ a viable hypothesis.  Crocs,
inasmuch as they are archosaurs and that they start out small and end up
big, are very good analogues for this issue.

> 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.

I realize my insectivore dino babies argument does not last forever.  But
it does extend dino survivability appreciably.  Even if juvenile diets are
utilized for only the first two months, now you are getting into a
dangerous time for all the mammals and birds involved.  My point is that
it is not a case of bolide comes and dinos all fall down.  I would also
emphasize that dinosaur trophic relationships were probably not as simple
as you suggest.  I would guess that mammals were a staple of several
non-avian dinosaur species.  Even if prey populations crashed you are
getting into another level of complexity, i.e., how big a population crash
do you need to kill of predators.  And all this has to make the impact of
the impact just so to achieve the magical targetting effect on non-avian
dinosaurs.  I appreciate _you_ making the mental effort to justify the
details.  Many do not and are content to argue circularly that since
these are the effects we see, these are the effects of the impact.

> >OK.  But plenty of other explanations for turtle and croc survival.  
> 
> I'D BE INTERESTED TO KNOW OF ANY THAT ARE AS PLAUSIBLE. 

Actually, crocs win, crocs win, under just about any extinction scenario.  

Geographical changes which may have affected dinos, may have enhanced croc
survival.  For example, the great increase in river systems at the end of
the Cretaceous provided much more habitat. (But I forget why this was bad
for dinos--possibly because loss of inland sea meant loss of coastal
habitat--just a guess).

> <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).

I wasn't really thinking too hard when I wrote that.  But here are some
questions regarding crocs under bolide scenario.

Is acid rain part of the extinction scenario you favor.  If so, what
happens to fish eggs? And wouldn't this break the food chain?

Were Late Cretaceous crocs (I believe crocs and alligators as we know them
today were pretty new then) dependent on only fish or were they like many
extant species which seem to rely on animals coming to drink at water
holes, etc.  If they were dependent on these vistors--dinosaurs, birds,
mammals--doesn't this mean the food web was relatively stable among
terrestrial vertebrates, i.e., mammals were slowly replacing dinosaurs
at water holes rather than a cosmic crash and gradual reestablishment of
prey numbers.  This is just a trial balloon.

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

Again, how long.  If some species were dependent on dinosaurs that became
suddenly extinct, one (if one were a croc) would have to wait a long time.
I would think the croc fossil record should show a sudden crash in
dinosaur-eating crocs.  Alternatively, if dinosaur extinction were
gradual, perhaps a slower turnover rate might be seen.

Certainly more questions than answers.  But, I argue, this is appropriate.