Re: Extinction

[John Bois <jbois@umd5.umd.edu>]

Nathan said:
>   1.  It is estimated that whatever caused the KT extinction killed 90%
of extant taxa in all groups in the biosphere (i.e. land and
sea).

I wasn't aware there was actual population data--even estimates from
fossil data.  Where does this figure come from?

>   2.  There seems to have been a systematic bias against animals of
large body mass.   So, it was not a completely random 90% - your odds of
survival were greater if you were smaller.

Depending on the duration of the window of destruction, there were plenty
of small non-avian dinosaurs--hatchlings.  One of the great unmet
challenges to this hypothesis is to explain the conditions and timing of
destruction that would produce the patterns of extinction and survival.
    
>   3.  There may be (this is more controversial) a bias in favor of
survival for animals that hibernate, have a dormant life cycle for part
of the year, or burrow.   This includes many small mammals, and it also
includes many frogs and amphibians. 

But mammals on all continents seem to have survived (right?).  Hibernation
would not have occurred at the same time globally.  And mammals must
prepare to hibernate.  Also, while many may have burrowed, most probably
did _not_ hibernate due to equable global climate. 

>   Taken together, these three effects - random chance that some groups
would have some survivors, and a bias against body size and a bias toward
burrowers/hibernators are probably the best explanation to date. 

Says Ptolemy.

 >   Note that these arguments are not specific to an impact scenario - it
is the statistical properties of any KT mechanism that matter.

...and the pattern of survival and extinction.