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Impact QUestion - Re: The birds vs. the pterosaurs

>Where was the equator at the time of the impact and where was the impact
located in relation to the equator? If the impact occurred in the summer of
the impact's 
>associated hemisphere, would this not have given an advantage to the
organisms in the hemisphere that was in winter? (Side question: did summer
and winter exist as
> we know them?) I say "advantage" as these organisms would already have
been in a "powered down" mode... 

This is an important question, and one that has been little considered in
the literature. There is some (not very strong) evidence that the Chicxulub
impact happened during the northern spring or summer and likewise some, also
not very strong, indications that the effects were slightly less devastating
in the southern hemisphere. 

However whether the effects at high latitudes (where organisms have distinct
"summer" and "winter" modes) would be greater or lesser for a summer or
winter impact is not at all simple and depends critically upon the character
and particularly the duration of the "impact winter". 

Consider a spring or early summer impact (which would of course be
autumn/early winter impact in the other hemisphere). This would essentially
obliterate the summer in one hemisphere, followed by a bad winter while the
next summer (one year after the impact) might be reasonably normal. In the
other hemisphere the result would be a *very* bad winter followed by a
*very* bad summer and then another more-or-less normal winter, in effect
three winters in a row, something I think very few organisms except seeds
and spores would survive.

Incidentally the argument often seen that high-latitude organisms which are
adapted to hard winters would not be much affected by an "impact winter" is
completely erroneous for at least two reasons:

1) Many organisms (e. g. trees, hibernating animals) have complex mechanisms
for surviving winter which require extensive physiological and behavioral
preparations, usually triggered by seasonal cues. A winter "out of season"
would be disastrous for these.

2) Winter is usually a season of extreme stress and a severe bottleneck for
high-latitude organisms. Most probably do not have the capability to survive
a winter of much greater duration or severity than normal. For example most
trees would be killed by drought if a winter was greatly prolonged since
they cannot absorb moisture when the ground is frozen.

Tommy Tyrberg