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Koeberl et al. (1997 - Mod Geol 25:731-734) claim to have discovered a giant
South African impact crater at or near the Jurassic/Cretaceous boundary. Its
size is uncertain, it may be from 70 to as much as 340 km across. In
comparison the Yucatan crater is 180-280 km across. It is therefore possible
that the biggest impact during the nonavian dinosaur era did not occur at the
end of their reign, but in the middle of it. Of course, it is possible that
other super impacts occured that we yet no nothing of because the crater was
on the ocean floor and has long been subducted, and no one has sampled the
entire Mesozoic sequence for iridium etc. layers so far as I know. 

If the size of the J/K impact proves to match or exceed that of the K/T
event, then the problem with the impact hypothesis of dinosaur extinction is
an obvious one. One that I pointed out in Bioscience (1989: 162-172).
Although dinosaur diversity fluctuated during the Mesozoic, the basic trend
was for them to always be diverse in all size ranges over the entire time,
indeed their diversity tended to increase over time. For example, although
sauropods may have suffered some extinctions at the J/K boundary (no one
really knows because the period is poorly known), they remained a large group
into the Cretaceous. 

If dinosaurs experienced one of more impacts approaching the K/T hit in
scope, then why did they not either undergo an extreme drop in diversity -
and loss of the giant bodied species - at those times. Conversely, if they
did so well at surviving mid Mesozoic impacts, why did they roll over and die
so easily when central America got hit by an extraterrestrial mountain. This
is fishy. 

In fact, the impact hypothesis has always had a certain whiff to it. Its
proponents keep emphasizing that the atmospheric pollution that resulted from
the blast would easily kill of the dinosaurs. Yet a number of birds and
amphibians - which are both so vulnerable to pollutants that they are
considered sensitive environmental indicators - survived the K/T impact so it
could  not have been THAT bad. It is also claimed that half the world's
forests burned at the time. Perhaps so, but that means that half (mainly in
the southern hemisphere) the woodlands did not burn, and they made it into
the Tertiary OK. So why did not the dinosaurs living in those forests

It is very hard to see how anything could have killed off the entire global
population of dinosaurs. This is especially true because most of them appear
to have been "r-strategists" that laid large numbers of eggs that hatched
fairly independent dinosaurs (see my paper 244-255 in Dinosaur Eggs & Babies
1994 Cambridge Univ Press). This made dinosaurs "weed species" with very high
potential recovery rates (unlike large mammals, which drop only a few calves
that have to be carefully tended to). Only a limited number of juvenile
dinosaurs needed to stay alive and they could have started a new radiation of
dinosaurs. It therefore seems impossible to quickly wipe out every single
species of nonavian dinosaur when much of the global ecosystem was not
totally destroyed, and birds, mammals, and herps survived. 

The impact hypothesis of dinosaur extinction remains important, but it has
never provided the exact mechanism/s by which every last dinosaur population
was destroyed. Confirmation that equally large Mesozoic impacts occurred
without driving dinosaurs into extreme declines will, ironically, further
weaken the impact hypothesis as the sole explanation for the demise of