More complex studies of all five mass extinctions that paleontologists know about have shown that, with the exception of, maybe, the last one, four global extinction events (GEE) have no plausible scientific explanation. In the researchers' own words, a rock falling out of the sky simply doesn't cut it. There are lots of inconsistencies between what scientists believe happened during the last extinction, which occurred 65 million years ago, and the one before that, which took place about 200 million years ago.
 

David Bottjer, USC earth scientist, suggests that the extinctions that wiped out life on Earth at the end of the Permian age (250 million years ago) and between the Triassic and Jurassic eras (200 million years ago) occurred simply because "the earth got sick." He says that life forms do not necessarily have to die as a consequence of a catastrophic event. Instead, continuous deterioration of the atmospheric conditions, coupled with periods of global warming and the acidification of the oceans could have very well destroyed all living creatures on the planet, with the possible exception of a few simple organisms.

 
Further questions are raised about the impact that such extinction events would have on coral reefs. While reefs nowadays are very similar to those that lived millions of years ago (and, most likely, are the same ones), oceanologists argued that other types of reefs were present in the past as well. Rowan Martindale, a USC doctoral student, said "The coral reefs look actually very similar to modern coral reefs. At the end-Triassic mass extinction, you lose all your reef systems. And nobody's figured out why that is."

Martindale also said that the second type of reefs could have been made up of mud and other debris, held together by some type of bacteria. If that was indeed the case, and both types of reefs lived at the same time, a meteorite hitting the Earth simply wouldn't have had what it took to destroy them both, both in terms of impact velocity, and in what regards power and effect.

 
The new theories were made public at the 2008 Joint Annual Meeting of the Geological Society of America held between the 5th and 9th of October in Houston, Texas. Though they may seem unusual and blatantly contradict the "comet impact" assertion the international scientific community held to be true until now, it's very likely that future research will be directed on exploring them even further.