--A major extinction of land and marine organisms occurred abruptly at the rock boundary between the Cretaceous Period and the Cenozoic Era.
--Many forms of life that had played major ecologic roles for tens of millions of years disappeared.
--The most prominent to us is the disappearance of the dinosaurs However, flowering plants which now dominate the terrestrial environment suffered extremely in the extinction event.
--In the ocean all the large reptilian swimmers (mosasaurs, plesiosaurs and giant turtles) all disappeared.
--The base of the food chain (the small marine plants in the ocean) declined to such an extent that most of the larger organisms also collapsed---a chain reaction.
In the early 1980s scientists from the University of California at Berkeley discovered a layer of sediment at the boundary between the Cretaceous and the Cenozoic that contained 30% more Iridium than occurred in rocks previously or after the boundary.
The scientists argued that since iridium is so rare on Earth the enriched layer at the boundary must have come from space. They advanced the hypothesis that the extinction of dinosaurs and other organisms occurred abruptly when a large meteorite hit the Earth and spewed vast amounts of dust and water vapor into the atmosphere. The size of this meteorite was estimated to be 10km in diameter (about 6 miles).
The dust from such an explosion would have been spread all over the Earth. Indeed, the iridium layer has now been found in places all over the world.
Other scientists argued against the hypothesis, suggesting that the iridium came from volcanic explosions. It is true that iridium is found in higher concentrations within the molten interior of the Earth. We know that there was a massive eruption of lava near the end of the Cretaceous in India.
However, two lines of evidence have convinced scientists that the extinctions and the iridium anomaly were caused by a meteorite impact.
Cooling seems to be the biggest problem at the boundary. The extinctions seem to have hit hardest in those organisms and groups of organisms that were adapted to warm conditions. Plants in the higher southern latitudes seem to have suffered less than those in the northern higher latitudes.
The location of the impact is uncertain. But, there is a large crater in the Yucatan Peninsula that may be large enough and of the right age to be the location where the impact occurred. This is supported by the occurrence of larger glass spheres (see photo above) near the crater. The larger sphere would have fallen out of the atmosphere sooner, near the source while the smaller ones would have been transported farther from the impact site.
An interesting outcome of the winter scenario is the concept of "nuclear winter". Scientists now contemplate the effect that nuclear explosions would have on the environment. These explosions would, like the Cretaceous impact, cause huge amount of dust to be put into the atmosphere which would act to block sunlight and cool the Earth.
Finally, the meek inherited the Earth.