Dissatisfactionwith conventional explanations for dinosaur extinctions led to a surprisingobservation that, in turn, has suggested a new hypothesis. Many plants andanimals disappear abruptly from the fossil record as one moves from layers ofrock documenting the end of the Cretaceous up into rocks representing thebeginning of the Cenozoic . Between the last layer ofCretaceous rock and the first layer of Cenozoic rock, there is often a thinlayer of clay. Scientists felt that they could get an idea of how long theextinctions took by determining how long it took to deposit this one centimeterof clay and they thought they could determine the time it took to deposit theclay by determining the amount of the element iridium it contained.
It has notbeen common at Earths since the very beginning of the planets history.Because it usually exists in a metallic state, it was preferentiallyincorporated in Earths core as the planet cooled and consolidated. lr is foundin high concentrations in some meteorites, in which the solar systems originalchemical composition is preserved. Even today, microscopic meteoritescontinually bombard Earth, falling on both land and sea. By measuring how manyof these meteorites fall to Earth over a given period of time, scientists canestimate how long it might have taken to deposit the observed amount of lr inthe boundary clay. These calculations suggest that a period of about onemillion years would have been required. However, other reliable evidencesuggests that the deposition of the boundary clay could not have taken onemillion years. So the unusually high concentration of lr seems to require aspecial explanation.
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