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Question: How did dinosaurs die?
The dinosaurs dominated the land for over 140 million years during the Mesozoic era. However, by the end of the Cretaceous Period (about 65 million years ago) they had all died out, the apparent victims of what has been called the K-T extinction or the Cretaceous-Tertiary extinction. In fact, up to 75% of the species of life on the Earth died out in this period. Included were most of the plankton and many of the invertebrate animal species in the oceans. In contrast, many groups survived, including the mammals, birds, insects, and the flowering plants. In the oceans, the fishes and most mollusks continued, diversified, and prospered to the present day. A number of hypotheses have been advanced for cause of the K-T extinction. Currently, the most seriously considered are 1) asteroid impact and 2) massive volcanic eruptions. In both, the climate of the Earth is drastically altered, leading to collapse of the food chains in both the land and marine ecosystems, resulting in the extinction.
In the first scenario, a large asteroid (6 miles in diameter) hits the Earth. Larger pieces of debris reentering the atmosphere could trigger massive firestorms, adding soot and ash to the atmosphere. Such an impact on land or in the ocean would kill any life in the general area. An impact in the ocean would also generate a mega-tsunami that would devastate costal areas. But the real killer would be the dust and soot injected into the atmosphere. This would soon circle the Earth, blocking the light of the Sun from reaching the surface for possibly several years. The resulting drop in temperature ("impact winter") and the loss of light (preventing photosynthesis) would lead to the collapse of the food chains in both land and marine ecosystems. The asteroid impact hypothesis is supported by the finding of a spike in the abundance of the element iridium at the K-T boundary layer of clay found across the globe. Iridium is a rare at the surface of the Earth, but is much more abundant in meteorites and asteroids. Vaporization of the asteroid upon impact would spread this element worldwide. At least two massive impact craters have been found that were formed more or less coincident with the K-T extinction – the Chicxulub crater on the Yucatan coast off Mexico, and the Shiva crater off the coast of India, suggesting that multiple impact events, rather than a single impact, may have triggered the extinction.
In the second hypothesis, gigantic volcanic eruptions cloud the atmosphere with ash, while the release of sulfurous gases could lead to acid rain. The prime candidate in this scenario is the volcanism that produced the Deccan Traps in India. These huge lava beds, up to 1.5 miles thick, were the result of a series of eruptions centered around the K-T boundary.
Choosing between these hypothesis is made difficult by 1) problems in correlating these events in time, and 2) the fact that we really don't know how fast the extinction actually occurred. Rather than a short intense episode, the extinction of the dinosaurs more likely extended over a periods of millions of years. Indeed, there is some indication that the dinosaurs as a group were already in decline in the later part of the Cretaceous. It seems likely that the end of the Cretaceous extinction was produced by a series of events, both asteroid impacts and volcanism, that led to drastic changes in the Earth's climate, perhaps spread over hundreds of thousands or even millions of years.
Finally, it should be noted that the K-T extinction was not the only major extinction episode, or even the worst, to challenge life on Earth. Other episodes of extinction have been identified at the ends of the Ordovician, Devonian, Permian, and Triassic periods (approximately 450, 370, 250, and 205 million years ago, respectively). Indeed, we may very well be in the midst of a major extinction event today, with some estimates projecting 50% of the species on Earth to be extinct within 100 years.
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