Dinosaurs extinction Theories.
Whatever factor or factors caused it, there was a major, worldwide biotic change at about the end of the Cretaceous. But the extermination of the Dinosaur is the best-known change by far and has been a puzzle to paleontologists, geologists, and biologists for two centuries. Many theories have been offered over the years to explain Dinosaur extinction, but few have received serious consideration. Proposed causes have included everything from disease, heat waves and resulting sterility, freezing cold spells, and the rise of egg-eating mammals, to X rays from a supernova exploding nearby. Since the early 1980s, attention has focused on the so-called asteroid theory put forward by the American geologist Walter Alvarez, his father, the physicist Luis Alvarez, and their coworkers.
The ongoing debate pits catastrophists against gradualists. Were the extinctions simultaneous and instantaneous, or were they nonsynchronous and spread over a long time period? The precision with which geologic time can be measured, by either radiometric means, paleomagnetic reversal stratigraphy, or the more traditional approach of measuring the fossil content of stratigraphic layers, leaves much to be desired. Only rarely does an ³instantaneous² event leave a worldwide‹or even regional‹signature in the geologic record in the way that a volcanic eruption does locally. Attempts to pinpoint the KT boundary event, even using the best radiometric dating techniques, result in a margin of error on the order of a half million years. Consequently, the actual time involved in the mass extinctions of that period, or in any of the preceding or subsequent extinctions, has remained undetermined.
The asteroid theory
The discovery of an abnormally high concentration of the rare metal iridium at, or very close to, the KT boundary, however, provides what has been recognized as one of those rare instantaneous geologic time markers that seem to be worldwide. This iridium anomaly, or spike, was first found by Walter Alvarez in the CretaceousTertiary stratigraphic sequence at Gubbio, Italy, in the 1970s. The spike has subsequently been detected at localities in Denmark and elsewhere, both in rock outcrops on land and in core samples drilled from ocean floors. Iridium normally is a rare substance in rocks of the Earth's crust (about 0.3 parts per billion). At Gubbio, the iridium concentration is more than 20 times greater (6.3 parts per billion), and it is even greater at other sites.
Because the levels of iridium are higher in meteorites than on the Earth, the Gubbio anomaly is thought to have an extraterrestrial explanation. The level of iridium in meteorites has been accepted as representing the average level throughout the solar system, and by extension, the universe. Accordingly, the iridium concentration at the KT boundary is widely attributed to a collision between the Earth and a huge meteor or asteroid. The size of the object is estimated at about 10 kilometres (6.5 miles) in diameter and one quadrillion metric tons in weight; the velocity at the time of impact is reckoned to have been several hundreds of thousands of miles per hour. The crater resulting from such a collision would be some 100 kilometres or more in diameter. No crater of this sort has been recognized, but, in view of the fact that the Earth's surface is two-thirds ocean, it is likely that such an impact site (called an astrobleme) would be hidden on the ocean floor.
The KT mass extinctions, however, do not seem to be fully explained by this hypothesis. The stratigraphic record is most complete for extinctions of marine life‹foraminifera, ammonites, coccolithophores, and the like. These life-forms apparently died out suddenly and simultaneously, and their extinction accords best with the asteroid theory. The fossil evidence of land dwellers, however, suggests a gradual decline in dinosaurian diversity, and possibly abundance, rather than a sudden change at the KT boundary. Alterations in terrestrial life seem to be best accounted for by environmental factors such as the consequences of seafloor spreading and continental drift. With the rearrangement of the continental masses, disrupting and deflecting oceanic current patterns and causing repeated changes in sea level, there undoubtedly occurred many climatic changes, which in turn would have affected terrestrial organisms and their distribution.
Finally, in the controversy between the gradualist and catastrophist explanations of the dinosaurs' extinction, it should be noted that one phenomenon does not preclude the other. It is entirely possible that a culmination of ordinary biological changes and some catastrophic event both took place around the end of Cretaceous time.
Dinosaur descendants
Contrary to the commonly held belief that the Dinosaur left no descendants, the rare (seven) specimens of Archaeopteryx (see photograph ) provide compelling evidence that birds are closely related to, and probably are direct descendants of, small theropod dinosaurs. In fact, today Archaeopteryx actually is classified as a Dinosaur and a bird by many experts, and some even categorize all living birds as dinosaurs.
Many ornithologists do not agree with this taxonomy, although many (if not most) accept a dinosaurian origin of birds.
The specimens of Archaeopteryx contain particular anatomic features that also are exclusively present in certain theropods, such as Ornitholestes, Coelurus, Velociraptor, Deinonychus, Saurornithoides, Troodon, and Sinornithoides.
All these Dinosaur existed too late in time to have
given rise to Archaeopteryx and thereby to later birds. However, it is believed that
they arose from a common theropod ancestor that possessed the beginnings of these
shared anatomic features found in Archaeopteryx, the oldest-known unquestionable
bird.
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Alternative Theory On Dinosaurs
Paleontology
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