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Dinosaur Extinction: Currents Theories and Explanations
Mass extinctions of plants and animals have occurred many times in the history of the earth, one of the most widely known being that of the dinosaur over 65 million years ago. Many theories as to why the dinosaurs became extinct exist, including volcanic eruptions, changes in climate, diseases, radiation form a nearby supernova, and giant meteor and/or comet impacts. Although loyal believers back all of these theories, no one single theory has been accepted by the scientific community as a whole (Blanchard, 1999).
Most theories surrounding dinosaur extinction revolve around the era of time known as the K-T boundary, the period between the Cretaceous period and the Tertiary period. Supporting evidence of this time frame exists in the fossils found throughout the Mesozoic era, while no such fossils exist for the Cenozoic era (Blanchard, 1999). Yet using fossils to either prove or disprove a theory creates problems in itself in that carbon dating can only produce reasonable results when used with organic material that is less than about 50,000 years old. Considering that the K-T fossils are approximately 65 million years old, the carbon dating method of dating bones raises more questions than it answers. Also, much of the evidence cited for the K-T boundary is located in North America, leaving one to wonder exactly what was occurring during the same time frame in other parts of the world (What Killed the Dinosaurs, 1995).
Two of the most widely accepted theories concerning the extinction of the dinosaurs are a sudden catastrophic event and gradual, environmental conditions including that of volcanism. While scientists from both camps claim to have evidence to back up their theories, the gaps that exist in both adds fuel to a battle that could go on indefinitely.
Before the two main theories can be contrasted in regards to believability and scientific evidence, it is first important to examine the general elements that most scientists agree upon. First, around the time of the mass extinction of the dinosaurs there was a global climate change. During this time, the environment changed from the warm and mild one present in the Mesozoic era to the cooler one of the Cenozoic. Second, evidence exists to support a massive terrestrial disturbance at the end of the Cretaceous period that included soot in the air, acid rain, the emission of poisonous gases, and a cooling similar to that of a nuclear winter. This terrestrial disturbance caused periods of darkness on the earth as well as a global greenhouse effect. Third, the dinosaurs were only one species that became extinct during this time. According to scientific evidence, approximately 60% of all species below the line of the K-T boundary did not exist above that line. While some life forms at the K-T boundary survived, there were mass casualties in life forms found in the oceans as well as many on land. Along with the above, scientists agree that a thin layer of clay with an usually high iridium content dates back to the time associated with the K-T boundary (What Killed the Dinosaurs, 1995). Both the scientists who believe in the catastrophic event theory and the environmental change theory use all the elements above to support their augments.
The theory of a catastrophic event, mainly that of an asteroid or comet striking the earth, was proposed in 1980 by Luis and Walter Alvarez, Frank Asaro, and Helen Michel, a group of scientists at the University of California at Berkeley. According to these scientists, a large extraterrestrial object collided with the Earth. As a result of the collision, dust filled the atmosphere prompting an immediate climate change. Although these authors give no exact timing for these extinctions, the implication is that they occurred over a few years. According to Alvarez, "at least one [extinction event] at the K-T horizon is basically instantaneous" (Archibald, 1996, pp. 13-14).
The theory of an asteroid strike is supported somewhat by scientific evidence. For example, the impact of the 10k asteroid proposed by these scientists would account for the iridium and soot found in the clay layer. The massive global fires caused by such an impact would account for the soot found. Further, the quartz crystals found, known as shocked crystals, support the extreme temperature conditions caused by the impact. Additional evidence includes that of the microtektites found that support the theory of a violent explosion, like that of a meteor or comet hitting the earth (What Killed the Dinosaurs, 1995).
Many who support this theory claim that not all of the dinosaurs were killed instantly by the explosions caused by the meteor impact itself. Instead, the ones who survived impact later died of starvation when the cloud of dust killed off the plant life they survived on.
Although the sudden impact theory seems to have supportive evidence, problems exist that keep it from being universally accepted. For one, while a mass extinction did occur, many species at the K-T boundary survived. To date there is not enough scientific evidence to provide a clear reason for this. Second, scientists disagree on the effect that the dust levels and prolonged darkness had on the various species of the time. Third, the iridium found could also be accounted for by the volcano theories. And finally, this theory fails to explain the gradual die out of foraminiferans and dinosaurs, although this flaw is covered in the alternate theory of gradual decline due to starvation after impact (Blanchard, 1999).
An article published by Science Magazine in 1986 by David Raup and J. John Sepkoski, Jr. suggests an alternate sudden impact theory. According to these scientists, an undiscovered planet nicknamed Nemesis exists in our solar system. The passing of this planet through the Oort causes the disturbance of comets, thus sending many of them crashing to earth. According to Raup and Sepkoski, these comet showers occur approximately every 26 million years, thereby accounting for other mass extinctions as well.
The second theory accepted by many scientists' centers around a gradual decline of dinosaurs rather than a catastrophic event. This gradual decline would have been due to environmental factors that led to the ultimate demise of the dinosaurs. One such hypotheses for this theory is that increased volcanic activity at the end of the Cretaceous period created enough dust and soot to block out the sunlight, thereby producing a climatic change. A second hypothesis centers on the belief that major changes occurred in the organization of the continental plates at the K-T boundary. This shift in plate tectonics would have resulted in a change in climate as well. Both of the above hypothesizes support the argument that the extinction of the dinosaurs occurred over a long period of time and was the direct result of changes in the environment (What Killed the Dinosaurs, 1995).
The supporters of gradual decline claim that the changing of the geography of the earth left many dinosaurs homeless, and caused changes in the vegetation that affected the plant-eating dinosaurs. As these plant-eating dinosaurs lost their food source, they died out. In turn, the flesh-eating dinosaurs lost their food source due to the decline of the plant-eating dinosaurs. The combination of the changes in climate, geography, and plant life became too much for the dinosaurs, and as a result, they died-out over a span of several million years.
Essays in the book The Natural History Reader in Evolution seem to support the theory that a gradual die out occurred. For instance author Niles Eldredge commits, "though written rather recently, (they) tend to see extinctions mostly as a function of ecosystem collapse, brought on by the most part by changes in the physical environment" (p. 168). Yet even though the gradual die-out theory seems to have a number of supporters, it too, like the catastrophic event theory, lacks enough evidence to emerge as 'the' reason dinosaurs became extinct.
One problem found in this theory is the same one found in the sudden impact theory; the fact that while many species were wiped out, others survived. Further, the problems associated with dating fossil records makes evidence produced by the rocks somewhat questionable, meaning that it is almost impossible to conclude for a certainty whether the dinosaurs decline was gradual or sudden (What Killed The Dinosaurs, 1995).
Other than the two most widely accepted theories above, many other theories surrounding the extinction of the dinosaurs exist, although the scientific community at large has ruled out most due to lack of supporting evidence. These other theories include: 1) mammals out-competed the dinosaurs, 2) mammals ate all of the dinosaur eggs, 3) the devastating effects of cosmic rays, and 4) extinction due to disease. All of these theories have been rejected on the basis on being impossible such as in the case of mammals eating all of the dinosaur eggs to highly doubtful according to existing evidence. For instance, in order for the cosmic ray theory to be true, irradiated dinosaur bones would exist. To date, no such bones have…[continue]
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