Introduction
For more than 150 million years, dinosaurs roamed across the earth. However, most dinosaurs became extinct suddenly within a short time. The last of the dinosaurs are estimated to have died about 65 million years ago. The cause of their extinction continues to be a mystery, with a number of factors such as diseases, climate change, geologic events and changing plant communities being thought to have played a part in the demise of dinosaurs. Of late, theories of dinosaur extinction have been the subject of much controversy and debate. The explanation that seems to have the support of many scientists spells out that dinosaurs became extinct when a big meteorite crashed to Earth near what is today the Gulf of Mexico. The reasoning is that its impact kicked up enough debris and dust that blocked out sunlight for an extended period of time. This led to a chain of events which were deadly. Therefore, as paleontologists dig further for clues to strengthen their theories, this paper examines some leading theories of dinosaur extinction.
Theories of Dinosaur Extinction
According to Archibald (102), no sooner had the first known dinosaurs been discovered and named in the early 19th century than people began to speculate on what had happened to them. There were about 80 dinosaur theories by the 1980s. With such many theories, there were bound to be some which were highly reasonable while others were absurd. Archibald (102) identifies the 3 best explained and testable of these theories. These three theories tend to argue that changes in climate, whether slow and cumulative or sudden and drastic, caused the extinctions.
The Deccan Traps Theory
According to this theory, massive flood basalts eruptions in the Indian subcontinent, known as the Deccan Traps, happened at the time of the K-T boundary. The K-T boundary is described as a geologic signature that defines the Mesozoic era’s end. The name K-T is derived from a combination of the terms Cretaceous and Tertiary. The former is where dinosaur fossils are found while the latter is the time in which the modern groups of mammals’ first appearance is seen. Flood basalts flow from volcanoes and great fissures with moderate quantities of explosive power. They seem more like lava flows experienced in the Hawaiian Islands and not like the explosive eruption of Mount St. Helens. However, according to Gawle and Czech (13), for the Deccan Traps, the eruption occurred over 4 or more millions years ago, and it produced enough lava that can cover both present day Texas and Alaska to a depth of 610 meters.
While the effects of such massive volcanic activity have not been studied well as those effects that have been fronted in other theories of extinction, it is clear that such activity can greatly increase the quantities of fine-grained material in the atmosphere. The dust would in turn reduce the amount of sunlight that reaches the surface of the earth, and this would cause long-term global cooling. This cooling and the decrease in the amount of sun energy reaching the surface of the earth would over an extended period of tome change the vegetation and thus affect the animals that feed on it, dinosaurs included. The second theory is marine regression
Marine Regression
This theory relates dinosaur extinction to marine regression. According to Archibald (103), marine regression can be described as the process by which shallow seas that occupied most of the low-lying areas of continents drain away back into the deeper ocean basins. Among such marine regressions, one of the greatest was recorded in rocks near the end of the Cretaceous period, about 65 mya or 65 million years ago. Unlike the Deccan Traps’ eruption which occurred over millions of years, the cretaceous marine regression took place over a lesser period of time which is put at only tens or hundreds of thousands of years. According to Fastovsky and Weishampel (34), it is estimated that about 11.2 million square miles of land were exposed during this period, and this is more than twice the next largest addition of land during the past 250 million years. The mass of land exposed can be approximately equated to the area covered by modern day Africa.
Therefore, as the shallow seas the size of a continent drained away, vast areas of low coastal habitat became fragmented into smaller isolated areas. As the habitats for terrestrial coastal species reduced in size and became more isolated, the size of populations decreased. In addition, as land emerged from the oceans, land bridges were exposed. Examples include the Bering land bridge that is between North America and Asia. This new land enables the migration of terrestrial vertebrates and thus there was increased competition among species that were previously separated. Also, river systems that once flowed over short distances grew in length with the recession of the shoreline. This offered greater habitat for several fresh water organisms. With new land areas exposed by the regression of the interior seaways, climatic extremes increased as climate cooled, and this further put strain on the already stressed environment (Archibald 103).
Asteroid Impact
According to this theory, an asteroid 6 miles wide struck the earth about 65 million years. This spewed very fine-grained material high into the atmosphere, spreading around the globe. This resulted in the blockage of much of the sunlight, greatly reducing or possibly stopping photosynthesis (Gawle and Czech 14). Many plants were killed or became stunted, and many plant species became extinct. Herbivorous dinosaurs as well as other vertebrates that fed on these plants then disappeared thus causing the extinction of the carnivorous dinosaurs that fed on these herbivores. According to Archibald (103), this process happened over a few thousand years. The evidence for this asteroid impact theory is derived from 3 sources.
First, the possible crater for this impact has been located to be near the tip of Central America’s Yucatan Peninsula. The crater was given the name Chicxulub, meaning the horns of the devil in the local dialect (Alvarez 56). The crater is 110 miles across and it was initially deemed to be the 2nd largest such structure on earth. Second is the increase in the levels of iridium in the K-T boundary. The presence of high levels of iridium in rocks that were there during that period in various places on earth is a strong pointer to the fact that there was an extraterrestrial source of this iridium, e.g. an asteroid falling on earth. Usually, iridium is rare at the earth’s surface and is mainly found in areas concentrated by deep volcanoes. Third, quartz grains that show shocked parallel layers in 2 directions at a microscopic level are an indication of a great quantity of rapidly applied pressure, such as may be caused by an asteroid’s impact.
Also, there is a post-impact environmental scenario that has been fronted to try to explain the demise of the fear-fully great reptiles. According to this scenario, following the K-T impact, ejecta that was reentering the atmosphere created an intense infrared radiation blanket that covered the entire surface of the earth (MacLeod and Archibald 2). This pulse of intense thermal radiation thus killed any unsheltered organisms including the dinosaurs. The scenario also postulates that since aquatic organisms were sheltered from the searing thermal radiation by a layer of water, then that explains why most aquatic species survived. This means that terrestrial forms such as the dinosaurs were killed and hence their extinction.
Conclusion
There are many theories that try to explain why and how the dinosaurs became extinct. Paleontologists are ever digging for more clues so as to advance and support their theories. This paper has addressed the three major theories that have been put forward to explain the demise of dinosaurs over 65 million years ago. These three theories are marine regression, the Deccan Traps and the Asteroid impact. Other theories such as the increase in the earth’s gravity and an infrared thermal radiation wave are also existent. What is similar about these major theories is that there is an aspect of changes in climate. In testing these theories, vertebrate fossil records are usually used. In these tests, marine regression explains more about the selective pattern of survivals and extinctions through the K-T transition in Western America. Therefore, it is probable that massive volcanism, an asteroid impact and marine regression all had a role to play in the mass extinction of the dinosaurs.
Works Cited
Alvarez, Walter. T. rex and the Crater of Doom. New Jersey: Princeton University Press, 1997.
Archibald, David J. Dinosaur Extinctions: Changing Views. New York: Columbia University Press, 1996.
Fastovsky, David E, and Weishampel, David B. The evolution and extinction of the dinosaurs. Cambridge: Cambridge University Press, 1996.
Gawle, Ryszard, and Czech, Zbigniew. The Energy-Based Theory Explaining Dinosaur Extinction and Selectivity of K-T Extinction Event Coincided with a large Meteorite Impact. International Letters of Chemistry, Physics and Astronomy, 2012; 3: 12-20.
MacLeod, Norman and Archibald, David J. Dinosaurs: Extinction Theories. Amsterdam: Elsevier Inc, 2007.
