Dinosaur Extinction: Current Theories and Explanations

Introduction to Dinosaur Extinction

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 dinosaurs over 65 million years ago. Many theories as to why the dinosaurs became extinct have been proposed, including volcanic eruptions, changes in climate, disease, radiation from a nearby supernova, and giant meteor and/or comet impacts. Although loyal supporters back all of these theories, no single theory has been accepted by the scientific community as a whole (Blanchard, 1999).

Most theories surrounding dinosaur extinction revolve around the era known as the K-T boundary — the period between the Cretaceous period and the Tertiary period. Supporting evidence for 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, since carbon dating can only produce reliable results when used with organic material less than about 50,000 years old. Considering that the K-T fossils are approximately 65 million years old, the carbon dating method raises more questions than it answers. Additionally, much of the evidence cited for the K-T boundary is located in North America, leaving open the question of what was occurring during the same time frame in other parts of the world (What Killed the Dinosaurs, 1995).

The K-T Boundary and Points of Scientific Agreement

Two of the most widely accepted theories concerning the extinction of the dinosaurs are a sudden catastrophic event and gradual environmental change, including volcanism. While scientists from both camps claim to have evidence supporting their theories, the gaps that exist in both fuel a debate that shows no sign of resolution.

Before the two main theories can be contrasted in terms of believability and scientific evidence, it is 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 period, the environment shifted from the warm and mild conditions present in the Mesozoic era to the cooler conditions of the Cenozoic. Second, evidence supports 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 cooling similar to that of a nuclear winter. This disturbance caused periods of darkness on the Earth as well as a global greenhouse effect. Third, the dinosaurs were not the only species that became extinct during this time. According to scientific evidence, approximately 60 percent of all species below the K-T boundary line did not exist above it. While some life forms at the K-T boundary survived, there were mass casualties among ocean life forms as well as many on land.

In addition to the above, scientists agree that a thin layer of clay with an unusually high iridium content dates back to the time associated with the K-T boundary (What Killed the Dinosaurs, 1995). Both proponents of the catastrophic event theory and the environmental change theory use all of these elements to support their arguments.

The Catastrophic Impact Theory

The theory of a catastrophic event — specifically, 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, Berkeley. According to these scientists, a large extraterrestrial object collided with the Earth, and as a result of the collision, dust filled the atmosphere and prompted an immediate climate change. Although these authors give no exact timing for the 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 by a degree of scientific evidence. The impact of the 10-kilometer 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 explain the soot. Furthermore, the quartz crystals found — known as shocked crystals — support the extreme temperature conditions that such an impact would produce. Additional evidence includes microtektites that support the theory of a violent explosion consistent with a meteor or comet strike (What Killed the Dinosaurs, 1995). Many supporters of this theory also argue that not all of the dinosaurs were killed instantly by the explosions; those that survived the initial impact later died of starvation when the cloud of dust killed off the plant life they depended upon.

Although the sudden impact theory has supportive evidence, problems prevent it from being universally accepted. For one, while a mass extinction did occur, many species at the K-T boundary survived, and to date there is not enough scientific evidence to explain why clearly. Second, scientists disagree on the effect that elevated dust levels and prolonged darkness had on the various species of the time. Third, the iridium found in the clay layer could also be accounted for by volcanic activity. Finally, this theory fails to fully explain the gradual die-out of foraminifera and certain dinosaur lineages, although proponents address this flaw through the alternate explanation of starvation following the initial impact (Blanchard, 1999).

An article published in Science Magazine in 1986 by David Raup and J. John Sepkoski, Jr. suggests an alternative sudden-impact scenario. According to these scientists, an undiscovered planet nicknamed Nemesis exists in our solar system. The periodic passing of this planet through the Oort Cloud disturbs comets and sends many of them crashing into the Earth. According to Raup and Sepkoski, these comet showers occur approximately every 26 million years, thereby accounting for multiple mass extinctions throughout Earth's history.

Conclusion

The debate over exactly how the dinosaurs became extinct is one that is likely to remain active in the scientific community for some time. It can neither be proved nor disproved that the dinosaurs became extinct due to the events that occurred around the K-T boundary, or whether they were merely weakened by them. Although popular opinion tends to lean toward the sudden impact theory, without some type of as-yet undiscovered evidence, a consensus among scientists as to the exact cause of dinosaur extinction remains unlikely in the near future.