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Dinosaurs continue to fascinate us. The suddenness of their disappearance has given rise to a number of theories, some more far-fetched than others. Below are ten of the most common - though not necessarily the most plausible - reasons cited for the mass extinction. Many of these theories work in conjunction with one another: volcanic dust produces a greenhouse effect, leading to a rise in temperature that then leads to a higher incidence of disease due to a rise in the insect population; or a stray asteroid collides with the Earth, producing a giant dust cloud that not only blocks the sun's light, but also causes widespread asphyxiation. In almost all of the theories, some external event produces a cataclysmic environmental change.
The asteroid theory claims that a large-size asteroid, perhaps 2 miles in diameter, collided with the Earth at the end of the Cretaceous period. What happened after the collision is still the subject of debate. The prevailing opinion is that such a high-intensity impact would have sent enough dust up into the atmosphere to choke off the sun's light. The resulting drop in temperature would have been particularly deadly to cold-blooded animals such as the dinosaurs. The brunt of the evidence for this hypothesis rests on the existence of iridium lodged in the Earth's crust at or near the same geological level as fossils. Its presence suggests a large and violent impact with a stellar object. Iridium is an element common in stellar objects but rarely found on the surface of the Earth.
The volcano theory supposes large-scale volcanic activity in the Deccan Traps of India. Multiple and steady eruptions flooded the atmosphere with carbon dioxide, and consequently produced a greenhouse effect. The sudden change in the global environment proved too much for the giant reptiles. What is particularly interesting about this hypothesis is that it accounts for the existence of iridium in the Earth's crust, while providing a viable alternative to the asteroid theory. The Web site above engages the asteroid versus volcano debate point by point.
Like most other scientists, proponents of this theory hold that climate change was the principal culprit for the disappearance of the dinosaurs. What is unique about this theory is the cause of the change. The "galactic dust" theorists hold that the Earth's solar system was passing through a massive cloud of galactic dust during the Mesozoic period. The dust acted as a kind of fuel for the sun, causing it to burn hotter and brighter. This would account for the higher temperatures associated with the period. Then, at the end of the Cretaceous period, the solar system passed out of the cloud. The result was a sharp drop in the Earth's temperatures, and the rest is history. The major drawback of this theory is the impossibility of its verification. As yet, no definite means exist to determine whether our solar system passed through any such cloud.
This is definitely one of the more far-fetched theories of the K/T extinctions. Defenders of this hypothesis - and there are not many - claim that during the continental drift of the Mesozoic period, the Arctic Ocean was isolated for a period of time. This isolation produced a colder, brackish, and less salty water mass. Eventually, this water reintegrated with the warmer, saltier waters of the Atlantic. The result was a massive drop in the surface temperature of the oceans. This, scientists claim, could have produced a worldwide temperature decline of as much as 10 degrees Celsius.
Champions of the "predatory mammals theory" feel that the emergence of mammals at the end of the Cretaceous period directly resulted in the demise of the giant reptiles. Mammals posed a new threat to the dinosaurs because they fed directly on their unborn. Unable to contend with these predators, the dinosaurs eventually fell to the small, warm-blooded creatures. This theory is interesting. However, it seems unlikely that mammals could have been the sole cause of the extinctions. The Dinosaur Mailing List Archive, above, includes an ongoing discussion regarding the major causes of extinction, including the predatory mammals theory.
The Earth's magnetic field protects it from outside forces such as cosmic radiation. Periodically, the Earth's magnetic field shifts and changes its polarity. The last such shift occurred about 700,000 years ago. Supporters of the magnetic reversals theory - and they are not easy to find - hold that during the interim between shifts, the Earth was exposed to high levels of cosmic radiation. It is interesting to note that this theory works well in conjunction with the supernova theory. Unfortunately, neither can be easily proven with any certainty.
7. Orbital Tilt
The orbital tilt theory holds that the major cause of climactic change was not a sudden traumatic action like a meteor, but the slow, periodic change in the tilt of the Earth's orbit. Changes in orbital tilt have been responsible for ice ages in the past, the last one occurring almost 10,000 years ago. Supporters of the theory contend that it is no great leap to assume that a similar ice age would have proved disastrous for cold-blooded species. The benefit of this hypothesis is that it does not have to rely on an external agent such as an asteroid, galactic dust, etc., but upon the natural movements of the Earth.
8. Cosmic Radiation from a Nearby Supernova
As its name implies, this theory supposes that a nearby supernova generated a tremendous amount of radiation, sufficient to kill off the dinosaurs. Unfortunately, this view suffers from two stumbling blocks. The first is the impossibility of its validation. No demonstrable proof has been constructed to verify the existence of such a supernova. Secondly, no proponent of the theory has been able to explain why only the dinosaurs died and not everything else.
9. Disease
Another frequently discussed explanation is the rise of epidemics at the end of the Cretaceous period. Perhaps due to a mutation in a common virus, disease spread rapidly in the dinosaur communities. While disease may not, in and of itself, have been enough to kill all the dinosaurs, it may have been enough to significantly reduce the gene pool, thereby guaranteeing their eventual extinction. Critics question the nonspecificity of the epidemic. They ask why disease struck all the dinosaurs and not just a few related species. Proponents respond by claiming that the elimination of only one or a few dinosaur species would have been enough to upset the delicate ecological equilibrium upon which all the dinosaurs depended.
10. Suffocation
The theory of suffocation holds that the dinosaurs died due to a sudden drop in the oxygen levels of the atmosphere. Proponents of the "Pele hypothesis" argue that the dinosaurs evolved physiologically to prosper in an oxygen-rich environment. Air samples trapped in amber have established the elevated levels of oxygen present in the Mesozoic and Cretaceous periods. Scientists further cite as evidence the tiny nostrils of the dinosaurs and their lack of a diaphragm. Given these conditions, the obvious question then is, What caused the dramatic decline in atmospheric oxygen levels? The sharp fall is attributed to the rise of the continental shelf, which exposed oxygen-absorbing organic materials to the air. The resulting decline in oxygen proved too great for the dinosaurs.
Nikheel Dhekne holds a masters in philosophy from the State University of New York at Stony Brook. He is currently a freelance Web publisher and developer.
Caleb Brown is an illustrator and biologist living in Montana. By day he drives a delivery van, and by night he draws pictures with his computer.
