by Steve Brusatte

Sixty-six million years ago the dinosaurs were thriving. Today their bones and fossils are confined to natural history museums. Exactly what happened? Sixty-five million years ago the earth went through a period a rapid death, called a mass extinction.

A mass extinction is most simply defined as a period of time in which the number of extinctions drastically rises (Raup 801). The extinction of the dinosaurs is a classic example, but this episode is not the only mass extinction recorded in the fossil record. In fact, there are many more, and in recent years several paleontologists have suggested that these mass extinctions are periodic, or occur in a regular cycle. Exactly what factors may cause these mass extinctions? Scientists have offered both extraterrestrial and earthly factors.

The original theory of periodic mass extinctions was proposed in 1977 by Princeton geologists Alfred Fisher and Michael Arthur, who suggested, based on minimal evidence, that mass extinctions have occurred every thirty-two million years (Wilford 251). because of the minimal evidence, though, the hypothesis was not taken seriously in scientific communities until nearly ten years later, when University of Chicago paleontologists J. John Sepkoski, Jr. and David Raup announced their findings on prehistoric biodiversity and extinctions in 1984. The monumental paper of Sepkoski and Raup was ultimately based on the dinosaur extinction research of the father-and-son combo of Luis and Walter Alvarez.

While researching plate tectonic data in Gubbio, Italy, the younger Alvarez, Walter, became intrigued by a sixty-five million-year old clay layer. This layer marked precisely the point in time when the dinosaurs disappeared from the fossil record (Lessem 290). Although he was not originally involved in a paleontological research project, Walter Alvarez took a sample of the clay back to his University of California - Berkeley laboratory to show his father, Luis Alvarez, a Noble Prize winning physicist. The clay layer was outside Luis Alvarez\'s area of research, too, but like his son he became deeply intrigued and vowed to solve the mystery of dinosaur extinction.

Before the Alvarez team dug into the mystery of dinosaur extinction, most paleontologists believed that the dinosaurs disappeared because the climate deteriorated, their diets deteriorated, or that diseases suddenly became widespread (Powell XIII). Little thought had been given to catastrophic and sudden causes. That changed when the Alvarez team published the highly renowned, and still controversial, paper entitled \"Extraterrestrial Cause for the Cretaceous-Tertiary Extinction\" in 1980 (Alvarez, L. 1095). In this paper the team directly blamed the Cretaceous extinction on an asteroid or comet impact. This impact, they reasoned, would have, among other effects, have caused large clouds of dust to block out the sun (thus halting the photosynthesis of plants), global cooling, worldwide wildfires, monstrous tsunamis, and the extinction of the dinosaurs and most other forms of Cretaceous life. Not only did this paper offer a solid solution to the mystery of dinosaur extinction, but it also opened up an entire new age for scientific research.

Suddenly, scientists from all disciplines, including paleontologists, geologists, physicists, astronomers, and chemists, were working together in an attempt to further understand massextinctions. In the following years after the Alvarez announcement hundreds of papers focusing on mass extinctions were published. Among these was the dramatic paper written by Raup and Sepkoski entitled \"Periodicity of Extinctions in the Geologic Past.\" This paper was the first solid account of regularly occurring mass extinctions. The concept of the Raup-Sepkoski paper started much like the Alvarez research-by accident. For several years, beginning before the Alvarez team even discovered the Gubbio site, Sepkoski had been compiling a massive set of data on biodiversity and extinctions, the most complete record of which had ever been developed.

In his research Sepkoski prepared a listing of about a quarter of a million species of sea-dwelling creatures, both extinct and current, noting the point in time where they appeared and became extinct. Sepkoski confined the study to marine organisms because, \"The oceans are receptacles of sediment. They are areas of net sedimentation, whereas land is an area of net erosion, which means fewer rocks and fewer fossils\". Although the study was originally devised to be a broad study of the distribution of marine life through geologic time, it soon turned into yet another part of the mass extinction debate. David Raup, a senior colleague at the University of Chicago, suggested that Sepkoski examine the date for any evidence of a pattern in the timing of the mass extinctions. They doubted they would find much, but Raup recalled the Fisher/Arthur report and hoped that Sepkoski\'s exhausting study may prove that mass extinctions were actually periodic. To further study extinction periods Sepkoski decided to concentrate on the last 250 million years of geologic time and to throw out animals whose point of appearance and disappearance were debated.

He was left with about 500 of his original 3,500 marine families (250,000 species). Sepkoski put the data through computer analysis and was surprised to find that life disappeared in great numbers approximately every twentysix million years. They checked for errors and confirmed the pattern; life seemed to disappear drastically exactly every 26.2 million years. When scientists discover an unexplained cycle or events they like to correlate the unexplained cycle with other, more wellunderstood cycles.


Naturally, the first thing the Sepkoski and Raup team did was attempt to find a known cycle that could accurately explain the 26.2 million year intervals in mass extinctions. There was one major problem, though; there were no known earthly cycles that require as long as a million years, much less 26.2 million years. It was then when Sepkoski and Raup turned to the astrophysicists to find possible extraterrestrial causes. The astrophysicists responded eagerly and inventively, and their enthusiasm led to several possible and interesting theories that may explain the periodic mass extinction cycles. Early theories held that possibly the sun somehow erupts every twenty-six million years or that the solar system perhaps encounters some periodic perturbation in its passage through the Milky Way Galaxy, interacting with other stars or intergalactic dust.

The sun eruption theory quickly died because of lack of evidence, but the perturbation theory lived on, however so briefly. Indeed, our solar system does rotate through the Milky Way Galaxy, and this motion often takes us through the center of the universe, known to scientists as the galactic plane. Because the galactic plane represents the absolute vertical center of our galaxy, it is the point where the collective gravity of all of the matter in the galaxy is the strongest. This explains why cosmic debris, such as dust and meteoritical particles, tend to settle out in the galactic plane. Author Christopher Lampton compares this settling to the way \"bilge water settles out in the lowest portion of a boat because that puts it closest to the earth\'s center of gravity\" (Lampton 61).

Two months after Sepkoski and Raup first presented their results astronomers Richard B. Strothers and Michael R. Rampino of NASA calculated that during a passage through the galaxy\'s galactic plane the solar system should encounter one of the massive interstellar clouds of dust and gas in the region. The gravitational force of such a cloud would be sufficient, they argued, to dislodge a swarm of comets from their normal orbits at the outer edge of our solar system. The abundance of comets, they reasoned, would drastically increase the probability of earth being struck by a bolide object. Rampino and Strothers reasoned that our solar system traveled through the galactic plane every thirty-one million years, but other scientists discredited their calculations, saying that the solar system is now moving up to the galaxy\'s central plane, when it should be in the position of cosmic bombardment. But, as we can attest to, nothing of that sort has happened yet. If the Strothers/Rampino theory held true, humans should have long ago been added to the list of extinct animals.


Geologists Erik M. Leitch and Gautam Vasisht have devised their own theory, strikingly similar to the galactic plane hypothesis. They reason that most major mass extinctions of the past 500 million years occurred when the solar system passed through the galaxy\'s spiral arms. Several pieces of evidence lend credence to this link. Interstellar space within the Milky Way\'s spiral arms differs from that between them. Also, several astrophysicists have suggested that supernovae are likelier to explode near the solar system when the sun is in a spiral arm. Moreover, the arms of our galaxy are rife with giant molecular clouds that can disturb the outer solar system\'s comets and asteroids and send them plunging toward earth. And, in comparison to the galactic plane theory, the Leitch/Vasisht team\'s dates seem to work out.

According to their calculations, we have another 140 million years or so before we again pass through the arms of the galaxy. In addition to these two galactic explanations are two theories that center around our own solar system. Both hypotheses, one focusing around a companion star and the other around a 10th planet, hold true the belief that a giant cloud of comets, called the Oort Cloud, revolves around the sun on the outer fringes of our solar system. Both theories have attempted to explain how this giant cometary cloud is disrupted, and how the comets from the cloud come hurling towards earth on a cosmic death march. The first of these two theories focuses around a distant companion star to our sun whose orbit disrupts the Oort Cloud on a 26.2 million year cycle. Most stars that we see in the sky are part of multiple-star systems, otherwise known as binary-star cycles.

This name implies that there are actually two \"suns\" in each solar system, held together by their collective gravitational attraction. For years astronomers believed that our sun was not part of such a system, but because of the Sepkoski-Raup hypothesis, they are being forced to reconsider. The theory of a binary star system was first suggested by Walter Alvarez\'s Berkeley colleague, Rich Muller. Before the Sepkoski-Raup paper went to press the team sent an advance copy for Luis Alvarez to review. Upon his first reading Alvarez was convinced that the theory had to be wrong, but his son and Muller, upon reading the paper, urged him to reconsider. Luis Alvarez challenged the astrophysicist Muller to explain just how these periodic extinctions occurred.

After a few days of intense research and calculations, Muller suggested that the sun may have a companion star that comes very close to the inner solar system every 26 million years, jarring loose a stream of comets from the Oort Cloud. To further understand just how a companion star could cause these extinctions Muller teamed up with fellow astronomers Piet Hut and Marc Davis, who wrote a paper explaining the theory. The paper, entitled \"Extinction of Species by Periodic Comet Showers,\" explained basically what Muller first proposed to the Alvarez team, that the possible companion star has a highly eccentric orbit that takes it far out in space but periodically brings it back nearer the outer boundaries of the solar system, where the Oort Cloud lies.

As the companion passes the Oort Cloud, its gravity could jar streams of comets loose and send them towards the inner solar system. In the paper the Muller/Davis/Hut team named the possible death star Nemesis, after the Greek goddess who punished earthly beings for attempting to usurp the privileges of the gods. One of the major rebuttals to the companion star theory is the question of why we haven\'t been able to see it, even after searching endlessly through several of the world\'s most powerful telescopes. Most likely, the Muller team hypothesized, the companion star would either be a white dwarf or brown dwarf. Both star types are very small and cold, which makes them increasingly harder to see, even through powerful telescopes. And, if there is a companion star, it is probably more than 30,000 times as far from the sun as the earth is.

The other solar system hypothesis that has been offered to explain these periodic mass extinctions is the theory that there is a tenth planet in our solar system, nicknamed Planet X. Daniel P. Whitmire and John Matese, of the University of Southwestern Louisiana, first offered up this theory. Actually, the entire idea of Planet X itself is not new. Certain oddities in the orbit of Neptune have long suggested that it might by disturbed in some way by the gravitational pull of some unseen object at the edge of our solar system.

When Pluto was discovered by Clyde Tombaugh in the 1930\'s, it was thought to be the source of some of those disturbances, however, it is now regarded by some astronomers to be too small to have such an effect on Neptune. This has led to the theory of a possible tenth planet. Whitmire and Matese simply implied this theory to the periodic mass extinction debate. They reasoned that this so-called Planet X may be so far out on the fringes of the solar system that it periodically crosses through the Oort Cloud and knocks comets out of their orbit. This planet would probably be about 100 times as far from the sun as earth is, compared to the 30,000 times distance exhibited by the possible companion star. According to Lampton, the theory of Planet X is probably the most superior, compared to the galactic plane, galactic arms, and companion star theories.

He reasons that the idea of Planet X is not new and evidence for it has already been recorded. Secondly, he writes, its orbit would be much more stable that that of a companion star because it would be closer to the sun and less likely to be snagged by the gravitational pull of a passing star. Coupled with the numerous extraterrestrial theories that have been offered to explain periodic mass extinctions are the earthly theories. Several scientists have suggested that the solar system has nothing to do with extinctions, but the culprit is likely to be found on earth, most likely due to volcanism or glaciation .

The more likely of these two earthly culprits is probably the volcanism theory, because of the simple fact that three of the greatest mass extinctions have been involved with periods of intense and chronic volcanism. The Siberian Lava Traps, in Asia, have been associated with the Permian mass extinction, or the greatest extinction of them all, which occurred 250 million years ago and ended with over ninety percent of sea life going extinct. The Deccan Traps of India are associated closely with the Cretaceous-Tertiary, or dinosaur extinction. Furthermore, the newly discovered Central Atlantic Magmatic Province has recently been associated with the Triassic-Jurassic extinction. These three extinctions are regarded as three of, if not the three, largest in the past 250 million years. It is obvious that there is a periodicity in times of intense volcanism, but there seems to be no reason why large areas of the earth would simply open up and start spewing out huge amounts of lava exactly every 26.2 million years. Because of this the theory is not treated as seriously as the extraterrestrial ones.

The second major terrestrial theory of periodic mass extinction causes is the hypothesis of glaciation. The Ordovician extinction, which occurred approximately 438 million years ago (out of Sepkoski\'s range) was associated with a long period of rapid glaciation, caused by the southern supercontinent called Gondwanna\'s gradual drifting over the south pole. Furthermore, paleontologist Steve Stanley of Johns Hopkins University believes that global cooling, which causes glaciers, leads to most mass extinctions, including the infamous dinosaur episode (Gore 689). But, as with the volcanism theory, the glaciation hypothesis bites the dust, too, because there are simply no known factors that would suddenly cause the seas to close up and glaciers to form exactly every 26.2 million years.

In hindsight, the mass extinction debate has caused a considerable amount of excitement in the field of science. Are mass extinctions periodic? If so, what factors cause them? Enthusiastic scientists have suggested both earthly and extraterrestrial causes in an attempt to explain the extinctions, but what is absolutely correct is anyone\'s guess. But, if some of these theories hold true, we may see ourselves, the entire human species, wiped out. If Sepkoski et. al are correct, the cycle they propose is one that even our greatest technology cannot interrupt or terminate.

In other words, no matter how intelligent, intellectual, and omnipotent we have, or think we have, become, we are still ruled by the intense and sometimes gruesome laws of Mother Nature. Another 13 million years, that is all we have, so I suggest that we make the most of it.

References

Alvarez, L.W., W. Alvarez, F. Asaro, and H. V. Michel. \"Extraterrestrial Cause for the Cretaceous-Tertiary boundary.\" Science, v. 208: 1095- 1108.


Alvarez, W. T. rex and the Crater of Doom. New York: Vintage Books, 1997.

Gore, R. \"Extinctions.\" National Geographic, June 89: 679-689.

Lampton, C. Mass Extinctions-One theory of why the dinosaurs vanished. New York: Franklin Watts, 1986.

Lessem, D. Dinosaurs Rediscovered. New York: Touchstone, 1992.

Olsen, P. \"Giant Lava Flows, Mass Extinctions, and Mantle Plumes.\" Science, v.284: 604.

\"Our Galaxy\'s Deadly Spiral Arms.\" Sky and Telescope, July 98: 20. \"Patterns\" Patterns of Extinction,
http://www.wf.carleton.ca/Museum/ extinction/patterns.html: 16 November 1999.

Powell, J.L. Night Comes to the Cretaceous. New York: Harcourt Brace, 1998.

Raup, D. and J.J. Sepkoski, Jr. \"Periodicity of Extinctions in the Geologic Past.\" Proceedings of the
National Academy of Sciences of the United States of America, v. 81: 801- 805.
Wilford, J.N. The Riddle of the Dinosaur. New York: Knopf, 1985

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