by Steve Brusatte
 

Since the dawn of the paleontological revolution in the 60\'s the mysteries surrounding mass extinctions have captivated us all. In 1980 Luis and Walter Alvarez took the mysteries to new heights when they made the controversial announcement that an asteroid was directly responsible for the extinction of the dinosaurs. This paper resulted in intense debate, with certain scientists pointing to a gradual extinction, while others agreed with the Alvarezs.

From the Silurian to the Permian to the infamous K-T, mass extinctions have been found nearly everywhere in the fossil record? Just what was the reason for these mass dyings? In the early 1980\'s J. John Sepkoski, Jr. and David Raup, both of the University of Chicago, began an immense study of extinct organisms, hoping to find a pattern to help explain these mysterious mass extinction.

Before I go into detail with their findings, I must make the somber announcement that Dr. Sepkoski suddenly died last month at the young age of 50. The doctors said it was heart failure. In all reality, his death was very sudden. Just three months before Dr. Sepkoski passed along the news to me that his wife was gravely ill, and I was shocked to hear the news that he was gone. Jack was one of my first interviewees for the book I am compiling. I will always remember him as a friendly and outgoing man. I remember him offering to call me from his home, in an attempt to save me money on my phone bill. He would always call me Mr. Brusatte, a title I surely did not reserve in a field with so many giants.

And, Dr. Sepkoski will always be remembered as one of these giants. His theories will live on, many of them still engulfed in controversy, even though he is gone. His most famous hypothesis will always be his theory regarding mass extinctions. Beginning in the early 80\'s he began to partake in an intense study of the life spans of thirty-five hundred families of sea dwelling animals over a 250 million-year period. He would then catalog the point in time at which each family of organisms appeared in the fossil record and the point where they then disappeared. Following his analysis Sepkoski and Raup began to analyze this information mathematically.

After analysis the two were surprised to discover that mass extinctions, from the Permian to the present, occur at exact 26 million-year intervals. In simpler terms, mass extinctions occur every 26 million years, with the most recent occurring 13 million years ago. Just years before their announcement a team of Princeton paleontologists announced that extinctions occurred at 32 million year intervals, but their theory lacked evidence. Sepkoski and Raup\'s hypothesis was backed by the past 250 million years of the fossil record.

Immediately after their announcement colleagues called for an explanation. Just what would cause mass extinctions to occur every 26 million years? To attempt to explain Sepkoski\'s results scientists attempted to correlate the extinction cycle with other, well understood cycles. The problem, though, was that there is no known cycle that requires 26 million years. The longest known cycle is the precession of the equinoxes, when the earth\'s axis gradually changes its orientation, and this takes only 26 thousand years.

The only explanation, many paleontologists theorized, had to lie in deep space. Only galactic cycles could take 26 million years. In their 1984 paper describing the study in \"The Proceedings of the National Academy of Sciences,\" Sepkoski and Raup wrote: \".we favor extraterrestrial causes for the reason that purely biological or earthbound physical cycles seem incredible.\" Immediately after the paper was published hordes of scientists set out with the goal to prove and disprove the hypothesis. One of these groups suggested that, possibly, the earth was bombarded by comets every 26 million years, which would have led to the extinction of many life forms. But, noted several scientists, we rarely see comets in the inner solar system. How could it be that these rare bodies could strike earth?

To solve this question, another group of scientists raised this simple question: what if the number of comets in the inner solar system were to increase? If this indeed could happen the chances for extraterrestrial strikes would increase drastically. And, with them the chances for extinction would sky rocket. But, just how would our chances of extraterrestrial impact increase? Many astronomers point to the Oort cloud.

In 1950 the great Dutch astronomer Jan Oort suggested that our solar system is surrounded by a vast cloud of comets. Based on the observation that many comets have orbits that take them as far as two light years away from the sun, Oort postulated that all comets must originate in a shell of material that, like the planets, also orbits the sun. This shell, which I must remind you is only a theory, has become known as the Oort cloud. Could it be possible that the heavilycometed Oort cloud is disturbed every 26 million years? Some astronomers, and paleontologists alike, believe that gravity may play a key role in the 26 million year extinction hypothesis. Many physicists postulate that, at the center of our galaxy, lies a galactic plane. This galactic plane is best defined as the vertical center of our galaxy. Because of its position, it is the point where the collective gravity of all the matter in galaxy is at its strongest.

Many astronomers hypothesize that, since matter tends to spread out in the galactic plane, a large cloud of gas and dust is present. In addition to this theory, many scientists believe that our solar system passes through this galactic plane on regular intervals, possibly twice every 62 million years. Now, they say, if the strong gravitational pull of this galactic plane cloud was strong enough to disturb the Oort cloud, it would send loose a shower of comets.

Because of the volume of comets, the chance of earth being struck by an extraterrestrial object would increase drastically. This increase in the chance of comet strikes, many paleontologists say, may be enough to cause mass extinctions on a 26 million year interval. The major problem with this theory, though, is that if the earth passes through the galaxy every 62 million years wouldn\'t these mass extinctions have to occur every 31 million years? Scientists theorize that our solar system \"bobs\" across the galaxy, making one complete trip through every 62 million years. If this is correct one half of this trip, or enough to reach the center of our galaxy, would take half of the 62 million years. Half of 62, as you know, is 31 million years. The return trip would also take 62 million years, with the solar system reaching the center 31 million years later.

While 31 million years appears to be rather far apart from 26 million, geologically speaking they are very close, which warrants a further investigation. According to Christopher Lampton, \"the two periods were sufficiently similar to imply the possibility of some kind of relationship.\" The problem is, though, this relationship has not yet been found. While this galactic plane theory gives shaky information, the death star hypothesis does not. This romantic theory suggests that our own sun may have a companion star, which every 26 million years may pass through the Oort cloud, sending hordes of comets on a collision course with earth. But, say several scientists, why don\'t we see this death star?

According to astronomer Piet Hut, the star may not be the yellow giant our sun is, but instead a brown dwarf. As you may know, a brown dwarf is a star that could not generate enough heat for fusion reactions to start. Instead, it is simply a mass of cooling matter. Could this mass of cooling matter have enough gravitational pull to disturb the Oort cloud?

Astronomers Daniel Whitmire and John Matese have suggested a theory similar to the death star hypothesis-the planet X theory. They argue that our solar system is home to not nine, but ten planets. This tenth planet, which they call planet X, although not seen may have enough gravity to disturb the Oort cloud. Although Sepkoski and Raup have generated much controversy with their hypothesis, it is safe to say that their idea has contributed greatly to the current paleontological revolution. But, in all reality, this theory does not define the entire career of Jack Sepkoski. Dr. Sepkoski is also noted for his work on marine evolution.

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