A portrait of Peter Sheehan

by Steve Brusatte

Paleontology is truly a fascinating field. Defined as the study of fossils, paleontology encompasses everything from the smallest foram to the fearsome Tyrannosaurus to the most ancient human. Paleontology, contrary to popular belief, is NOT the study of dinosaurs. That is a common misconception among many. True, the field does include dinosaurs, but also so much more.

When you see the word paleontology the first thought that comes into your mind is dinosaurs, and there is a good chance that you automatically focus on the K-T extinction. Yes, the extinction of the dinosaurs is one of the oldest and most fascinating paleontological dilemmas. Just how did the dinosaurs, the majestic beasts that ruled the prehistoric landscape for over 150 million years, meet their demise? Was it sudden, possibly caused by an asteroid? Were the terrible lizards gradually declining, by a result of global cooling or a drastic environmental change? The most interesting part is that nobody knows for sure, and that nobody will
probably ever know with 100 percent certainty.

In 1980 Luis Alvarez and his son Walter presented an interesting hypothesis. After studying K-T (Cretaceous-Tertiary) boundary rocks in Italy, the two noticed high levels of iridium, a mineral often present in comets and extraterrestrial bodies. They used these observations to announce that they believed that the dinosaurs died as a result of a sudden asteroid striking the earth. Ever since this surprise announcement, the world has been struck by dino fever.

But, how can we prove that this truly happened? Once we prove that an asteroid did strike the earth, how can we prove that it actually killed off the dinosaurs? Both of these questions have been plaguing paleontology since the father and son presented their thesis. Now, the question may be solved.

Peter Sheehan is the Curator of Geology at the Milwaukee Public Museum in Milwaukee, Wisconsin, USA. Ever since he left the University of California-Berkeley with his Ph.D., Sheehan has been working to try to discover how the dinosaurs did die out. For those of you familiar with the June 1989 National Geographic (with an Apatosaurus on the cover) you probably recognize the name of Peter Sheehan. At the time of that publication, Sheehan was in the midst of one of the great paleontological field expeditions of all time, a three year trip to the Hell Creek formation in the western United States.

The point of the trip was simple, Sheehan and his colleagues were to map each and every K-T fossil they could possibly find in the three layers of the Cretaceous rock formation. After every fossil was mapped, Sheehan was to compare them. If the species were changing, Sheehan stated, there must have been serious environmental stress that was forcing them to evolve rapidly. If evidence of environmental stress was present, he stated, the dinosaurs most likely died out gradually. After the three years of field work, Sheehan took the evidence to his lab and compared the specimens. After much research and time, Sheehan concluded that the ecologic diversity did indeed remain constant up until the K-T boundary, proving that the dinosaurs did indeed die out abruptly (Dino Fest Volume, Rosenberg and Wolberg, 1994).

But, this evidence did not convince everyone. Angry paleontologists were outraged that a geologist was trying to reconstruct the extinction. Instead of a rock expert, an ancient life expert should try to plot out the problem, some paleontologists stated.

Sheehan then went to work again, trying to convert the nonbelievers. This time he joined a group from Syracuse University. The team studied the rocks of the Yucatan Peninsula in Mexico, the site where a large asteroid did indeed hit at the end of the Cretaceous (basically, paleontologists accept that an asteroid did hit the earth at the end of the Cretaceous, but the debate is whether it suddenly killed the dinosaurs or finished off a declining species).

While studying the formation, the group located a mineral called anhydrate, which contained sulfur. This anhydrate would have vaporized when the asteroid hit the earth, the team hypothesized. When the mineral vaporized, it would have spread out over the planet, and, if there was a high enough level, it would have led to a great flow of acid rain, which may have helped kill off the dinosaurs. The team reconstructed the impact, and calculated how much anhydrate would have been vaporized. Once they figured out the total amount, they then calculated how the sulfur would have been spread evenly over the earth. The team concluded that yes, indeed, there was enough sulfur to cause high amounts of acid rain.

Still, many were not convinced, and Sheehan went back to one of his original theories, published in the journal Geology in 1986, to further prove his point. In his 1986 journal article, Sheehan and his colleague Thor Hansen presented a simple theory.

The theory stated that the majority of organisms that became extinct at the K-T boundary, were tied to food chains which included living plant matter. Furthermore, they went on to say that most organisms that survived were involved in food chains that included the consumption of dead plant matter (detritus matter).

This theory ties into the original asteroid extinction theory. The Alvarezs hypothesized that the asteroid would have thrust a large cloud of dust into the atmosphere, which would have blocked out the sun for about three months. This, of course, would have halted
photosynthesis, which would have killed most live plants. This would cause a severe disruption of not only the herbivorious dinosaurs, but the carnivores which relied on the plant eaters for food.

The chronic dying of the plants would have caused an excess of dead plant matter, which would have increased the detritus food chain member chances for survival. The small mammals, alligators, turtles, birds, and animals that you currently see today survived because they relied on detritus food chains.

With these three hypothesis Sheehan and his colleagues have shown that the sudden extinction of the dinosaurs was highly probable. The theory of acid rain and the hypothesis of detritus food chains further enhanced his work.

While some paleontologists still do not credit the work of Sheehan, it is hard to ignore it. While the topic of dinosaur extinction is
still heavily debated, Peter Sheehan of the Milwaukee Public Museum has added a wealth of new information to the current
thinking on dinosaurs.

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