The Extinction of the Dinosaurs
A Research Program at the Milwaukee Public Museum
by Peter M. Sheehan
Curator of Geology
EDITOR'S NOTE: This article is reprinted without illustrations from LORE magazine, a benefit of museum membership. © 1996 Milwaukee Public Museum, Inc.
The reign of dinosaurs ended about 65 million years ago. The search for the cause of this extinction has brought together a diverse array of scientists -- astronomers, chemists, ecologists, physicists, evolutionary biologists, oceanographers and geologists. The Milwaukee Public Museum is playing a central role in this research.
Hundreds of hypotheses have been proposed to explain the extinction of dinosaurs including: competition from the mammals, diseases, and even allergies to the newly evolved flowering plants. But these kinds of explanations could not explain all events associated with the extinction. More importantly, because the extinction happened to both land and sea animals, the cause has to be a global event which effected both environments. In the late 1970s, paleontologists were certain they know the cause of extinction -- climatic change. Vast inland seas retreated, and the climate became harsher over several million years.
The Dig-A-Dinosaur Program Begins
In 1978, the Geology and Exhibition Program sections at MPM began to plan the Third Planet Hall which was to feature a dinosaur diorama. Dr. Robert (Mac) West began a series of expeditions to Montana to collect fossils for the diorama. Thus, the Dig-A- Dinosaur Program was born. Since then, hundreds of volunteers have assisted the Museum with field work.
A New Extinction Hypothesis
In 1980 a team from the University of California led by Nobel Laureate Physicist Dr. Louis Alvarez and his son, geologist Dr. Walter Alvarez, mad an exciting discovery. Working in Italy in rock deposited in a deep ocean environment, they found a clay layer at the horizon where marine organisms became extinct. The clay had remarkably high content of the element iridium. This element is extremely rare on the surface of the earth but meteorites have abundances similar to that in the clay layer.
The California team reasoned that the iridium came from an asteroid that collided with earth. Asteroids are believed to have the same composition as meteorites. When the asteroids hit the earth, the impact demolishes them and creates a huge crater. The iridium layer apparently came from a cloud of dust and debris that was thrown above earth's atmosphere, forming a dense dust cloud that encircled the earth. As the dust settled, it formed a layer of clay over the entire surface of the earth. Physicists modeled the impact and found that the iridium dust layer would have required an asteroid six miles in diameter that excavated a crater 100 miles across. As the dust settled, over five months or more, the cloud blocked sunlight from the earth's surface. Photosyntheses would have stopped, and plants would have died both on land and in the oceans, causing animals feeding on the plants to become extinct.
The new evidence created a controversy between scientists who believed the extinction event was caused by the asteroid impact and those who believed it was caused by gradual climatic change. The gradualist group reasoned that even if an impact had occurred, it was only a nail in the coffin for the ecosystem which had already deteriorated beyond hope for recovery.
The MPM Extinction Project
Back in Milwaukee, the Third Planet exhibit opened in 1983. The Dig-A-Dinosaur Program had many devoted volunteers who were anxious to continue field work. They were already trained by the museum staff to work in the Hell Creek Formation which -- in a remarkable case of serendipity -- turned out to be the only known rocks in the world that preserved a diverse dinosaur assemblage at the end of the age of the dinosaurs. Another piece fell into place when we became associated with Dr. David Fastovsky who was completing his Ph.D. dissertation a the University of Wisconsin on the sedimentology of the Hell Creek Formation.
A five-person team was assembled to study the extinction. My interest began with studies of a much earlier extinction event that occurred about 400 million years ago. Diane Gabriel and Claudia Berghaus in the Geology Section had extensive knowledge of the vertebrate fossils from Hell Creek. Dr. Fastovsky, now at the University of Rhode Island, became our sedimentologist, and Dr. Raymond G. Hoffmann, Head of the Division of Biostatistics at the Medical College of Wisconsin became our statistician.
Expedition volunteers, through the Friends of the Milwaukee Public Museum, were our primary source of funding. Terry and Mary Kohler and the Windway Foundation, Inc. were our primary sponsors contributing their own time in the field; substantial funds; resources from field equipment to a helicopter for reconnaissance and invaluable staff members (especially Ken Wendt who set up an incredible computer system to record data and provide computer generated maps). Additional funds and in-kind donations were provided by Marquette Electronics Foundation, Beatrice Foods, Johnsonville Sausages, Coleman Outdoor Products and the Igloo Corporation.
The Hell Creek Formation
About 300 feet of sediments were deposited over eastern Montana and western North Dakota during the last 2.2 million years of the existence of dinosaurs. These sediments have become the banded rocks called the Hell Creek Formation. The iridium layer was found at the top of the formation. The formation was formed from sands and silts laid down in stream channels and on vast flood plains were silt and clay were deposited during floods. Occasionally, dinosaur bones were entombed in these sediments. Much later. the entire region was uplifted and rivers cut down through these sediments exposing them in the badlands.
Tests of the two hypotheses were developed. If the gradual extinction theory was true, we should find that dinosaurs gradually declined during the last few million years of the Cretaceous. If the asteroid theory was true, dinosaur communities should be stable until their abrupt demise.
Data were collected by volunteers who spread out in search parties exploring the badlands for dinosaur remains. Moving from older beds at the bottom to younger beds at the top of the formation, we went over rocks deposited during the final 2.2 million years of the existence of dinosaurs. Each dinosaur fossil was identified. Records were made of the position in the formation and the kind of sediment in which the fossil was found.
During three long, hot summers we collected data, essentially conducting a census of dinosaurs just prior to their extinction. Volunteers found more than 2,000 dinosaur fossils.
Each fossil bone was assigned to one of the families of dinosaurs. Different families of dinosaurs have distinct functions in the ecosystem. For example, members of the family Ceratopsidae were herbivores, while members of the families Tyrannosauridae and Dromaesauridae were carnivores. By recording the number of individual animals belonging to each family we were able to compare the relative proportions of each of the families found in dinosaur communities through Hell Creek time. The Shannon Index was used to measure ecologic diversity.
A Sudden Extinction
The first results of the project were published in the journal SCIENCE in October, 1991. Our studies of ecologic diversity showed that communities of dinosaurs were not deteriorating during Hell Creek time as the gradualist hypotheses predicted. The results are consistent with the asteroid impact hypothesis.
Analysis of the vast data base continues. We are now examining the relative abundance of individual dinosaurs. Many months of analysis are needed before we will have a definitive answer. However, based on the results of our community study we expect to find that dinosaur abundance was not deteriorating during the latest Cretaceous as would be expected if dinosaurs had died gradually.
What Allowed Some Animals To Survive?
Not all Animals died when the dinosaurs became extinct. In fact, land mammals survived and soon flourished. Why did some animals survive, while others became extinct? the answer may lie in the feeding habits of the two groups. Most of the extinct land vertebrates were animals that lived in food chains that relied directly on living plant matter. These groups became extinct when the dust cloud caused by the asteroid impact blocked sunlight and killed living plants.
Other animals, including many of the mammal that lived alongside the dinosaurs, were in a different food chain. During the age of dinosaurs no mammals were largely herbivores or predators. Most mammals were very small, shrew-like insectivores. These animals ate insect larvae, worms and small arthropods which, in turn, feed on dead and decaying plant matter. The temporary loss of sunlight had little effect on this food chain that was dependent on dead rather than living plant matter.
Tags: Geology Science Education