Extinctions: the earthly argument.

Extinctions: The earthly argument

Paleontologist Robert T. Bakker of the University of Colorado Museum in Boulder has a new candidate for "death star," the great unknown cause of mass extinctions (SN:4/21/84, p. 251). He does not believe, as some scientists do, that gigantic meteorites, crashing into the earth and blackening the sky with dust and ash, killed the dinosaurs and other extinct creatures. Bakker instead pins the death star, symbolically, on a man, the British Army general, Lord Horatio H. Kitchener, who in the late 19th century brought Asian cattle to Egypt, inadvertently setting off a plague of rinderpest that continues to this day and is slowly killing off the great herds of African antelope.

Bakker's idea is that species immigration -- which, like the Asian cattle immigration, often introduces disease into stable populations -- may account for a great share of the mass extinctions.

"It seems to me dinosaurs went out not with a bang, but with [illnesses such as] diarrhea," Bakker told participants at the meeting.

For one afternoon at the conference, Bakker and others who believe extinctions occurred gradually, not instantaneously by meteorite impact, held sway. At a session entitled "Death by Earthly Causes," they brought out their best arguments for the idea that extinctions were caused by such things as disease, changes in climate and changes in ocean temperature and salinity.

Bakker points out that most mass extinctions were preceded by a decline in "evenness" among species. For example, for a half-million years before the great extinctions at the end of the Cretaceous period (about 65 million years ago), there was an overwhelming abundance of Triceratops among the dinosaurs, Bakker says. "What we see before the end is the premonition of mass extinction," Bakker says. "The giga-fauna [largest animals] are reduced to low evenness."

Biologist J. David Archibald of San Diego State University describes evidences showing that 16 of 19 turtle species in Montana survived the extinctions at the Cretaceous-Tertiary (K-T) boundary. Such evidence supports the concept of gradual or "stepwise" extinction, but not one large catastrophe, Archibald says.

Jack A. Wolfe and Garland R. Upchurch Jr., paleobotanists at the U.S. Geological Survey in Denver, describe evidence from leaf fossils suggesting that the North American climate changed from dry to wet at the K-T boundary. Leaves from the Cretaceous period are small and have many hair follicles, which appear to be adaptations to drought, Upchurch says. But the leaves from the Tertiary period are much larger and lack evidence of the hairs that would help them retain water. Thus, Upchurch says, "The extinctions at the K-T boundary must be due to a complexity of ecological changes rather than a single factor."

Jonathan R. Bryan of the Florida State Museum in Gainesville reports a systematic study of sediment layers spanning the K-T boundary near Braggs, Ala. The fossil evidence from this site suggests that only 36 percent of the 83 species present at the site went extinct at the boundary, Bryan says.

William B. Gallagher of the New Jersey State Museum in Trenton reports a similar study of K-T boundary layers at Sewell, N.J. Noting the apparent gradual extinction of species in the layers, Gallagher does not suggest a specific cause of extinction, but proposes the more general "double-whammy theory: Things got bad, and then they got worse."

Although the gradualists dominated the afternoon's discussion, their counterparts in the extinction debate were present at the conference. One of them, David M. Raup of the University of Chicago, earlier in the day suggested that paleontologists may be too quickly judging the impact theories by "Roman law," -- that is, by saying, "They're guilty." He proposes that paleontologists learn more about extraterrestrial phenomena that affect the eart. "Luis Alvarez [who came up with the impact theory] is learning paleontology as fast as he can," Raup says. "And it would be far easier for us to learn astronomy."