The intriguing issue of what did in the dinosaurs continues to roil paleontology. Was it an asteroid, volcanoes on the moon, or more down-to-Earth causes?
Sixty-five million years ago - during the transition between what geologists call the Cretaceous and the Tertiary or Paleocene periods - half the life forms on Earth, including the dinosaurs, died out. It's one of several puzzling ''sudden extinctions'' in the fossil record - one which, in recent years, has become the subject of lively scientific controversy.
Proponents of various ''explanations'' were able to air their views during a session of the recent annual meeting of the American Association for the Advancement of Science (AAAS).
Geologist Walter Alvarez, his physicist father, Luis, and their colleagues at the University of California at Berkeley, who started the debate by invoking an influence from outer space, pressed their claim that an asteroid did it. The effects of its collision with Earth could have caused the mass extinctions in as short a time as three months, they say.
However, skeptics who give more credence to fossil evidence than to speculations based on physics said they are becoming more firm in their disbelief. Paleontologist William A. Clemens, also at Berkeley, and Leo Hickey of the Smithsonian Institution insisted that, whatever the cause, the ''sudden extinctions'' were not that sudden. They took place over tens of thousands or hundreds of thousands of years -- a short time geologically, but a long time in terms of biological processes.
Thomas J. M. Schopf of the University of Chicago summed up this line of argument by saying, ''It does not appear that the demise of the dinosaurs can be used as evidence for a single catastrophic event.'' The extinctions, he explained, are most likely the result of a deteriorating environment as climate cooled and land forms changed.
Still other interested scientists, acknowledging that the extinctions were neither simultaneous nor instantaneous, appealed to volcanism -- either on Earth or on the moon. John A. O'Keefe of the NASA Goddard Space Flight Center, who favors the moon, suggested that an erupting lunar volcano sent a stream of particles toward Earth. These, he explained, formed a ring around the planet. By filtering incoming sunshine, they cooled Earth's climate. Over many thousands of years, this could have had disastrous consequences for the dinosaurs and many other organisms, Dr. O'Keefe suggested.
In the background of the discussion was the mysterious ''iridium anomaly.'' Its discovery had led the Alvarezes and their colleagues to start the debate over the dinosaurs' fate in the first place.
A thin layer in sedimentary rocks marking the end of the Cretaceous period in some deposits has an unusually high concentration of the element iridium. At Gubbio, Italy, in a formation with one of the most complete geological records for the period, the concentration is some 25 times that of rocks above and below the Cretaceous-Tertiary boundary layer. Since iridium is rare on Earth compared with other parts of the solar system, this suggested an extraterrestrial source for the iridium anomaly -- a source that might also have been the ultimate cause of the Cretaceous extinctions.
At first, the Alvarez team thought in terms of a nearby supernova explosion, perhaps within a tenth of a light-year. A star explosion that close could be expected to have had severe environmental effects on Earth, including exposing the planet to dangerous radiations. However, it should also have left a special form of plutonium -- plutonium 244 -- along with the iridium in the anomalous sedimentary layer. No such plutonium has been found. So the Alvarez group looked to other extraterrestrial causes and settled on an asteroid impact.
Such an impact on land would have raised vast dust clouds that could have blocked the sun and perhaps cooled the climate, these scientists say. An impact in water would have injected large amounts of water vapor into the air. This gas traps heat being radiated outward by the planet's surface. Its presence could have warmed the climate enough to have led to the extinctions.
The Alvarez team also notes that the energy released by the impact could have warmed the atmosphere rapidly, producing a ''quick burn'' that would have caused rapid worldwide extinctions of many land and ocean species. At the AAAS session, Walter Alvarez explained that he believes this effect could have been realized over a period as short as three months.
Such a quick catastrophe is hard for many paleontologists to swallow. As Clemens, Hickey, and Schopf pointed out, it simply doesn't square with the fossil record as they read it. But not all interested scientists are ready to abandon the asteroid hypothesis on that account.
Kenneth J. Hsu and colleagues at the Swiss Federal Institute of Technology suggest that an asteroid or comet could have introduced poisonous material into the sea. In a paper Dr. Hsu read at the meeting, they speculated that this could have killed off many organisms. Loss of such organisms, which help take carbon dioxide (CO2) out of the atmosphere, could in turn have led to an increase in the atmospheric CO2 concentration. Since CO2 also traps heat, this could have warmed the climate enough to have led to dinosaur extinction over a period of some thousands of years, the Swiss scientists said. Dinosaur reproduction rates could have been impaired by the warmer climate.
Skeptics, however, say they can do without such exotic speculation. Dewey M. McLean of Virginia Polytechnic Institute and State University explained that he thinks the Cretaceous extinctions could more easily be attributed to global effects of a major volcanic outbreak, called the Deccan volcanism, in what is now India. This, he said, could have led to massive loss of marine organisms and a rise in atmospheric CO2, with subsequent climatic warming and loss of the dinosaurs.
Compared with the colorful speculations of the Alvarezes or the O'Keefe, such a scenario, McLean said, ''offers simplicity based on natural, earthly processes.'' Be that as it may, it was evident from the AAAS session that the extraterrestrial speculations are the subject of some devoted and lively research. The debate over the fate of the dinosaurs is not likely to quiet down for some time.