The oldest known samples of air -- trapped for 80 million years as tiny bubbles in amber -- have led researchers to suspect that Earth's atmosphere may have been 1 1/2 times richer in oxygen in the days of the dinosaurs than it is today.

The finding, reported last week at a meeting of the Geological Society of America in Phoenix, is said to be preliminary (interactions with the amber may have altered the air's composition), but if further work confirms its reliability, it could lead to a major reassessment of how the atmosphere evolved.

The finding also raises several other questions. Did such high oxygen levels allow forms of life to evolve that could not survive in today's air? Where did the oxygen go when the atmosphere changed to today's lower concentration?

Some scientists also speculate that because of the high oxygen levels, chemical phenomena in the Earth's atmosphere, seas and soil may have been different than they have assumed. Some say the high oxygen level could also have turned small fires into unusually hot conflagrations.

The finding challenges the generally accepted view that the oxygen level has grown gradually over the ages.

Earth's primordial atmosphere has been thought to have been largely devoid of oxygen until plants appeared and carried out photosynthesis, releasing oxygen from carbon dioxide. Although animal metabolism and organic decomposition both consume atmospheric oxygen, plants make so much that, it was thought, air's oxygen content could only increase.

The amber samples contained air that appeared to be about 30 percent oxygen, compared with the slightly less than 21 percent it accounts for in today's air. The rest of today's air is 78 percent nitrogen, and argon accounts for slightly less than 1 percent and carbon dioxide about 0.03 percent. Several other gases are naturally present in trace amounts. The nitrogen, argon, carbon dioxide and trace components of the ancient air appeared in about the same relative proportions as today.

The amber air study was done by Robert A. Berner of Yale University and Gary P. Landis of the U.S. Geological Survey in Denver.

Amber is the hardened resin of certain trees. When the thick resin flows over the bark, it can trap small animals such as insects as well as small air bubbles, which may remain entombed for millions of years. The size of the bubbles that yielded the ancient air ranged from microscopic to that of a pinhead. Berner and Landis released the air by crushing the amber in a vacuum chamber.