Researchers have found evidence of an early major mass extinction of animals, the sudden disappearance of three-quarters of the plankton in the sea 650 million years ago at a time when such single-celled creatures dominated life on earth, according to a National Science Foundation report that will be released today.

About 70 percent of algae species disappeared, and almost all protozoan life was wiped out in the event, including all the most complex creatures living near the sea surface, said Andrew H. Knoll of Harvard University, who discovered the extinction along with Gonzalo Vidal of the University of Lund in Sweden.

The algae that disappeared were spherical, one-celled animals with a variety of spiked, knobbed or otherwise "decorative" surfaces, Knoll said. The protozoa were larger and came in a variety of shapes.

At that time the earth was inhabited only by one-celled animals, including bacteria, protozoa and algae.

The population crash came during an era of glacial expansion, and so Knoll speculates that the movement of the glaciers may have caused it.

Of the approximately 40 species of algae then alive, 34 vanished, and virtually all the half-dozen protozoans of the sea disappeared from the fossil record for 100 million to 150 million years.

Until now, the earliest known mass extinction was 450 million years ago, when many species of shell-covered marine animals disappeared.

The evidence for the mass extinction of 650 million years ago came from the analysis of sedimentary rocks from several locations, including Scandinavia, the Baltic region and Greenland.

"Extinctions are a fact of evolutionary life," Knoll said. "The fossil record clearly shows that most of the species that have sat, run or swum on the earth during its history no longer exist."

The extinctions of many species are not spread evenly through history, however. They sometimes come in spectacular crashes of animal life, such as the sudden disappearance of the dinosaurs, or in this case the vanishing of most of the earth's species of algae and plankton.

Knoll said he could not tell what the effect of the disappearance of the algae and plankton was. He said it may have been very small, since similar species eventually appeared again on earth.

Or, since the extinction occurred at about the same time as the appearance of the first multi-celled animals, the two events might be connected.