A UC Davis researcher has discovered in the Earth's early fossil record evidence of what he believes to be the first mass extinction,which occurred hundreds of millions of years ago. Called the Botomian mass extinction, this previously overlooked event marked the elimination of at least 90 percent of the animal species that lived early in the Cambrian Period more than 500 million years ago, according to Phil Signor, a paleobiologist and associate professor of geology at UC Davis, who unintentionally stumbled upon this major biological event in the midst of another study. He presented his findings at the recent North American Paleontological Convention in Chicago. Like a handful of other known mass extinctions, this biological upheaval knocked large numbers of organisms out of the evolutionary race and left relatively few survivors to become the ancestors for life on earth. The most severely affected animals were tropical reef-forming archaeocyathans, a group of ancient sponges that lived on the ocean floor and filtered tiny organisms from water currents for food. As to why the extinction happened, Signor cannot say. "Just identifying a mass extinction isn't enough to tell us about possible mechanisms," he said. "We must search for common ecological patterns among the victims and survivors that might point to a specific mechanism."