A University of California, Davis, researcher has discovered what he believes to be the first mass extinction, hundreds of millions of years ago, documented by the Earth's early fossil record. Called the Botomian mass extinction, this previously overlooked event marked the elimination of probably more than 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. "This was more than just an extinction, it was a diversity crisis," Signor said. "Often in the history of life, extinctions are balanced by originations of new species and genera. Not at this time. It's the combination of extinction and origination that controls diversity." As with other mass extinctions later in the planet's history, fewer new types of animals appeared during the several million years immediately following the Botomian mass extinction than during any other time in the early Cambrian, Signor said. Estimated to have begun 530 million to 600 million years ago, the early part of the Cambrian Period marks the time when most animals with skeletons first appeared. "This might reinforce the notion that a lot of sorting occurred early in the fossil records," Signor said. "Some groups made it, others didn't." In addition to its evolutionary implications, the Botomian mass extinction might be the most severe extinction of the geological past, in terms of absolute numbers of animal groups lost, Signor said. More than 80 percent of the many different groups of animals were lost. However, this -more- 2-2-2 Mass Extinction mass extinction occurred during a time of turmoil and high turnover among the types of creatures living on Earth. Accounting for the high background extinction rate characteristic of early life on Earth, Signor estimated that the net loss due to the Botomian mass extinction might be comparable to the extinction that wiped out the dinosaurs 65 million years ago. Just as mammals inherited the terrestrial realm after the most recent mass extinction rid the planet of dinosaurs, trilobites seemed to have been the big winners in the Earth's earliest mass extinction. Today, scorpions and horseshoe crabs are the closest living relatives to the thick-skinned, many-legged, multisegmented trilobites. Trilobites fared so well, according to Signor's global data base of shelled marine organisms during the early Cambrian Period, because the extinctions of certain trilobites were balanced by the appearances of new types of trilobites. The diverse groups of trilobites subsequently accounted for an estimated 75 percent of Cambrian animal species. Trilobites thrived for about 300 million more years before they became victims of another mass extinction. "The dominance of trilobites began with the Botomian mass extinction," Signor said. 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. About 90 percent of the archaeocyathan genera disappeared from Earth. "That has been known for a decade," Signor said. "But people didn't understand that it was part of a general mass extinction that affected the whole fauna." Small shelly fossils that constitute a category of taxonomic orphans also fared poorly in the Botomian extinction. On the other hand, a group of shell-bearing creatures called brachiopods nearly broke even, eventually diversifying and becoming the dominant group of animals for the next several hundred million years. "Contrary to conventional wisdom, this early extinction suggests that animals were not radiating into an empty world, free of biological stresses, a metaphorical Garden of Eden," Signor said. "It suggests that life was hard from the start, that there was no grace period when animals first invented the various ways of making a living that seem so familiar to us now." The Botomian extinction was an accidental discovery. Hoping to learn more about how the ancient continents were positioned at that time, Signor and a colleague, UC Davis geology -more- 3-3-3 Mass Extinction professor Eldridge Moores, had been examining the biogeographic distribution of organisms during different stages of the Cambrian Period. Signor has assembled in one data base what he believes is the most comprehensive documentation of the early Cambrian Period in any one place, incorporating new work by other researchers in the past decade based on fossil records in Siberia, China, Iran, Newfoundland and Eastern Europe. The five other extinctions generally recognized as major biological events happened at the end of the Ordovician Period about 439 millions years ago, during the late Devonian Period about 377 million years ago, at the end of the Permian Period about 245 million years ago, at the end of the Triassic Period about 208 million years ago, and finally at the transition between the Cretaceous and Tertiary periods (known as the K-T boundary) about 65 million years ago. Other aspects besides the loss of diversity convinced Signor to put the Botomian extinction in the same category as these other big ones. The extermination of reef-forming organisms, enhanced extinction and depressed origination of organisms, and a sea level drop all matched the pattern observed at most other mass extinctions, Signor said. Signor's paleobiological colleagues reportedly reacted with enthusiasm and some skepticism about the magnitude of the extinction when he presented his findings earlier this month. 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."