Mass Extinction

Extinctions occurring at an average or normal rate are distinguished from mass extinctions according to the circumstances. In other words, there is no firm line over which a large "normal" extinction becomes suddenly a mass extinction. Mass extinctions are characterized by the loss of large numbers of species in a relatively brief span of geological time. Usually, a brief span would be interpreted as a range of a few thousand to a few million years. The concept of a "large numbers of species" is usually expressed in terms of the percent of known fossil species becoming extinct (or disappearing from the rock record) over a brief span of time. A large percent might be in the fifty to ninety percent range. Some paleontologists prefer to express faunal and floral loss in mass extinction as a percent of genera, rather than species, because it is thought that this may be a more accurate way of accounting for death in mass extinctions. A genus is a group of species and is the next higher taxonomic level above species. On average, a fossil genus has about five fossil species (of course, some have more, some less), so when mass extinction is expressed in terms of genera (plural of genus) lost, it is close to, but not exactly the same as speaking of species loss.

The implication of mass extinction is that there was no time for a species to adapt to change. Sudden change was upon them and they died or could not effectively reproduce, therefore, the species was terminated. With so many enduring the same fate at once, scientists initially thought was that some overwhelming or global-scale force might have been at work. With such mind-boggling possibilities being entertained, some paleontologists stepped back and began to wonder if the fossil record was somehow deceptive. In other words, could the mass extinction or sudden death event be more apparent than real? There was considerable concern about the possible role of poor or selective fossil preservation, especially in the less abundant and more fragile species. Much study was devoted to this matter (and such studies continue today), but the global nature of such mass extinction events and their repetition through the rock record at selected intervals are a strong arguments in favor of a "more than just a lack of preservation" explanation for validity of the mass extinction record.

In 1992, paleontologist J.J. Sepkoski put together a graph that has been widely cited in many recent papers on mass extinctions. Sepkoski carefully researched paleonto-logic literature and from his reading, plotted the percent of extinction of genera (of marine organisms only) versus geologic time. Sepkoski chose the operative increment of geologic time as the geologic stage, which is a relatively small interval of time, averaging about five million years. He looked at loss of genera at the boundary between geo-logic stages, from 570 million years ago to present. The resulting graph showed varying levels of "normal" or background extinctions and also strong peaks of extinction that rise above the background level. The graph clearly illustrated the modern view of what is a mass extinction. The most obvious of these peaks occurs at the following levels (dates in millions of years before present): 530; 515; 510; 478; 448; 438; 421; 374; 367; 333; 320; 286; 253; 245; 225; 208; 193; 144; 91; 65; 36.6; 11.2; and 1.64. Of these 23 peaks, five were much greater than the rest (530, 438, 245, 208, and 65). These are known as the great mass extinctions in the history of life.