Detecting mass extinctions in the fossil record
Mass extinctions were first identified by the obvious traces they left in the fossil record. In the strata corresponding to these time periods, the lower, older rock layer contains a great diversity of fossil life forms, while the younger layer immediately above is depauperate in comparison. Often, the rock layers bookending the mass extinction are noticeably different in their compositions. These changes in the rocks show the effects of environmental disturbances that triggered the mass extinction and sometimes hint at the catastrophic cause of the extinction.
|The Cretaceous-Tertiary boundary, as recorded in the rocks at Gubbio, Italy. At left, the later Tertiary rocks appear darker almost orange and the earlier Cretaceous rocks appear lighter when viewed with the naked eye. At right, magnification reveals few different sorts of microfossils in the later Tertiary layers (top), but a wide variety in the earlier Cretaceous sample (bottom).
Such dramatic changes in adjacent rock layers make it clear that mass extinctions were geologically rapid and suggest that they were caused by catastrophic events (e.g., a period of intense volcanic activity). Exactly what do we mean by "geologically rapid?" Even in cases where a mass extinction seems to have been triggered by a near instantaneous event (e.g., a massive asteroid colliding with Earth), the impact of this event on Earth's systems and biota may have taken much longer to play out. Figuring out exactly how much real time passed during a mass extinction is difficult and requires the use of state-of-the-art radioisotopic dating techniques. Using such techniques, geologists estimate that some of these massive extinctions took place in 200,000 years or less. Current techniques for dating such ancient rocks cannot pinpoint dates more specifically than this so we can't be sure if some mass extinctions took place in 150 years or 150,000 years. Either way, this represents a sudden event when compared to life's 3.5 billion year history. For example, if you compressed the entire history of life into a human lifespan of 80 years, a mass extinction would zip by in less than a day!