Major Extinctions
Major extinctions refer to significant periods in Earth's history when a large number of species disappeared in a relatively short timeframe, sharply contrasting the normal rates of extinction. Paleontologists broadly recognize at least five major extinction events that have resulted in the loss of more than 50% of Earth's species, with the most catastrophic occurring at the end of the Permian period, possibly wiping out up to 95% of species. The causes of these mass extinctions are still debated, with theories ranging from asteroid impacts, as suggested by the discovery of the Chicxulub Crater, to volcanic activity and gradual environmental changes. These events have not only shaped the course of life on Earth but also created opportunities for new species to thrive, such as the rise of mammals after the dinosaurs' extinction. Current research seeks to understand both the specific triggers of historical extinctions and the potential for a sixth extinction linked to human activities and climate change. Ongoing discussions in the scientific community highlight the complexity of extinction dynamics and the importance of understanding these patterns to inform conservation efforts today.
Major Extinctions
Introduction
Paleontologists have identified periods in the earth's history in which large numbers of species became extinct in a relatively short period of time. These periods contrast with the standard rate at which new species emerge and become extinct. Scientists call these peaks in extinction rates “mass” or “major extinctions.” The precise definition of a major extinction is the subject of significant disagreement and debate. The scarcity of fossils from distant periods leaves gaps in the fossil record that complicate efforts to measure both the timing and magnitude of extinction events.
Paleontologists generally agree that there have been at least five major extinction events, identified according to their position in the geologic time table: (1) the Triassic–Jurassic, (2) the Permian–Triassic, (3) the Devonian–Carboniferous, (4) the Ordovician–Silurian, and (5) the Cretaceous–Tertiary extinctions. Each of the major extinctions resulted in the loss of more than 50 percent of the earth's species. The largest extinction event, at the end of the Permian, may have resulted in the extinction of up to 95 percent of species.
Some scientists study mass extinctions in an effort to predict or prevent factors that may lead to a sixth major extinction, potentially linked to human activity. Mass extinctions have been an important feature in the history of life on earth, causing major shifts in the types of organisms that dominate the environment. The extinction event at the end of the Cretaceous, for instance, resulted in the death of all species of dinosaurs (except birds) and allowed mammals to expand into the roles once filled by dinosaur species.
Key Terms
Background Extinctions: A lower, more average rate of species extinction that occurs during periods when a mass extinction is not the cause.
Chicxulub Crater: Large crater in Mexico's Yucatan Peninsula that scientists believe resulted from the impact of a large meteor at the end of the Cretaceous.
Radioisotope Dating: Also called “radiometric dating,” it is the process of using the decay of radioactive isotopes, found in minerals, to estimate the age of a geologic sample.
Extinction: The complete disappearance of a species occurring when the last living representative of the species dies.
Fossil Record: The collective record of life on earth, as revealed from the discovery of fossilized remains of organisms.
Geological Time Scale: A chronologic scale for placing events that have occurred at different times in the earth's history.
Impact Events: The collision of an extraterrestrial object, most often a meteor, with the earth's surface, often cited as a potential cause or precipitating event leading to a mass extinction.
K–T Boundary: Layer of sediment deposited at the end of the Cretaceous in which paleontologists have discovered evidence of a meteorite impact that may have played a part in the extinction of most dinosaur species.
Press/Pulse Model: Theory that mass extinction occurs as a result of two different causes—long-term pressure from environmental sources (called the “press event”) and a precipitating shock, such as a sudden catastrophe, to the local or global ecosystem (called the “pulse event”).
Volcanic Theory of Mass Extinction: Theory that a rapid increase in volcanic activity, resulting in periods of major volcanic eruptions, could change environmental conditions enough to lead to a major extinction event.
Key Players
Norman D. Newell: Pioneering paleontologist Norman D. Newell was one of the first scientists to argue that mass extinctions occurred in the earth's history. Newell's put forth this argument in the article “Revolutions in the History of Life” (1967), which drew upon research from a number of studies published in the late 1950s and early 1960s. As a professor, Newell trained many paleontologists who went on to become key figures in the study of extinction and its effect on evolution, including Stephen Jay Gould and Niles Eldredge.
Luis and Walter Alvarez: Father-and-son research team Luis and Walter Alvarez were the first to suggest that the extinction of the dinosaurs at the end of the Cretaceous was initiated by the impact of a large meteor. The Alvarezes collected data supporting this theory with the discovery of iridium deposits at the K–T boundary, a layer of sediment marking the end of the Cretaceous. Their article on the subject, “Extraterrestrial Cause for the Cretaceous-Tertiary Extinction” (1980), precipitated a number of alternative theories explaining mass extinction in relation to extraterrestrial causes.
Jack Sepkoski and David M. Raup: Raup and Sepkoski were among the first paleontologists to provide thorough proof of the existence of mass extinctions, identifying five distinct extinction events in “Mass Extinctions in the Marine Fossil Record” (1982). In “Periodicity of Extinctions in the Geologic Past” (1984), Raup and Sepkoski proposed that extinction events were not random, but instead occurred at regular intervals of approximately 24 million years. Both scientists later pioneered a number of theories suggesting that extraterrestrial factors, including asteroid and meteor impacts, were the most likely cause of mass extinctions. Raup and Sepkoski's research initiated a worldwide effort to explain periodicity in mass extinction.
Shanan Peters: Geologist Shanan Peters published the paper “Environmental Determinants of Extinction Selectivity in the Fossil Record” in 2008, proposing the idea that changing sea levels are the most important factor in determining whether a mass extinction will take place. Peters believes that factors such as increased volcanic activity and extraterrestrial impacts may have played a smaller role. Peters's work has led to new efforts to match extinction events with patterns in sea levels and the accompanying environmental changes.
History
Early Study of Extinction: In the early 1800s, paleontologists began to notice that fossils common at one level in the sediment were not present in subsequent layers. At first, these gaps were thought to be discrepancies in the fossil record but, as paleontologists compared data from multiple sites, patterns began to emerge, leading some to speculate that there had been periods when thousands of species became extinct in a relatively short period of time. Some supported this theory of “mass extinctions,” but others, including Charles Darwin and paleontologist Charles Lyell, argued that extinction was a gradual process and that mass extinctions were an artifact of an incomplete fossil record.
Paleontologist Norman Newell was among the first to consolidate evidence for the existence of major extinctions, drawing on a number of studies from the 1950s. Newell's research, published in 1967, left little doubt among most paleontologists that extinction events had occurred in the past and were not the results of gaps in the fossil record. The advent of radioisotopic dating strengthened the case by helping scientists to more accurately estimate the age of geological samples, thereby create a better working model of geologic time.
Cause and Periodicity of Extinctions: In 1980, researchers Walter and Luis Alvarez proposed that an asteroid impact was the best potential cause for the extinction of the dinosaurs. Evidence for this theory came largely from the appearance of iridium at the K–T boundary, an element uncommon in the earth's crust, but often found in minerals from meteors. They reasoned that a massive meteor might have jettisoned iridium into the atmosphere, leaving it to settle in the surrounding settlement. The discovery of the Chicxulub Crater in Mexico gave credence to the Alvarezes’ hypothesis.
Over the next several years, paleontologists David Raup and Jack Sepkoski published a series of studies indicating that extinction events occurred on regular cycles of between 24 and 30 million years. The authors compiled a variety of evidence to support their argument and also supported the idea that extinction events might be tied to extraterrestrial phenomena, such as asteroid impacts. (See Raup 1992.)
One theory proposed during this period suggested that a belt of comets orbiting the sun, called the “Oort cloud”, periodically released comets that might fall into earth's orbit, leading to impact events. Evidence from the K–T boundary supported this theory, but paleontologists were unable to find definitive evidence of impact events coinciding with other extinction events. An alternative theory, developed in the 1990s, suggested that extinction events may be linked to periods of increased volcanic activity resulting in the formation of “super volcanoes” capable of creating massive shifts in environmental conditions.
Current Research and Implications
Combination of Causes: In the twenty-first century, many paleontologists are moving away from the “single cause” approach to understanding extinction and, instead, believe that the factors causing both background extinctions and major extinctions are similar. The press/pulse theory, proposed in 2006 by Nan C. Arens and Ian D. West, is one of a new breed of theories suggesting that in order for a mass extinction to take place, there must be a steady increase in extinction pressure, brought about gradually by changing environmental conditions. This constant pressure is brought to a peak by the addition of a “pulse” event, potentially in the form of a major environmental shock, such as an impact event or massive volcanic episode.
In 2008, geologist Shanan Peters published research suggesting that each of the earth's extinctions, including the five major extinctions, can be linked to falling sea levels. Peters's research does not preclude other causal events, but may instead indicate that falling sea levels represent a gradual pressure that is later exacerbated by another major environmental change. (See Peters 2008.) In 2011, researchers from the California Institute of Technology published new research utilizing recently developed methods of measuring temperature in sedimentary samples. The study indicates that climate cooling played a major role in the Ordovician extinction and constitutes the first time that scientists have found definitive links between climate change and extinction.
Continued Disparities: Despite decades of dedicated research, new findings occasionally indicate that many of the theories on extinction are in need of major revision. For instance, in 2005, physics researchers at the University of Berkeley published the results of a four-year study indicating that mass extinctions occur on a regular cycle of approximately 62 million years—a significant variation from the cycle of between 24 and 30 million years developed by Sepkoski and Raup.
Another current debate regarding extinction is the potential for a sixth extinction in the future, perhaps linked to human impact on climate change. The scientific community is divided on the subject of the sixth extinction, though the potential for a massive global catastrophe has provided strong motivation for research and adds to concerns about climate change and other effects of human life on the environment.
Bibliography
Books
Adams, Jonathan S. Species Richness: Patterns in the Diversity of Life. New York: Springer, 2009.
- Chapter 3 examines extinction events from a modern perspective, informed by studies drawing from the life sciences and geology. Contains information pertaining to the history of research into the nature of mass extinctions.
Lieberman, Bruce M., and Roger Kaesler. Prehistoric Life: Evolution and the Fossil Record. Hoboken, NJ: Wiley-Blackwell, 2010.
- A review of the literature, discoveries, and major concepts in paleontology. Contains sections on mass extinctions and discussions of extinction theories.
Raup, David. Extinction: Bad Genes or Bad Luck? New York: W. W. Norton, 1992.
- An overview of the debates and theories surrounding mass extinctions, presented by one of the pioneering scientists in the field. Discusses many potential contributing factors to extinctions, as well as the potential benefits of extinction to the overall diversity of global ecosystems.
Ridley, Mark. Evolution. Hoboken, NJ: Wiley-Blackwell, 2004.
- Presents many of the major issues that arise when researching evolution. The section of “macroevolution” contains information on the major extinctions and their role in evolution.
Journals
Jablonski, David. “Mass Extinctions and Macroevolution.” Paleobiology 31 (2005): 192–210.
- Article from one of the pioneers in extinction research—discusses the role of mass extinctions in the evolution of new species, as well as some of the current research into the timing and causes of mass extinctions.
Peters, Shanan E. “Environmental Determinants of Extinction Selectivity in the Fossil Record.” Nature 454 (2008): 626–29.
- Presents evidence suggesting that changes in ocean levels are closely linked and potentially important causal factors behind extinction events. Also discusses the relationship of ocean levels to other possible causes of extinction.
Additional Works Used
Finnegan Sean, et al. “The Magnitude and Duration of Late Ordovician-Early Silurian Glaciation.” Science 331 (2011): 903–906.
Green, Waltan A., Gene Hunt, Scott L. Wing, and William A. DiMichele. “Does Extinction Wield and Axe or Pruning Shears?: How Interactions Between Phylogeny and Ecology Affect Patterns of Extinction.” Paleobiology 37 (2011): 72–91.
McKie, Robin. “Bad News—We Are Way Past Our ‘Extinct By’ Date.” Observer. 13 Mar. 2005. <http://www.guardian.co.uk/science/2005/mar/13/research.science>.