Mass Extinction
Mass extinction
Periodically, every fifty to one hundred million years or so, the earth has experienced mass extinctions, the relatively rapid, large-scale disappearance of many if not most living creatures. The world may be experiencing this phenomenon now at a far more rapid pace than ever before.
Of the five largest mass extinctions, the oldest occurred during the lower Ordovician period some 435 million years ago, when approximately one-quarter of all ocean families, and half of ocean genera, disappeared. (Creatures living in the sea are often used to measure the severity of mass extinctions, because the fossil record of marine sediments is more complete than that for terrestrial animals.) Among the creatures lost during this time were many types of trilobites, cephalopods, and crinoids, all of which are thought to have died out in shallow tropical waters because of sudden changes in ocean levels.
During the late Devonian period, about 357 million years ago, over a fifth of marine families and more than half of marine genera gradually died out over what may have been a ten-million-year interval. Climate and sea-level changes apparently doomed many types of corals, trilobites, fish, and brachiopods.
The most severe mass extinction of all took place at the end of the Permian period 250 million years ago. This destroyed as much as 96% of all plant and animal species , probably over an interval of at least a million years. Over half of all ocean families were wiped out, as were up to 80% of the marine genera. This was fatal for the remaining trilobites and most land species. But the "great dying," as the event is called, also created ecological niches for future life forms, including the dinosaurs.
In the Permian extinctions the suspected but unproven cause is a massive and extremely violent volcanic eruption in Siberia, which catapulted huge quantities of sunlight-blocking dust and aerosol droplets into the atmosphere . This, in turn, cooled the climate abruptly, expanding the polar icecaps, shrinking the oceans, and causing a new ice age . The rapid fluctuations in sea levels decimated the marine creatures of this period. To make matters worse, volcanic explosions could also have turned sulfate minerals into sulfuric acid and sulfur dioxide gas, which would have produced a ruinous rain of acidic precipitation. Over a 600,000 year period, these eruptions spewed a flood of molten basalt that created rock formations 870 miles (1400 km) in diameter next to Lake Baikal , an area called the Siberian Traps.
Another mass extinction occurred about 198 million years ago, during the late Triassic period, when a quarter of marine families and half or more of marine genera disappeared. Cephalopods, gastropods , bivalves, brachiopods, and reptiles, were destroyed, as were the conondonts, the fish from which vertebrates may have descended.
The most recent and best known mass extinction occurred a mere 65 million years ago, at the end of the Cretaceous period. It brought about the end of many creatures, including the dinosaurs and ammonites (shellfish). Sixteen percent of marine families and up to 46% of marine genera were lost at this time.
Scientists are still debating the causes of this catastrophic event. It could have coincided with (and may have been caused or accelerated by) a massive volcanic eruption and lava flood in what is now India, where it created a rock formation called the Deccan Traps. Some scientists suggest that a catastrophic climate change and a drastic cooling of the atmosphere made survival impossible for the dinosaurs, since they had no fur, feathers, or hibernating dens to shield them from the cold, nor could they migrate to a warmer climate. Other experts believe that the explosion of a nearby star cooled the atmosphere and emitted deadly radiation for thousands of years. Or perhaps herbivorous dinosaurs starved because they could not adapt to the new types of vegetation that developed. Their extinction would have killed off the carnivorous dinosaurs that preyed on the plant-eaters. Maybe the dinosaurs could not compete for food with the newlyemerging mammals, which ultimately replaced them as the dominant creatures on the planet.
In truth, much is unknown about the causes of the past mass extinctions. A theory with the weight of some evidence behind it raises the possibility that one or more comets or asteroids may have hit the earth, flinging billions of tons of dust and other debris—or ice crystals, if the object impacted in the oceans—into the atmosphere. By blocking out the sunlight for several months, and plunging the earth into darkness, the collision could have lowered temperatures below freezing, killing off the dinosaurs and the plants on which they fed. This hypothesis is based in part on the 1979 discovery, by Dr. Walter Alvarez of the University of California at Berkeley, of the unusual presence of the precious metal iridium in sediments from the time of the dinosaurs' extinction. Iridium, which belongs to the platinum family of elements, is much more plentiful in meteorites than planetary rocks, but it is also frequently found in strata from volcanic eruptions. Other scientists contend that virtually all mass extinctions could have been caused by volcanic eruptions and the huge floods of basalt resulting from such explosions. Evidence supports both theories. University of Chicago paleobiologist Dr. David M. Raup may have put it best when he wrote, "The disturbing reality is that for none of the thousands of well-documented extinctions in the geologic past do we have a solid explanation of why the extinction occurred."
Mass extinctions are not just a phenomenon of ancient geologic history; the last few thousand years have witnessed several such events, albeit on a far smaller geographic scale than those previously discussed. For example, the extinction of the giant animals of North America appears to have coincided with the arrival of humans on the continent, with the changing climate at the end of the last ice age also a possible factor. Crossing the Bering land bridge linking Siberia and Alaska about 12,000 years ago, primitive tribespeople, possibly from Siberia, found a population of large mammals, including giant beavers, bison , camels, mammoths, mastodons, and even lions in North America. These prehistoric hunters, moving south from Alaska towards South America, may have wiped out most of these species within 1,000 years.
Fossil remains of these Pleistocene mammals, some with spear points imbedded in them, reveal such creatures as the elephant-like mastodon; the huge-tusked, shaggy redhaired mammoth; a giant beaver weighing over 400 pounds (181 kg); the fabled saber toothed "tiger"; a 7 foot (2.1 m) wide, long-necked camel; 20-foot (6 m) long ground sloths; a bison 7 feet (2.1 m) high at the hump, with a 6 foot (1.8 m) spread between its horns; and huge wolves , lions, bears, and horses.
As the twenty-first century approaches, the earth faces another threat of mass extinctions which may occur over a few years or decades, instead of thousands or millions of years. Particularly endangered are those species that are less well known, and in many cases not yet identified by science. Many of these are plants, animals, and invertebrates found in tropical rain forests, which are rapidly being cut and destroyed.
For example, the 1980 Global 2000 Report to the President, compiled by the President's Council on Environmental Quality and the State Department, with the help of other federal agencies, projected that "...between half a million and two million species—15–20 percent of all species on earth—could be extinguished by the year 2000." And a report issued by the World Resources Institute in 1989 predicted that "between 1990 and 2020, species extinctions caused primarily by tropical deforestation may eliminate somewhere between 5 and 15% of the world's species...This would amount to a potential loss of 15,000 to 50,000 species per year, or 50 to 150 species per day." In 1988, Harvard biologist Edward O. Wilson put the annual rate of species extinction at up to 17,500.
Many of these lost species may be potential sources of food or medicines, and may perform essential ecological functions critical to sustaining life on the planet. It is impossible to predict precisely what the effects of these modern mass extinctions will be, but the ecological disruptions that will inevitably occur could have drastic consequences for human welfare and survival.
See also Acid rain; Biodiversity; Endangered species; Food chain/web; Greenhouse effect; Ice age refugia; Radioactivity; Radiocarbon dating; Rare species; Volcano
[Lewis G. Regenstein ]
RESOURCES
BOOKS
Donovan, S. K., ed. Mass Extinctions: Processes and Evidence. New York: Columbia University Press, 1991.
Tudge, C. Last Animals at the Zoo: How Mass Extinction Can Be Stopped. Covelo, CA: Island Press, 1992.
PERIODICALS
Hecht, J. "Global Catastrophes and Mass Extinctions." Analog: Science Fiction-Science Fact 111 (May 1991): 78–89.
Mass Extinction
Mass Extinction
Greater and lesser mass extinctions
Extinction, the death of all members of a species, is a natural process that has been occurring since the beginning of life on Earth. Nearly all species that have ever existed are now extinct—about 99.9% of them—and extinction is an important factor in the evolution of new species. Mass extinction, the death of large numbers of species over a relatively short span of geologic time, is also a natural process, but one that is less common than what can be called the background extinction rate, the extinctions that are always occurring at a normal rate through time.
Mass extinctions have been recognized in the fossil record since the middle of the nineteenth century. Levels of mass extinction of species were selected as marker levels in the stratigraphic record because the death of index-fossil species provided a convenient marker to subdivide and correlate strata. The mass-extinction level called the Permian-Triassic boundary is so profound in terms of faunal and floral change that it was early on noted and chosen to represent the transition from Paleozoic to Mesozoic era. The mass-extinction level called the Cretaceous-Tertiary boundary is also quite distinctive in terms of faunal and floral change, and it too was noted early on and chosen to represent the transition from Mesozoic to Cenozoic era. Other less profound, but nevertheless distinctive levels of mass extinction of fossils have been selected to represent marker points in the stratigraphic record at which geological periods, epochs, ages, and other intervals of lesser temporal value are defined.
Much of this work was done in the nineteenth and twentieth centuries by stratigraphers and paleontologists who did not know or really concern themselves with the causes of such mass extinctions and the ramifications of such mass extinctions. In the middle nineteenth century (and in some quarters still today), mass extinctions were attributed to the actions of an angry deity who periodically swept away life on Earth to make room for new. This interpretation was made at a time when geological observations were forced to fit into Biblical accounts of creation. Geological scientists who expressed sentiments otherwise faced exclusion from polite society. As geological thought moved away from such ideas in the late nineteenth century, many pondered, but few began to understand that Earth has experienced many profound changes and that any life on Earth which could not adapt, has continually paid a price for such changes.
Identifying mass extinctions
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 paleontologic 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 geologic 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.
Greater and lesser mass extinctions
The classical explanation for the lesser and greater mass extinctions of life included climatic change (global greenhouse to icehouse shifts), sea-level shift, extensive volcanic activity (with resultant damage to atmosphere and ocean systems), disease, and plate-tectonic continental motion (convergence or divergence of land masses). With the 1980 publication of a widely cited paper in the journal Science (by American physicist L.W. Alvarez and others) that showed strong evidence of comet or asteroid impact at the 65-million year old mass extinction boundary (the Cretaceous-Tertiary boundary), many investigations shifted toward possible cosmic impact as the cause of other mass extinctions.
In a 1994 paper, geologists M.R. Rampino and B.M. Haggerty marshaled evidence that at least 16 of the 23 extinction peaks identified by J.J. Sepkoski were either strongly connected to impact events (with known craters on Earth) or indirectly connected to impacts on Earth (e.g., chemical traces of impact dust or shocked materials). Subsequent work on this issue has increased the number of mass extinction peaks associated with known impacts on Earth. In particular, there is now strong to good evidence of major impact events (perhaps more than one impact per extinction) at three of the great mass extinction events (i.e., the permian-Triassic boundary 245 million years ago; impact dust and shocked materials in some places); the Triassic-Jurassic boundary (208 million years ago; same age as Manicouagan crater, Canada); and the Cretaceous-Tertiary boundary (65 million years ago; same age as Chicxulub crater in Mexico and boltysh crater in Ukraine and global clay layer with impact dust and shocked materials).
Whether the main cause of mass extinctions, especially great mass extinctions over geologic time is more likely to be cosmic impacts or some more Earth-bound factor, the fossil record clearly shows that they have occurred in the past. There is no reason to assume that they will not happen on earth in the future. An understanding of these events, by careful study of the strati-graphic and paleontologic record, may help scientists better understand how to care for the world as it is known today. It has been suggested that the modern rate of species loss on earth is comparable to the great mass extinctions of the past. While it is difficult to relate observations of modern faunal and floral species loss to observations taken from the fossil record, there is a warning for all in the record of the past. Extinction is forever, and mass extinctions profoundly change the faunal and floral characteristics of Earth’s ecosystems after they occur.
See also Correlation (geology); Paleontology; Stratigraphy.
David T. King, Jr.
Mass Extinction
Mass extinction
Extinction , the death of all members of a species , is a natural process that has been occurring since the beginning of life on Earth . Nearly all species that have ever existed are now extinct, and extinction is an important process in the evolution of new species. Mass extinction, the death of large numbers of species over a relatively short span of geologic time , is also a natural process, but one that is less common than what one might call "back-ground" extinction, or extinction of species at a normal rate through time .
Mass extinctions have been recognized in the fossil record since the middle of the nineteenth century. Levels of mass extinction of species were selected as marker levels in the stratigraphic record because the death of index-fossil species provided a convenient marker to subdivide and correlate strata . The mass-extinction level called the Permian-Triassic boundary is so profound in terms of faunal and floral change that it was early on noted and chosen to represent the transition from Paleozoic to Mesozoic era. The mass-extinction level called the Cretaceous-Tertiary boundary is also quite distinctive in terms of faunal and floral change, and it too was noted early on and chosen to represent the transition from Mesozoic to Cenozoic era. Other less profound, but nevertheless distinctive levels of mass extinction of fossils have been selected to represent marker points in the stratigraphic record at which geological periods, epochs, ages, and other intervals of lesser temporal value are defined.
Much of this work was done in the nineteenth and twentieth centuries by stratigraphers and paleontologists who did not know or really concern themselves with the causes of such mass extinctions and the ramifications of such mass extinctions. In the middle nineteenth century (and in some quarters still today), mass extinctions were attributed to the actions of an angry deity who periodically swept away life on Earth to make room for new. This interpretation was made at a time when geological observations were forced to fit into Biblical accounts of creation. Geological scientists who expressed sentiments otherwise faced exclusion from polite society. As geological thought moved away from such ideas in the late nineteenth century, many pondered, but few began to understand that Earth has experienced many profound changes and that any life on Earth which could not adapt, has continually paid a price for such changes.
Identifying mass extinctions
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.
Greater and lesser mass extinctions
The classical explanation for the lesser and greater mass extinctions of life included climatic change (global greenhouse to icehouse shifts), sea-level shift, extensive volcanic activity (with resultant damage to atmosphere and ocean systems), disease , and plate-tectonic continental motion (convergence or divergence of land masses). With the 1980 publication of a widely cited paper in the journal Science (by American physicist L.W. Alvarez and others) that showed strong evidence of comet or asteroid impact at the 65-million year old mass extinction boundary (the Cretaceous-Tertiary boundary), many investigations shifted toward possible cosmic impact as the cause of other mass extinctions. In a 1994 paper, geologists M.R. Rampino and B.M. Haggerty marshaled evidence that at least 16 of the 23 extinction peaks identified by J.J. Sepkoski were either strongly connected to impact events (with known craters on Earth) or indirectly connected to impacts on Earth (e.g., chemical traces of impact dust or shocked materials). Subsequent work on this issue has increased the number of mass extinction peaks associated with known impacts on Earth. In particular, there is now strong to good evidence of major impact events (perhaps more than one impact per extinction) at three of the great mass extinction events (i.e., the Permian-Triassic boundary 245 million years ago; impact dust and shocked materials in some places); the Triassic-Jurassic boundary (208 million years ago; same age as Manicouagan crater, Canada); and the Cretaceous-Tertiary boundary (65 million years ago; same age as Chicxulub crater in Mexico and Boltysh crater in Ukraine and global clay layer with impact dust and shocked materials).
Whether the main cause of mass extinctions, especially great mass extinctions over geologic time is more likely to be cosmic impacts or some more Earth-bound factor, the fossil record clearly shows that they have occurred in the past. There is no reason to assume that they will not happen on Earth in the future. An understanding of these events, by careful study of the stratigraphic and paleontologic record, may help scientists better understand how to care for the world as it is known today. It has been suggested that the modern rate of species loss on Earth is comparable to the great mass extinctions of the past. While it is difficult to relate observations of modern faunal and floral species loss to observations taken from the fossil record, there is a warning for all in the record of the past. Extinction is forever, and mass extinctions profoundly change the faunal and floral characteristics of Earth's ecosystems after they occur.
See also Correlation (geology); Paleontology; Stratigraphy.
Resources
books
Montanari, A. and C. Koeberl. Impact Stratigraphy Berlin: Springer-Verlag, 2000.
Raup, D.M. Extinction: Bad Genes Or Bad Luck? New York: Norton, 1991.
Sepkoski, J.J., Jr. "Phanerozoic overview of mass extinctions." In D.M. Raup and D. Jabloknski (eds), Patters and Processes in the History of Life, Berlin: Springer-Verlag, 1986, 277–95.
Ward, P. The End of Evolution, on Mass Extinctions and thePreservation of Biodiversity New York: Bantam Books, 1994.
periodicals
Alvarez, L.W., et al. "Extra-Terrestrial Cause of the Cretaceous-Tertiary Extinction." Science no. 208 (1980): 1095–1108.
David T. King, Jr.