extinctions and mass extinctions Extinction of species is a continuous process, and evidence of its occurrence abounds in the fossil record. It has been estimated that marine species persist for about four million years, which translates into an overall loss of about two or three species each year. This is considered to be the background extinction rate and it is balanced by speciation events that result in the development of new species. Mass extinctions are events during which the rate of extinction rises dramatically above this background rate. A number of these have occurred during Phanerozoic times, i.e. since the end of the Precambrian, about 600 million years ago. Five major events have for some time been recognized: the end-Ordovician; late Devonian (Frasnian–Fammenian); end-Permian; end-Triassic; and end-Cretaceous events. These are best seen in the record of shallow-water marine organisms; in each event at least 40 per cent of the genera were eliminated. By using statistical methods it has been possible to estimate that at least 65 per cent of species became extinct at each of these events, 77 per cent being eliminated at the end-Cretaceous event and 95 per cent at the end-Permian event. Research by David Raup and Jack Sepkoski at the University of Chicago has demonstrated the presence of a number of other such mass extinctions and has shown that they occur with a regular periodicity of about 26 million years (Fig. 1). This periodicity has become a controversial topic because of its association with ideas about possible extraterrestrial causes of such events.
Causes of mass extinction
Attempts to explain the causes of mass extinctions have traditionally centered on terrestrial phenomena such as sea-level changes, climatic changes, or volcanism. Sea level has shown regular fluctuation on a global level during the Phanerozoic (Fig. 2). The term eustatic was coined for this by the Austrian geologist Edward Suess at the turn of the century. These eustatic changes appear to be related to the melting or formation of polar ice caps or to major tectonic events, such as continental splitting or collision and the uplift and subsidence of ocean ridges. Extinction events appear mostly to be correlated with periods of marine regression. (i.e. retreat of the sea from the land when the sea level was low). The reason for this appears to be that a withdrawal of the ocean leaves a much smaller habitat area for shallow-water marine organisms. This leads to increased crowding and competition, and ultimately to an increased extinction rate. Reduction of large terrestrial vertebrates during these regressions, as happened during the end-Permian, Triassic, and Cretaceous events, might be related to increased annual seasonality caused by the loss of the ameliorating influence of the shallow epicontinental seas. It has also been shown that some extinctions are related to transgressive events (rising sea level), possibly resulting from the spread of anoxic waters across epicontinental areas, and represented by extensive deposits of black shales. Climatic changes seem to be generally correlated with eustatic events, and the evidence implicating temperatures as the main cause of extinctions seems to be weak. For example, the most important extinction event, the end-Permian, occurred at a time of temperature amelioration marked by the disappearance of the Gondwana ice sheet.
Volcanism has been presented as a possible cause for the Cretaceous–Tertiary boundary extinctions. The lavas forming the present Deccan flood basalts of northern India were erupting at that time and would have produced large quantities of volatile emissions that could have resulted in global cooling, depletion of the ozone layer and changes in ocean chemistry. However, no evidence exists as yet for the involvement of volcanicity in other extinction events.
Although various extraterrestrial causes for mass extinction events have been suggested in the past, these ideas have gained greater credence since the publication in the early 1980s of work by Louis and Walter Alvarez of the University of California at Berkeley, who ascribed the Cretaceous–Tertiary boundary extinction event to the effects of the impact of a large bolide (extraterrestrial object) in the Caribbean region. The impact of such a large object, estimated to have been 10 km in diameter, is estimated to have resulted in some months of darkness caused by the global dust cloud that was generated. This would have halted photosynthesis and would thus have resulted in the collapse of both terrestrial and marine food chains. Although cold would initially have accompanied the darkness, greenhouse effects and global warming would follow as atmospheric gases and water vapour trapped infrared energy radiating from the Earth. Physical evidence for an impact rests on the presence in boundary layers of high concentrations of iridium and other elements that are generally rare at the Earth's surface but abundant in asteroids. In addition these layers often contain shocked quartz grains which are otherwise known only from impact craters and nuclear test sites, and microtektites, which are glassy droplets formed by bolide impacts. Soot particles are also present in some localities, which suggests that extensive wildfires may have raged across the continents. Although the evidence for extraterrestrial impacts at the other major extinction events is slight, this causal factor has been linked with the regular periodicity of extinctions demonstrated by Raup and Sepkoski. It has consequently been suggested that perturbation of the Oort cloud of comets by the regular passage of a large planetary body as yet unidentified (Planet X, or Nemesis the death star) would result in increased asteroid impacts and extinction events.
Mass extinction events
The first mass extinction event that can be recognized in the fossil record occurred in the middle Vendian about 650 million years ago. Although its study is hampered by the paucity of macrofossils, it is clear that micro-organisms such as acritarchs (resting stages of planktonic, eukaryotic marine algae) underwent a severe decline. This extinction event has been linked to climatic cooling related to the Varangian glaciation, which occurred during the Lower Vendian.
The extinction in the Late Ordovician was a major event in which 22 per cent of marine families became extinct; graptolites and corals were particularly hard hit. As there were two main pulses of extinction and no iridium anomaly is known, an extraterrestrial cause seems unlikely. Changes in sea level and temperature have been cited as likely causal factors. In addition oceanic overturn might have brought biologically toxic bottom waters to the surface during periods of climatic change.
The end-Devonian (Frasnian–Fammenian) event had a catastrophic effect on brachiopods, which lost about 86 per cent of genera, and on reef-building organisms such as corals and stromatoporoids. Shallow-water faunas were most severely effected; only 4 per cent of shallow-water species survived, but 40 per cent of deeper-water species survived; and cool-water faunas also survived better. This has been linked with a significant drop in global temperatures, of unknown cause, during this time period.
The end-Permian event was the most severe of Phanerozoic time: it resulted in the extinction of up to 95 per cent of all marine invertebrate species. Taxa that became extinct include rugose and tabulate corals, trilobites, goniatites, and many groups of crinoids, bryozoans, brachiopods and foraminifera. On land amphibians and therapsids (mammal-like reptiles) were both badly affected, while vascular plant diversity dropped by 50 per cent. Although extraterrestrial causes have been suggested, no iridium anomaly is present. The most likely explanation is climatic instability caused by continental amalgamation and the simultaneous occurrence of marine regressions, which would have resulted in trophic (nutritional) disruptions on a major scale.
The end-Triassic event was much less severe but still resulted in major reductions in ammonoids, brachiopods, and marine reptiles in the oceans. On land there was a major faunal turnover in which labyrinthodont amphibians, early reptile groups, and mammal-like reptiles died out and were replaced by archosaurs, lepidosaurs, and mammals. Again, no evidence of an impact event is present and the extinctions are generally correlated with widespread marine regressions.
The Cretaceous–Tertiary boundary (K–T) mass extinction has been hotly debated, largely because of the bolide impact hypothesis of Louis and Walter Alvarez. Although the broad pattern of extinctions is known for marine organisms, the detailed picture is known only for planktonic foraminifera and calcareous nannoplankton. Study of the ranges of these micro-organisms shows that the extinctions of foraminifera occur over an extended period of time, starting well before and finishing well after the boundary. Cretaceous species of calcareous nannoplankton likewise survived across the boundary and became extinct some tens of thousands of years later. The macrofaunal record as yet shows insufficient resolution, although it is evident that brachiopods clearly suffered badly across the boundary and were replaced almost entirely by new species in the earliest Tertiary. Although much has been made of the extinction of ammonites at the end of the Cretaceous, there are too few ammonite-bearing sections to show if this was gradual or abrupt. On land, the evidence for a dramatic increase in fern species just above the boundary suggests the presence of wildfires, for ferns are usually the first plants to recolonize an area devastated in this fashion. However, in many sections a return to the Cretaceous vegetation is seen above the fern ‘spike’, indicating little extinction. Among the vertebrates a picture of gradual change is seen for mammals, with drastic reductions occurring only in the marsupials. The boundary also does not seem to have been a barrier for turtles, crocodiles, lizards, and snakes, all of which came through virtually unscathed. The dinosaurs did become extinct, and much argument has centred on whether or not their extinction was abrupt or occurred after a slow decline. In this context it must be noted that there is only one area where a dinosaur-bearing sedimentary transition across the Cretaceous–Tertiary boundary can examined; this is in Alberta and in the north-western USA. Records of dinosaurs in this area during the later part of the Cretaceous show a gradual decline in diversity with a drop from thirty to seven genera over the last eight million years of Cretaceous time. Although explanations of the extinction of dinosaurs have ranged from mammals eating their eggs, through terminal allergies caused by the rise of flowering plants, to the current ideas about bolide impacts, the answer is probably related to climate. A major regression of the oceans occurred at this point, resulting in a drop in mean annual temperatures and an increase in seasonality. The bolide impact would have dealt the
coup de grâce to taxa that were already declining.
The major extinction event that occurred at the end of the Pleistocene primarily affected large terrestrial mammals. In North America, 33 genera of large mammals were lost; 46 were lost in South America, and 13 in Europe. In North America these included mammoths and mastodon, giant ground sloths, and glyptodonts. These extinctions coincided with a shift from a more equable to a more seasonal climate as a consequence of the end of the last glacial stage, thus suggesting a climatic cause. However, they also coincided with the arrival of humans in North America, which suggests that overpredation by these new and skilful hunters might have been the cause.
David K. Elliott
Bibliography
Elliott, D. K. (ed.) (1986) Dynamics of extinction.John Wiley and Sons, New York.
Nitecki, M. H. (ed.) (1984) Extinctions. University of Chicago Press.
Raup, D. M. (1991) Extinction: bad genes or bad luck? W. W. Norton, New York.
