Mass extinctions

Extinction happens all the time, but there are rare examples of short periods of time where extinction rates have been very high. The definition of the rate and duration of these events is subjective, but most researchers agree that five mass extinction events have occurred since the Cambrian (Fig. 16.6). These large faunal turnovers have been used by stratigraphers to define the boundaries of intervals of geological time (Fig. 16.7). They mostly occur at what we define as the end, or close to the end, ofthe following geological periods: Ordovician, Devonian, Permian, Triassic, and Cretaceous (Table 16.3).

These mass extinctions are unique in detail but share the following characteristics:

1 A significant proportion of species became extinct -between 30 and 95%.

2 The extinctions seem to have operated across a wide range of environments and lifestyles.

3 The extinction events occurred rapidly and were probably precipitated by one or more physical factors.

One of the major problems with mass extinctions is defining cause and effect. By their nature, they are periods of time when conditions are unlike any modern analog. It is not known how ecosystems can be made to collapse in a catastrophic way, and it is unclear whether local disasters, such as a volcanic eruption, can be scaled up to help us understand global disasters. Intensive study of the boundary sections that represent mass

Sepkoski Histogram Intensity Extinction

Millions of years ago

Fig. 16.6 A plot of extinction rate with time showing a variable but low background rate and occasional sharp peaks, the mass extinctions of the Phanerozoic.

Millions of years ago

Fig. 16.6 A plot of extinction rate with time showing a variable but low background rate and occasional sharp peaks, the mass extinctions of the Phanerozoic.

extinctions rarely yield unequivocal data. Although extinctions have come to be associated with particular events, for example the assumed relationship between the end-Cretaceous event and a large meteorite impact, it is never possible to remove all uncertainty about whether the likely event actually caused the extinctions.

Permian Sea Floor
Fig. 16.7 Effects of the Permian mass extinction, comparing a late Permian tropical sea floor with one of the early Triassic, based on a section at Meishan, China, and showing the loss of reef-dwelling organisms.

Table 16.3 A brief description of the five major mass extinction events of the Phanerozoic.

Mass extinction

Biological expression

Probable causes

End Ordovician About 70% of marine species became extinct. Tropical faunas in general were badly damaged, especially reefs. Main affected groups were trilobites, graptolites, echinoderms, and brachiopods

End Devonian A series of events lasting about 10 million years. This extinction was characterized by low rates of origination, and extinction rates as such remained unexceptional. Cephalopods, fish, and corals were most affected

End Permian The largest mass extinction, removing 95% of marine species and over 50% of marine families. Trilobites, cephalopods, corals, bryozoans, and crinoids were badly affected. There was a major faunal and floral overturn on land. Marks the boundary between dominance by the Palaeozoic and Modern fauna

End Triassic A multiple event. It was most important on land, where the floral overturn exceeded 95%. Around 30% of marine species became extinct, mainly reef dwellers including ceratite ammonites, brachiopods, and bivalves

End Cretaceous Dinosaurs, marine and aerial reptiles, ammonites, and belemnites became extinct. Brachiopods, bivalves, and foraminifera were severely affected

A sudden, major glaciation, spreading from the South Pole. Most of the tropical belt disappeared. Sea levels fell, reducing the shallow shelf area, and cold water faunas moved to low latitudes, excluding warmer water ecosystems

The least understood mass extinction. Sea bed anoxia or extraterrestrial impact have been cited. Another suggestion is that the marine ecosystem was badly affected by the rise of land plants and a short-lived drawdown of carbon dioxide

Event may relate in some way to the formation of the supercontinent Pangea. Suggestions for this relationship include the abrupt release of huge quantities of methane from frozen submarine deposits called gas clathrates. Widespread oceanic anoxia is known to have occurred, and H2S may have built up in the atmosphere. Other possible causes of the event are enormous lava eruptions in Siberia and China, and controversial evidence for a meteorite impact close to the boundary

Recent work suggests that widespread submarine volcanic activity may have been responsible for massively elevated carbon dioxide levels at this time. This volcanic activity was a function of the break-up of Pangea

Most commonly attributed to a major meteorite impact into the Yucatan area of Mexico. Underlying sulfates and limestones would have vaporized to produce acid rain. Short-term cooling (from dust and sulfur dioxide) and long-term warming (from carbon dioxide) resulted. Also linked to the Deccan Igneous province in India

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