Permo Triassic Extinction

The Permo-Triassic extinction occurred 245 million years ago at the end of the Permian Period. It marked the close of the Paleozoic Era and was a time when perhaps 95 percent of all species went extinct. The extinction was ecumenical in scope. Species that went extinct belonged to every phylum, lived in a variety of marine and nonmarine habitats, and had many modes of life. Understanding the Permo-Triassic extinction, the greatest crisis the biota has faced, is important if we are to put into perspective the biodiversity crisis facing our modern world.

Studying the Permo-Triassic extinction is not easy. In many areas the Permian-Triassic boundary is marked by an unconformity, a surface of erosion where uppermost Permian rock was eroded before the first Triassic rock was deposited. Because all evidence in geology comes from the rocks and their contained fossils, where Upper Permian rock is absent, evidence of the mass extinction is lost forever.

Douglas Erwin (1993, pp. 51-73), who specializes in the Permo-Triassic extinction, discussed places in which Permian rock is not separated from the Triassic by an unconformity. These include northeastern Greenland, the southern Alps, Transcaucasia and Iran, the Salt Range in Pakistan, Kashmir, the

Table 1

Major Groups of Invertebrates Affected by the Permo-Triassic Extinction

Table 1

Major Groups of Invertebrates Affected by the Permo-Triassic Extinction

Period Protists

Corals

Brachiopods

Molluscs

Arthropods

Echinoderms

Fusulinids

Tabulate Rugose

Productids Spiriferids

Gastropods Ammonoids

Trilobites Ostracodes

Crinoids Blastoids

Cretaceous

Jurassic

Triassic

Permian

Carboniferous

Devonian

Cretaceous

Jurassic

Triassic

Permian

Carboniferous

Devonian

Complete

Extinction

Heavy losses

Moderate losses

Diminished importance

Abundant and diverse

Figure 1

Extinct Invertebrates at the End of the Permian Period

1. Fusulinid, a single-celled organism with a complex shell of calcium carbonate

2. Solitary rugose or horn coral

3. Spiriferid brachiopod

4. Productid brachiopod

5. Ammonoid

6. Palaeocopid ostracode

7. Crinoid

Sources: 1957, 1961, 1964, 1965, 1981. Treatise on Invertebrate Paleontology. Lawrence: University of Kansas in association with the Geological Society of America, Part L, fig. 65A, Part Q, fig. 102 1b, Part C, fig. 327 2b, Part H, fig. 370 2c, fig. 564 2d, Part F, fig. 162; Moore, Raymond C. 1962. "Article 29, Echinodermata 5." University of Kansas Paleontological Contributions. Lawrence: University of Kansas Paleontological Institute. (Reprinted with permission).

Himalayas, and southern China. Most of these areas are remote from the centers of paleontological research, and in a few civil war is raging. Thus some have not been studied in detail, and evidence of an unconformity is stronger in some than others.

Table 1 lists the major groups of invertebrates that were strongly affected by the extinction. Some of these kinds of organisms had dominated marine ecosystems for 300 million years. Nevertheless, they were extinguished in a short time, although the length of time over which the extinction took place is not known.

The fusulinids (Figure 1,1) were single-celled, animallike organisms that evolved in the Carboniferous and thrived to the end of the Permian. Their rapid evolution produced a high diversity, and in some rock units they are the most abundant fossils. Yet they went extinct at the end of the Permian, leaving no descendants. Two major groups of corals were abundant in the Paleozoic, the tabulate corals and the rugose or horn corals (Figure 1, 2). Both groups died out at the end of the Permian, leaving no descendants.

Brachiopods are present in the sea today, but during the Paleozoic they were a domi-

Figure 1

1. Fusulinid, a single-celled organism with a complex shell of calcium carbonate

2. Solitary rugose or horn coral

3. Spiriferid brachiopod

4. Productid brachiopod

5. Ammonoid

6. Palaeocopid ostracode

7. Crinoid nant element of the marine fauna. The spire-bearing brachiopods (Figure 1, 3) lost half of their families at the end of the Permian and went extinct in the Jurassic. The productid brachiopods were less successful (Figure 1, 4). Abundant and diverse during the late Paleozoic, none survived into the Triassic. In fact, following the extinction, brachiopods were never again dominant elements of the marine fauna.

Ammonoid cephalopods evolved in the Devonian and fluctuated in biodiversity for the rest of the Paleozoic and Mesozoic (Figure 1, 5). They were reduced in numbers at the end of the Permian but rebounded and became dominant predators until their extinction at the end of the Cretaceous.

Ostracodes—microscopic crustaceans related to lobsters—are abundant today, although they are often overlooked because of their small size (Figure 1, 6). Several groups survived the end-Permian extinction, but the major group of Paleozoic ostracodes went extinct, and post-Paleozoic ostracode faunas are quite different from those of the Paleozoic.

Many other groups of invertebrates were affected by the mass extinction. Two that are common in Paleozoic rocks are crinoids (Figure 1, 7) and trilobites. Crinoids live in the sea today, but they went almost extinct at the end of the Permian. Trilobites had been in decline for much of the Paleozoic, and the last species went extinct at the end of the Permian. Several groups of marine fishes were decimated by the extinction, but freshwater fishes and those tolerant of fluctuating salinity persisted.

Terrestrial tetrapods were affected as severely as marine forms of life. Nearly two-thirds of the tetrapod families went extinct. Many families of amphibians were exterminated at various times in the Permian, not only at the end of the period; and two-thirds of the remaining nine families of amphibians were lost in the extinction. The mammal-like rep tiles were very hard hit; they rebounded in the Triassic but were never again the dominant tetrapod group, being displaced by the archosaurs, the ancestors of the dinosaurs.

The global turnover of plants from the Permian to the Triassic required some 25 million years and, thus, scarcely qualifies as a mass extinction. Nevertheless, the dominant groups of Permian plants, the seed ferns and primitive gymnosperms, were replaced by the more modern seed ferns and gymnosperms with which we are familiar today: conifers, ginkgoes, and cycads.

The cause of the Permo-Triassic extinction is unknown. Erwin suggested that it may have been the result of the coincidence of a regression of the sea and massive eruption of basalt in Siberia. The regression would have caused ecological instability and increased the concentration of carbon dioxide in the atmosphere, causing global warming and anoxic oceans. Eruption of basalt also would have contributed CO2. We now know that the onset of extinction coincided with the first eruption of basalt. More recently, evidence of a bolide impact has been found, but work remains to be done to confirm the cause of the extinction.

The end of the Permian, marked by the extinction of many groups of organisms that had dominated marine and terrestrial environments for hundreds of millions of years, cleared the way for the expansion of the modern fauna of today.

—Roger Kaesler See also: Geological Time Scale; Mass Extinction Bibliography

Erwin, Douglas H. 1990. "End-Permian." In Palaeo-biology: A Synthesis, edited by Derek G. Briggs and Peter R. Crowther, pp. 187-194. Oxford: Blackwell; Erwin, Douglas H. 1993. The Great Paleozoic Crises: Life and Death in the Permian. New York: Columbia University Press; Hallam, Anthony, and P. B. Wig-nall. 1997. Mass Extinctions and Their Aftermath. Oxford: Oxford University Press.

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