The great extinctions

 Since life appeared on Earth 3,6 billion years ago, millions and millions of species have appeared, flourished, and then become extinct only to be replaced by new forms. Over the course of evolution, the extinction of species is a common, almost everyday event that is integral to the dynamics of biological evolution, its "background noise".

Aside from the natural death of species, paleontologists have noted that over geological time there have bee periods of crsis characgterized by the disappearance of many groups of plants and animals. Over the last 600 million years, marine and terrestrial ecosystems have been virtually wiped out in 27 different crises. Of these, five are called "great extinctions," because they were so much more vast than the others: during each of them at least 20 to 50% of the families of invertebrates and vertebrates living in the oceans disappeared. Each time, the overall biological diversity of the planet was greatly diminished. The causes and consequences of these "biological catastrophes" are still poorly understood and the subject of much debate. In the following, we list the principal groups of plants and animals affected by each of the five great extinctions, as well as what scientists think may be the main causes behind these events.

At the end of the Ordovician Period (438 million years ago), about 22% of the families of invertebrates living in the ocean disappeared. At that time there was one vast southern continent, known as Gondwana, located over the South Pole that included Africa, South America, the Indies, Antarctica, and Australia. A glacial cap covered northern Africa. This extinction event is primarily known from fossil remains found in Europe and North America, and it seems to have occurred in successive stages over a period of several million years. Its main cause was climatic: the glaciation at the end of the Ordovician lead to global cooling. Gradually, planktonic organisms such as certain conodonts and acritarchs disappeared, as did organisms living in reefs or on the continental shelves such as the graptolites, and certain corals, brachiopodos, nautiloids, and trilobites.

The second great extinction occurred eigthy-eight million years later when the concentration of oxygen in the world's ocean bodies diminished radically, literally suffocating a large number of species out of existence. This phenomenon was probably related to the general rise in the level of the oceans at that time. The extinction was brief, lasting only from 10.000 to several hundred thousand years -but afterward, the planet's biological diversity continued to be impoversished for more than one million years. This crisis has been well studied in North America, the Rhineland, and the southern portion of France's Massif Central. Research shows that the conodonts and certain planktonic foraminifers were affected, as were reef-dwellers and the invertebrate and vertebrate fauna on the continental plateau, including not only sessile animals such as coral, and deep bottom dwelling such as trilobites and brachiopods, but also free-swimming vertebrate species like the placoderms and the acanthodians.

The most devastating extinction of ll time occurred at the end of the Permian Period, 250 million years ago. Over half the families of marine invertebrates disappeared, which meant the extinction of 75 to 90% of species. This is one of the most poorly understood extinctions because deposits from this period are rare (Greenland, the Southern Alps, Armenia, Pakistan, southern China, and South Africa). At that time, all emerged land was part of one gigantic super-continent, Pangaea, and the oceans probably had little influence on the climate of this enormous land mass. Seasons were very clearly differentiated, and the presence of evaporites (such as gypsum) and red formations indicate there were long periods during which vast areas became increasingly arid. In addtion, layers of basalt discovered in Siberia show there was heavy volcanic activity at this period. The climate, volcanic activity, and the drop in the sea level were probably the major influences behind this extinction, which spanned several million years. The last trilobites disappeared from the seas. Reef communities were decimated: certain coral (Tabulata and Rugosa) and bryozoans, and the fusulinids vanished. Among the terrestrial species, certain insects and mammmalian reptiles (therapsids), as well as almost 80% of the families of amphibians disappeared. Numerous plant groups were also wiped out: Bothyropteridales, Marattiales, Cordaitales, Trigonocarpales.

The fourth gret extinction occurred several million years later, at the end of the Triassic Period (220 million years ago). But this crisis, too, es poorly understood because there are few fossil remains. It probably stretched over several million years, and it seems that terrestrial environments were affected before marine environments. Among the land vertebrates, the principal victims were the labyrinthodont amphibians and mammalian reptiles (dicynodonts and cynodonts). Among the marine reptiles, only the icthyosaurs survived. The super-continent Pangaea also began to fragment at this time.

The las great extinction occurred at the end of the Cretaceous Period (65 million years ago). Certain researchers think it was caused by a meteorite colliding into the Earth and that the crisis lasted only a few years. Others pinpoint hevay volcanic activity as the cause and theorize that the crisis lasted at least 500.000 years. The most well-known victims were the dinosaurs, but marine organisms were also affected: certain nannoplankton, and the ammonites and belemnites disappeared.

The study of geological crises shows that:

1) All major extinctions are selective and contingeut. On one hand, not all organisms are affected in the same manner, and on the other, nothing indicates that the forms which disappeared were more vulnerable than those which suvived. At any given time and regardless of their specie's age, all organisms have an equal chance of surviving.

2) All major extinctions are linked to the appearance of new species. The "vacuum" left by forms which disappear is quickly filled by adaptive radiations in other groups.

The presumed causes for these major crises continue to provoke intense debate. The causes may have been external, such as asteroids falling to earth or the passage of comets; or they may jave been internal to terrestrial systems, for example volcanic eruption or a drop in sea level; or they may have been due to a combinatino of both internal and external factors, as in the case of major climatic upheavals.

During these geological catastrophes, the biological diversity of plant and animal communities is drastrically reduced. But after each such event, diversity is restored and groups that had previously lacked variety undergo some remarkable adaptive radiations.

Jean-Lous Hartenberger

Director of Research

University of Montpellier.