Mass extinctions

Mass extinctions involve large numbers of species and many types of species undergoing global extinction in a geologically short period of time. None of these has a truly precise definition, because there are no fixed rules for mass extinctions.

Indeed, how do we know that there even were mass extinction "events" and how can we recognize them? A compilation of invertebrate extinctions through time (Figure B15.1.1) shows that, although extinctions characterize all periods (it is these that are termed background extinctions), there are intervals of time in which extinction levels are significantly elevated above background levels. Such intervals are said to contain the mass extinctions. Fifteen such intervals are recognized, of which five clearly towered above the others (Figure B15.1.1).

The 15 mass extinctions are classified into "minor," "intermediate," and "major" mass extinctions, on the basis of the amount of extinction that took place above background. In the entire history of life, only one extinction qualifies as "major"; that is, the Permian-Triassic (commonly called Permo-Triassic) extinction. The remaining four of the Big Five - including dinosaur extinction - are considered to have been "intermediate." The rest are considered "minor," although undoubtedly not to the organisms that succumbed during them.

Figure B15.1.1. A compilation of extinctions through time, taken from the work of D. M. Raup and J. J. Sepkoski (Raup, D. M. and Sepkoski, J. J. 1984. Periodicity of extinctions in the geological past. Proceedings of the National Academy of Sciences, 81, 801-805.) The five most significant were the Late Ordovician (438 Ma), the Late Devonian (374 Ma), the Permo-Triassic (251 Ma), the Triassic-Jurassic (200 Ma), and the Cretaceous-Tertiary (65.5 Ma). Tr, Triassic; J, Jurassic; K, Cretaceous.

Late Ordovician

Late Devonian

T. . Cretaceous-Tertiary Permo-Triassic . '

Triassic-Jurassic

Geological time (Ma): 600

Era: Paleozoic

Geological time (Ma)

Mesozoic Cenozoic

1. This is a simple statistical average for all of Dinosauria. It was not necessary for an older species to disappear before its descendant appeared.

Prehistoric Bivalvia

Another important group of invertebrates are the bivalves. Careful study has shown that, with one exception (which went extinct much earlier), 63% of all bivalves went extinct sometime within the last 10 million years of the Cretaceous. The record is, unfortunately, not more precise than this, but it does show that the extinction took place without regard for latitude: bivalves in temperate regions were just as likely to go extinct as those in the tropics.

Marine microorganisms. Because of the richness of their fossil record, foraminifera, marine microscopic, shelled, single-celled organisms that are either planktonic (living in the water column) or benthic (living within sediments) have dominated discussions of K/T boundary events (Figure 15.10). Micropaleontologists studying foraminifera have shown persuasively, since as early as the late 1970s (and in many studies thereafter, including the observations of the Alvarez team at Gubbio) that the planktonic foraminifera extinction was abrupt, with only a few species crossing the boundary into the Paleocene.

Marine Nanofossils
Figure 15.10. The carbonate shell of a modern planktonic (free swimming) foraminifer, Globorotalia menardii. The long dimension is 0.75 mm.

An entirely different group of marine microorganisms, calcareous nanofossils, also shows an abrupt extinction. It is safe to say that most paleontologists working with marine microorganisms are inclined toward a catastrophic view of the extinction.

The "Strangelove" oceans. Some of the most unexpected results came from a series of studies of K/T oceanic primary production; that is, the amount of organic matter synthesized by organisms from inorganic materials and sunlight.

At the K/T boundary there was observed a rapid and complete breakdown in nutrient cycling between surface and deep waters, to less than 10% of what it had been. For some oceanographers, this signalled that the world's oceans were effectively all but dead. For the succeeding 1.5 million years, nutrient cycling remained at levels well below those preceding the original drop.4

4. The moribund K/T oceans were called "Strangelove" oceans after Dr Strangelove, Peter George's brittle, grotesque character, played by Peter Sellers in the eponymous film, who was unconcerned about a scorched, post-nuclear world.

Obviously, nutrient cycling is fundamental to oceanic health. With oceans covering 75% of the Earth's surface (or even more during the many high sea levels experienced during Earth history), it would not be an exaggeration to state that Earth's marine and terrestrial ecosystems are dependent upon these great cycles.

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