Paleontologists generally divide extinctions into two categories. The first are the so-called background extinctions, isolated extinctions of species that occur in an ongoing fashion. The second type are called mass extinctions. The latter certainly have caught media and the public’s attention, and they appear to be something qualitatively as well as quantitatively different than background extinctions.
Although background extinctions are less glamorous than mass extinctions, they are essential to biotic turnover: University of Tennessee paleobiologist M. L. McKinney has estimated that as much as 95% of all extinctions can be accounted for by background extinctions. Isolated species disappear from a variety of causes, including out-competition (the edge), depletion of resources in a habitat, changes in climate, the growth or weathering of a mountain range, river channel migration, the eruption of a volcano, the drying of a lake, the spraying of a pesticide, or the destruction of a forest, grassland, or wetland habitat. Dinosaur populations had a species’ turnover rate of around 2 million years per species. This means that each species lasted about 2 million years, before a new one appeared and the old one disappeared. 1 Although some dinosaur extinctions coincided with earlier mass extinction events (such as those at the Triassic–Jurassic and Cretaceous–Tertiary boundaries), most dinosaurs fell prey to background extinctions. By far the majority of favorite and famous dinosaurs – Maiasaura, Dilophosaurus, Protoceratops, Deinocheirus, Styracosaurus, Velociraptor, Iguanodon, Ouranosaurus, Allosaurus (to name a tiny fraction) – were the victims of background extinctions. The ultimate dinosaur extinction didn’t wipe out the total number of species accumulated over 160 million years, it killed only the latest-evolved representatives of the group (see Figure 13.1).
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.