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evolved in the Late Devonian, and they survived this extinction event (Clack, 2002).

The Permian–Triassic extinction (≈251 Mya) was by far the worst of the five mass extinctions; 95% of all species (marine as well as terrestrial) were lost, including 53% of marine families, 84% of marine genera, and 70% of land plants, insects, and vertebrates (Jablonski, 1995; Erwin, 2001). Causes are debated, but the leading candidate is flood volcanism emanating from the Siberian Traps, which led to profound climate change. Volcanism may have been initiated by a bolide impact, which led to loss of oxygen in the sea. The atmosphere at that time was severely hypoxic, which likely acted synergistically with other factors (Huey and Ward, 2005). Most terrestrial vertebrates perished, but among the few that survived were early representatives of the three orders of amphibians that survive to this day (Marjanovic and Laurin, 2007; Cannatella et al., in press).

The End Triassic extinction (≈199–214 Mya) was associated with the opening of the Atlantic Ocean by sea floor spreading related to massive lava floods that caused significant global warming. Marine organisms were most strongly affected (22% of marine families and 53% of marine genera were lost) (Jablonski, 1995; Erwin, 2001), but terrestrial organisms also experienced much extinction. Again, representatives of the three living orders of amphibians survived.

The most recent mass extinction was at the Cretaceous–Tertiary boundary (≈65 Mya); 16% of families, 47% of genera of marine organisms, and 18% of vertebrate families were lost. Most notable was the disappearance of nonavian dinosaurs. Causes continue to be debated. Leading candidates include diverse climatic changes (e.g., temperature increases in deep seas) resulting from volcanic floods in India (Deccan Traps) and consequences of a giant asteroid impact in the Gulf of Mexico (Jablonski, 1995; Erwin, 2001). Not only did all three orders of amphibians again escape extinction, but many, if not all, families and even a number of extant amphibian genera survived (Vieites et al., 2007).

A SIXTH EXTINCTION?

The possibility that a sixth mass extinction spasm is upon us has received much attention (Novacek, 2007). Substantial evidence suggests that an extinction event is underway.

When did the current extinction event begin? A period of climatic oscillations that began about 1 Mya, during the Pleistocene, was characterized by glaciations alternating with episodes of glacial melting (Barnosky, Chapter 12, this volume). The oscillations led to warming and cooling that impacted many taxa. The current episode of global warming can be considered an extreme and extended interglacial period; however, most



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