Introduction

Before 1980s, the topic of mass extinctions was relatively unstudied and poorly known, with only a handful of papers published by paleontologists, and almost no testable hypotheses about their causes. But in the 1980s, mass extinction and its causes became the hottest topic not only in paleontology, but spilling over to earth sciences and even astrophysics. The crucial trigger for this change was the discovery in 1978 (published by Alvarez et al., 1980) of abnormally high levels of iridium at the Cretaceous/Tertiary (K/T) boundary near Gubbio, Italy. Alvarez et al. (1980) interpreted this discovery as evidence of an impact of a 10-km asteroid, which supposedly caused dust clouds that darkened and cooled the earth, and wiped out the non-avian dinosaurs, ammonites, and many other creatures that did not survive into the Cenozoic.

The appeal of this hypothesis was immediate, because it offered ways to test whether it had occurred, and what its effects might be, and also because it postulated a catastrophic mechanism that brought geologists and astrophysicists into a research area normally reserved for stratigraphers and paleontologists. The "K/T impact bandwagon" then got rolling, with many different scientists tackling the problem, and within a decade there was a scientific literature of several thousand papers, as well as numerous trade books. Soon the impact advocates were seeking evidence of iridium and other impact products in nearly every other mass extinction horizon, and claiming that they had found it in many of them (reviewed below).

Along with the "bandwagon" of impact advocates claiming they had found the cause for this or that mass extinction was another parallel argument. In 1984, Raup and Sepkoski (1984, 1986) made the claim that mass extinctions were periodic, occurring approximately every 26 million years. They based this on what they interpreted as major pulses of extinction in their database of fossil marine families and genera. As Raup (1986) details, astronomers jumped the gun on this idea, publishing explanations ranging from periodic comet showers (Davis et al., 1984), the oscillation of the solar system through the galactic plane (Rampino and Stothers, 1984; Schwartz and James, 1984), an unknown Planet X (Whitmire and Jackson, 1985), and an undetected companion star to the Sun dubbed Nemesis (Whitmire and Jackson, 1984). Most of these papers were written based on the preprint of Raup and Sepkoski (1984), before it had even undergone peer review, let alone much critical testing from the paleontological community.

Unfortunately for this exciting hypothesis, several ugly facts have completely discredited the periodicity model (see review in Prothero, 2004a, 94-95). No evidence for Nemesis or Planet X has ever been found, nor any evidence tying comet showers or the motion through the galactic plane to mass extinctions. Many statisticians challenged the statistical robustness for the periodicity model. The taxonomic basis for much of the "periodicity" has been debunked, since many of the "extinction peaks" of Raup and Sepkoski (1984) are not real, or are nowhere near the 26 million year spacing required by the model. Most importantly, the periodicity model requires some sort of regular external forcing factor in the environment (comets, Nemesis, Planet X), so each mass extinction horizon should show a common signal of its causation. As we'll see below, they don't.

After a decade of controversy, Stanley (1990) proposed a model that represents the general consensus among paleontologists for why extinctions appear spaced by at least 20 million years. According to his hypothesis, the "aftermath" world of major mass extinctions is inhabited by a low diversity of "weedy"

opportunistic survivor species, which are extinction-resistant and persist for a long time after the event. It takes a full 10 to 20 million years after a major extinction for life to recover its former high diversity, complete with vulnerable ecological specialists that might be prone to the next mass extinction. If there were a major impact too soon after a mass extinction, it would have little or no biotic effect.

For most scientists, the periodicity hypothesis is long dead. However, some do not give up. Rohde and Muller (2005) claim to have found a 62-million-year extinction periodicity in the long-discredited Sepkoski database. Firestone et al. (2007) recently postulated that the late Pleistocene megafauna of North America was wiped out by impact, although their model does not explain which so many large mammals (especially bison) survived while others did not. In addition, it does not address a whole host of other problems (Pinter and Ishman, 2008). Other elements of the mass extinction debate are still widely accepted among those who do not keep up with the paleontological literature or attend the professional meetings. Due to its high public profile and "sexy" nature, the impact hypothesis for the K/T extinction is still widely believed among the public, and among non-paleontologists. The idea that an impact killed the dinosaurs is hot enough to make the cover of Time magazine, whereas most of the public knows nothing else about the K/T extinction or any other extinction (including the much larger Permo-Triassic extinction). Most of the trade books and now even many of the textbooks repeat the idea from the 1980s that the K/T extinction, and most of the other major mass extinctions were caused by impacts. But research on these topics has not stood still, even if it no longer dominates the spotlight as it did in 1980s. A lot of work has been completed on the detailed record of the major mass extinction horizons, and the past decade of research results have not gone favorably for the impact advocates (Prothero, 1999, 2004a, b; Keller, 2005; Lucas, 2005; White and Saunders, 2005; Morrow, 2006; Ward, 2007).

In this paper, I will review the recent evidence for the impact hypothesis, and the current evidence for the causes of the major mass extinctions.

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