418 Extinction in the Fossil Record extinction, gastropod success did not differ especially from periods of more subdued extinction, but groups with planktotrophic larvae and geographically restricted groups suffered more than average (Erwin, 1993) Thus, as extinction intensity increases, some qualitative changes may emerge for some taxa and biogeographic/dispersal properties, but not for all Causes of Mass Extinctions Mass extinctions are associated in time with major environmental changes The problem, of course, is that other times of no mass extinction also mark the times of environmental change, and it is fair to say that we could not easily predict all mass extinctions with nonfossil data alone If environmental forcing, which transcends the abilities of species to survive or adapt, is a major cause of mass extinction, what are the factors? We can list them but finding smoking guns is often another matter Impact or a series of impacts of extraterrestrially derived objects Volcanism Climate change Lowering of sea level, which reduces available habitats for marine species Anoxia, especially transgressive spread of deep-anoxic waters onto the continental shelves Methane hydrate release, resulting in extreme global warming These causes, stem more from associations in time between inferred geological events and extinctions, and not from a solid model linking environmental change to extinction The best example of the latter is the Permian mass extinction The vast marine regression may have been the driving force behind a variety of environmental changes, including a rise in carbon dioxide, which led to increased temperature and oceanic anoxia At the end of the Permian, sea level dropped, perhaps about 200 m, which was followed by a transgressive rise of sea level in the Lower Triassic of similar magnitude in just My Seasonality and reduction of habitat complexity during the regression may also have begotten environmental instability, beyond the adaptive ranges of a number of specialized groups Robert Berner produced a solid model that demonstrated a remarkable drop in atmospheric oxygen from the end of the Permian to the beginning of the Triassic The drop may have been stimulated by a period of extensive volcanism, which in turn caused dry climates and the wide-spread drying of the planet, which reduced burial of carbon in swamps and released carbon dioxide to the atmosphere This might have caused extensive warming and temperature stress The reduction of oxygen might have been the trigger for extinction both on land and sea If oxygen in the ocean declined, hydrogen sulfide might have appeared, which would be poisonous to most marine life Volcanism might be a minor contribution to climate change at the end of the Permian, because calculations preclude much of a change in the large 13 C deviations at this time, due to outgassing However, the extensive volcanism in Siberia might have risen to the surface and heated up carbonates and coal deposits, liberating lethal methane, which might have triggered extinctions and caused larger 13C deviations The Siberian traps cover an enormous area of about two million square kilometers Paleontologists Norman Newell and Anthony Hallam have implicated sealevel change in a number of extinctions throughout the Mesozoic, but they are also often combined with other events, such as bolide impacts, anoxia, and temperature change Douglas Erwin likened this multicomponent explanation to Murder in the Orient Express by Agatha Christie, where twelve culprits were ultimately found to have conspired to murder the victim Great for murder mysteries but maddening for science Even this cast of characters ignores the hypothesis of global cooling triggered by glaciation, but this may be discounted as the glacial evidence can be dated much before the extinction begins The Pace of Mass Extinctions The end of the Cretaceous is not the most dramatic mass extinction in the Phanerozoic (Figures and 2) At the time, however, both major terrestrial and marine elements were lost, the fauna was sufficiently modern to be understood ecologically, and some of our favorites, such as dinosaurs and ammonites bit the dust Luis, Walter Alvarez, and colleagues set off a debate that has yet to flag by suggesting that a massive asteroid impact caused the extinctions by blanketing the earth with dust spread along ballistic trajectories outside the atmosphere Such catastrophes had been suggested before by paleontologists, but here was the first tangible evidence The fact of an End-Cretaceous impact is supported by a world-wide anomaly of high concentrations of the element iridium in rocks just at the End-Cretaceous boundary (K-T boundary) Although there still is some controversy about this, extraordinarily high iridium concentrations indicate an extraterrestrial origin for some of the materials in the rock Shock structures on quartz crystals suggest that an enormous crater should be present on a continent A possible piece of ejecta has been found in a core of the Pacific at the K-T boundary, which indicates that the bolide was likely a typical metal- and sulphide-rich carbonaceous chondrite rather than derived from cometary materials The site of impact has probably been located in the megacrater at Chixulub in the Yucatan of Mexico The crater harbors an armory of smoking guns, including shocked breccia clasts similar to shocked rock fragments found worldwide, tektite-like glasses, a pronounced iridium anomaly, and a radiometrically estimated geological age of 65.2 Ma, which matches the ages of world-wide K-T boundary samples with tektites The crater suggests a bolide of some 10 km in diameter If the impact were at an angle, presumably more material would be spattered into the atmosphere, but it is clear from the world-wide iridium anomaly that winds could have spread the calamity throughout the earth We at present can only speculate the possible biological consequences The dust cloud would exist for a time sufficient to severely disrupt climate by cutting off all light, and temperature might have been expected to drop precipitously A stable oxygen isotope anomaly at the boundary gives evidence for a sudden temperature change The impact should therefore