This is certainly the most widely known and probably also the best investigated of the major mass extinctions, simply because the popular (nonavian) dinosaurs disappeared at the Cretaceous-Tertiary (K-T) boundary. Yet it is, with a 16% loss of the families, a 47% loss of the genera, and an estimated loss of at least 70% of the species in the marine realm, the least severe among the five major mass extinctions in earth history (Jablonski 1994; Hallam and Wignall 1997; O Figure 16.11). Some marine groups disappeared completely at the end of the Cretaceous (e.g., the large marine reptiles), others suffered heavy losses (especially planktonic groups), but there were also groups that exhibited little or no reduction over the last Cretaceous to the lowermost Paleogene (MacLeod et al. 1997; Norris 2001).
On land, plants suffered evidently little long-term reduction (Niklas et al. 1983; Niklas 1997; Willis and McElwain 2002), but in many sections a short proliferation of ferns at the expense of angiosperms ("fern-spike'') is documented. Among the tetrapods, amphibians, turtles, crocodilians, and eutherian mammals were largely unaffected by the K-T boundary event, whereas lizards and marsupials suffered heavy losses (Archibald and Fastovsky 2004). For the
O Figure 16.11
Extinction patterns during the K-T mass extinction. Modified after Hallam & Wignall 1997; MacLeod et al. 1997
ornithischians and the (nonavian) saurischians, this event was of course the end of a long era. Land-dwelling species were more severely hit than freshwater inhabitants, endothermic tetrapods (including ornithischians and saurischians) more than ectothermic, and larger more than smaller species (Archibald 1996; Archibald and Fastovsky 2004).
Evidence for both a gradual decline in dinosaur species richness as well as for a catastrophic and (geologically spoken) instantaneous extinction of the dinosaurs was presented (Hurlbert and Archibald 1995; Archibald and Fastovski 2004), but the most recent data indicate again that the dinosaurs went abruptly extinct during the time of their highest diversity (Fastovsky et al. 2004). The picture is equally complicated in marine exposures in which again evidence for both a gradual as well as a sudden extinction was presented (see MacLeod et al. 1997). Yet resampling of formerly investigated sections extended taxonomic ranges upward and the reported gradual decline of many groups might well be a consequence of the Signor-Lipps effect (Ward 1990).
Among the possible causes for the K-T mass extinction that are still considered today are volcanism, climatic fluctuations, and marine regression, and an asteroid impact (Benton 1990). There is indeed overwhelming evidence that the earth was hit by a major asteroid perhaps 10 km in diameter that had a devastating impact on earth's life. This evidence includes molten sediment particles (glass spherules), shocked quartz grains, and a worldwide recognized enrichment in iridium in K-T boundary layers (Alvarez et al. 1980, 1995). The impact hypothesis received further support with the discovery of a 65-Ma old impact crater (Chicxulub) on the Yucatan peninsula, Mexico (Hildebrand et al. 1990). Yet it has also been demonstrated that the latest Cretaceous was a time of major climatic fluctuations (Skelton 2003) and the pronounced marine regression at the end of the Maastrichtian was recognized already a long time ago. In addition, intensive volcanism, which spanned less than 2 Ma over the K-T boundary, is documented from the so-called "Deccan Traps'' (Courtillot 1990, 1999). These represent immense outpourings of lava in what is today India and must have had a profound impact on the biosphere (Kelley 2003).
Currently there are two schools of thought to explain the K-T mass extinction. According to the gradualistic, multiple causes-scenario (Archibald 1996; see also Archibald and Fastovsky 2004), the climatic fluctuations and the marine regression near the end of the Cretaceous changed profoundly the available habitats in both the terrestrial and marine environments. This biotic stress led to a gradual decline in the dinosaurs and other terrestrial vertebrates. Likewise, the regression also imposed a major stress on the marine animals by reducing the available shelf environments. Further stress was imposed by the Deccan Trap volcanism that erupted large amounts of dust into the atmosphere, with general cooling of the globe, drying of many terrestrial ecosystems, and slowing of the photosynthetic activity as the most likely consequences. The asteroid impact at the K-T boundary with its devastating environmetal consequences is not disputed in the gradualist camp but is seen merely as the "last strike'' that led to the collapse of already weakened ecosystems and extinguished animal groups that were already in decline.
Those researchers that favor a single cause for the mass extinction at the K-T boundary emphasize the almost apocalyptic effects a bolide impact would have (Alvarez et al. 1995). This impact ejected considerable amounts of molten rock particles and dust into the atmosphere. Furthermore it produced huge tsunamitype waves that devastated the coastal plains, and an immense fireball ignited vast wildfires. The dust particles would remain in the atmosphere for months, perhaps even years, leading to global cooling and darkening. Photosynthesis came almost to a halt, at least in plants adapted to higher light intensities. The fern spike recorded from many terrestrial boundary sections testifies to this sudden decline in higher plants and the spread of the ferns, which could cope with darker conditions. As a consequence of the collapse of the ecosystems, first the consumers and then the carnivores died out within years. Those animals that did survive were preferentially small, unspecialized, opportunistic species that could feed on a variety of diets. That ecosystems were also severely and almost instantaneously hit in the marine realm is indicated by the patterns ofthe stable carbon isotopes across the boundary. These indicate an almost lifeless ocean after the K-T boundary ("Strangelove Ocean''; Hsu and McKenzie 1985; Zachos et al. 1989).
The normal succession in Caribbean coastal sections agrees well with this scenario (after Alvarez et al. 1995): above the Maastrichtian limestone, larger airborne particles (microtectites, glass spherules) were deposited first, then come tsunami deposits containing reworked Maastrichtian limestone and charcoal (from wildfires), then dust-borne iridium and shocked quartz, and finally we see a return to normal sedimentation. Yet in some sections, the succession of the different layers is incomplete, and sometimes there are multiple ejecta and iridium layers (some of them perhaps reworked). According to a new scenario, three impacts within 0.3-0.4 Ma around the K-T boundary best explain the patterns and all contributed to the mass extinction (Keller et al. 2003). The one producing the Chicxulub crater predated the K-T by 0.27 Myr.
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