Cretaceous Tertiary Extinction

The Cretaceous-Tertiary (K-T) mass extinction defines the K-T boundary and represents one of the five largest biotic crises that punctuate the Phanerozoic record. The broad consequences of this event on the earth's biota can be seen in that it demarcates the transition from the Mesozoic ("middle life") into the Cenozoic ("modern life"). During this interval numerous groups that had dominated both marine and terrestrial Mesozoic ecosystems disappeared or suffered substantial reductions, whereas the aftermath—especially within the terrestrial realm—is characterized by the rise to dominance of new animal and plant groups. Furthermore, interest in the K-T boundary has been focused on the question of causation. This mass extinction event has the clearest record of extraterrestrial impact, as well as additional evidence for flood-basalt volcanism. This evidence has given rise to a lively and often contentious debate on the nature of geologic data and how these data relate to the fossil record across the boundary.

Environmental Setting

The K-T boundary occurred during a period of gradually declining global warmth and retreating sea levels, although the overall conditions are still considered "greenhouse"—that is, the dominant climatic mode of the Cretaceous. Superimposed upon the general thermal and sea level trends are two important shorter-term events that influenced the paleoenvi-ronment: (1) the extrusion of the Deccan Traps, a voluminous flood basalt that erupted in and covered much of west-central India; and (2) a bolide impact. The Deccan Traps represent a series of eruptions that produced an estimated 512,000 cubic kilometers of basalt. Based on the most recent radiometric dates measured from the various flows, the eruptions commenced approximately 67 million years ago (that is, roughly 2 million years prior to the K-T event) and ceased at 65 million years ago (soon after the K-T boundary).

The evidence for an impact directly at the K-T boundary was initially proposed based on the large increase in iridium found within sediments from this interval. Iridium is generally found in extremely small quantities at the earth's surface, but in a study of the element through the Maastrichtian (the last stage of the Late Cretaceous) strata of Gubbio, Italy, extremely high concentrations were found. Because there are no known terrestrial sources capable of producing such an increase in iridium, an extraterrestrial source—namely, an iridium-rich meteorite—was suggested as a source. After this initial discovery, not only were iridium anomalies found in sections globally, but other indicators of impact—such as shocked quartz, potential soot layers, and evidence for acid rain—were also found. The primary missing component of impact was a crater—the so-called smoking gun. Finally, in 1991, scientists rediscovered a subsurface feature in the Yucatan, the Chicxulub structure, and after extensive investigation concluded that this crater was of the proper age and proper size, as hypothesized by the available data for the impact event.

A more detailed examination of the record suggests that the interval surrounding the K-T boundary was one characterized by substantial fluctuations in the atmosphere-ocean system, as reflected in various geochemical proxies, especially d18O and d13C, that are used to monitor paleoconditions. Approximately 1.5 million years ago, prior to the boundary, there was a pronounced oceano-graphic event that is believed to indicate an important decrease in deep-oceanic temperatures. Furthermore, the K-T boundary itself records a dramatic geochemical event termed the Strangelove Ocean, and this has been interpreted as a major disruption of carbon cycling in the oceans, tied to a pronounced reduction in primary productivity.

The Biotic Impact

Given the various changes across the K-T mass extinction that may have affected the fauna and flora through the interval, it is critical to examine the biotic record in detail to determine the nature of the biotic response. From a broader overview, however, estimates of the level of extinction of higher taxa across the K-T boundary suggest that approximately 15 percent and 37 percent of marine families and genera, respectively, went extinct, with potentially higher rates for the terrestrial biota. Furthermore, there is abundant evidence that both marine and terrestrial organisms were affected by the events, suggesting that the causal mechanism(s) had to have global impact. The boundary is marked by the complete disappearance of a number of important terrestrial and marine groups, including non-avian dinosaurs, ammonites (shelled cephalopods), rudistid bivalves (the primary reef builders of the Late Cretaceous), and inoceramid bivalves. Furthermore, many marine and terrestrial ecosystems and the species composing them—such as mammals, plants, bivalves, gastropods, brachiopods, bry-ozoans, and numerous members of the plankton family—suffered significant reductions, which in some cases led to the disappearance of various communities, such as reefs, for extended periods following the extinction.

Examining the available data in more detail reveals important extinction patterns, as well as geographic differences that offer insight into the event. First of all, there are important extinction events that precede the K-T boundary. The reef-building rudistids as well as the inoceramids, both mollusks, go largely extinct approximately 1.5 million years prior to the boundary. The virtual extinction of these groups—which dominated reefal as well as most epifaunal marine communities during the Late Cretaceous—suggests that there were important environmental changes occurring prior to the boundary that were beginning to stress a biota that had evolved under greenhouse conditions. Furthermore, a number of groups show accelerated levels of extinction prior to the boundary, and it is possible that this was coupled with a reduction in species origination that resulted in decreased biodiversity. Against this backdrop, there is also abundant evidence, especially from marine planktic groups, such as foraminifers and coccolithophores, that there was a dramatic extinction event directly at the K-T boundary. This event has also been documented in certain floral studies, suggesting that the perturbation affected both marine and terrestrial habitats. Therefore, the overall pattern is one of an initial interval of declining diversity punctuated by a dramatic biotic event directly at the boundary.

From a geographic perspective there are also several important trends that have been documented. First of all, within the marine realm, whereas planktic groups and other shallow-water marine organisms suffered significant reductions, deep-marine taxa, such as benthic foraminifers, were unaffected by the event. Additionally, within the affected environments, the data suggest that tropical regions suffered a greater degree of extinction than did higher latitudes. For example, whereas rudist-dominated, platform reef communities were completely destroyed, the mass extinction primarily reorganized the taxo-nomic composition of higher-latitude communities. Secondly, within the terrestrial realm, higher-latitude faunas, especially those in North America, suffered significant changes in diversity and abundance, although a similar pattern has been documented in other areas. In many cases, the so-called fernspore spike directly overlies the iridium layer, suggesting that the flora underwent a drastic, albeit fairly short-lived, compositional change. However, the tropical flora, which are generally less resilient to changes in the earth's climate, apparently suffered little to no change through the interval.

Analyzing the Evidence

One of the advantages to studying the K-T mass extinction is that, at least from the perspective of geologic time, it is relatively recent. This results in a much more completely preserved geologic record than for any of the other so-called Big Five mass extinctions. To evaluate the effects of these biotic crises, it is critical to have continuous sections that represent a wide spectrum not only of different geographic settings but also of various paleo-environments. This increases the likelihood that global effects can be distinguished from local changes. For the K-T boundary there are a wide variety of different sections that have been studied on all continents. Furthermore, because there are a large number of cores that have been recovered through deep-ocean sediments across the boundary in numerous places, the geographic distribution of fossiliferous sediments is excellent. Furthermore, these sections represent a variety of depositional environments that compose marine as well as terrestrial sections, making possible a comprehensive analysis of the extinction dynamics.

An important component to consider when attempting to unravel the dynamics of a mass extinction is how the pattern of biotic response (for example, biodiversity changes and variation in community structure) compares with what would be expected from the presumed effects of the hypothesized extinction mechanism. Different mechanisms should produce distinct patterns, with unpredictable, shortlived events like bolide impacts resulting in the catastrophic extinction of numerous taxa and the instantaneous disruption of widely variable ecosystems; more predictable, longer-term events, such as climate or sea-level change, would be expected to produce more gradual, ecologically graded mass extinctions. Discerning biotic patterns from the geologic record is, however, confounded by a number of thorny questions.

Firstly, the fossil record is notoriously incomplete, and this hinders attempts to accurately determine the stratigraphic ranges of the various taxa. Secondly, there are various measures that can be used to analyze the biota, ranging from compilations at familial and generic levels, to the stratigraphic ranges of individual species, to the abundance of individuals within individual taxa. The questions being asked play a large role in determining which approach offers the best data to answer the question. For addressing the issues related to the short-term biotic response to mass extinction events, stratigraphic ranges (used to investigate changes in local to global biodiversity)

as well as abundance (used to determine changes in ecologic structure) offer the best measures. However, it is also critical to have sufficient knowledge of these variables during "background" intervals, so that changes forced by the mass extinction can be differentiated from variability inherent within any ecosystem and its constituents. Finally, to evaluate the long-term evolutionary impact of a mass extinction event, it is important to be able to track individual ecosystems over time.

Biotic Recovery

One of the commonly overlooked elements of mass extinctions is the postextinction biotic rebound. It is obvious that the biota recovered from this mass extinction, and this implies that there must be a number of surviving stocks from which this new biodiversity is derived. Following the K-T event, most ecosystems, with the notable exception of shallow-water reefs, recovered geologically rapidly, especially in light of the intensity of the extinction. The biotic response appears to be largely controlled by the duration of the environmental perturbations, and to be based on various geochemical proxies; the environmental disruption at the K-T boundary, although severe, extended only into approximately the initial 500,000 years of the Paleocene. During that interval, taxa with broad ecologic tolerances thrived under highly variable environmental conditions. This is exemplified by the dominance of ferns, a group with a relatively long geologic history that today is often one of the first colonizers following environmental devastation. There are, however, important differences, especially in terrestrial ecosystems, between Late Cretaceous and Cenozoic ecosystems. The best-documented change is the transition from nonavian dinosaurs to mammals within terrestrial vertebrates. In addition, more subtle changes—but of poten tially critical ecologic significance—occurred in many other marine and terrestrial ecosystems, in which taxa that dominated Cretaceous ecosystems either went extinct or lost their dominance across the boundary.

Lessons for the Present Biodiversity Crisis

Given the current rate of biotic extinction, it is critical to look for an analog from the geologic past to use as a potential model for the future of the earth's biota. The biotic response to the K-T boundary shows that the extinctions occurred over an interval of approximately 1.5 million years, and that the bulk of them occurred in close association with the Chicxulub impact. This suggests that environmental conditions were deteriorating at the close of the Cretaceous, initiating the extinction of various biotic elements, such as the inoceramids and rudistids, and also stressing other biotic elements. Because conditions were already in flux, the addition of the short-term, intense disruptions caused by the impact had a pronounced effect upon the biota. Clearly, humans are causing various environmental and ecologic disruptions, and those disruptions, be they through overhunting or chemical emissions, are creating a biota at risk. However, because of the differences in causation, these analogies can be carried only so far. The K-T boundary and the other documented mass extinction events were all forced by changes in environmental factors. In the current case, the extinctions are being forced by a biotic element: humans. Therefore, the lessons learned from an analysis of the fossil record may best serve to show how life will rebound, although they will be unable to predict the path of the current crisis.

—Peter J. Harries and Neil H. Landman

See also: Adaptive Radiation; Mass Extinction; Mol-

lusca; Valuing Biodiversity

Bibliography

Alvarez, Luis W., et al. 1980. "Extraterrestrial Cause for the Cretaceous-Tertiary Extinction." Science 208: 1095-1108; Archibald, J. David. 1996. Dinosaur Extinction and the End of an Era: What the Fossils Say. New York: Columbia University Press; Landman, Neil H. 1984. "To Be or Not to Be?" Natural History 93: 4-41; Raup, David M. 1991. Extinction: Bad Genes or Bad Luck? New York: W. W. Norton; Ward, Peter Douglas. 2000. Rivers in Time: The Search for Clues to Earth's Mass Extinctions. New York: Columbia University Press.

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