Cretaceous-Paleogene boundary

The Cretaceous-Paleogene boundary, sixty-six million years ago, is the junction between the Mesozoic and Cenozoic eras. This boundary coincides with a major extinction of marine and terrestrial organisms, the most conspicuous of which were the ammonoid cephalopods in the sea and the dinosaurs on the land. A ten-kilometer-diameter bolide that collided with Earth at this time has been invoked by some as the cause of these extinctions.

Mass Extinction

The Cretaceous-Paleogene boundary is a point in geological time located sixty-six million years before the present. It corresponds to the junction between the geological eras known as the Mesozoic, of which the Cretaceous is the youngest subdivision, and the Cenozoic, of which the Paleogene is the oldest subdivision. This boundary coincides with (and, using fossils, is recognized by) a major extinction of marine and terrestrial organisms. This extinction is not the most massive known extinction—the Paleozoic-Mesozoic extinction, 250 million years ago, holds that honor. The extinction at the end of the Cretaceous is, however, the best-known extinction in Earth's history because it was during this time that the dinosaurs disappeared.

When British paleontologist John Phillips coined the terms Mesozoic and Cenozoic in 1840, he knew they represented time intervals in Earth's history characterized by very different types of organisms. However, paleontologists only recognized the full significance of the boundary between the Mesozoic and Cenozoic eras in the early twentieth century. By 1900, about a century of scientific collecting and study of fossils demonstrated that many organisms had become extinct at or just before the Cretaceous-Paleogene boundary. This extinction thus ended what is popularly termed “the age of reptiles,” setting the stage for the appearance and proliferation of the types of organisms that have inhabited the Earth for the last sixty-six million years, or what is popularly called “the age of mammals.”

In examining the extinctions that occurred in the late Cretaceous, scientists have learned that about 15 percent of the families (or approximately one hundred families) of shelled invertebrates became extinct. Particularly hard-hit groups were the ammonoid cephalopods, relatives of living squids and octopi, which suffered total extinction; clams and gastropods (snails), which endured significant losses; and the marine reptiles, the mosasaurs (giant marine lizards) and plesiosaurs (long-necked reptiles), which vanished altogether. Major changes occurred in the marine plankton, and the foraminiferans (microscopic shelled protozoans) also suffered heavy losses. On land, the flying reptiles (pterosaurs) and the dinosaurs became extinct, many marsupial mammals disappeared, and a few flowering plants, especially broad-leafed forms and those living in low latitudes, died out.

After the extinction, the land surface was populated by many placental mammals, which rapidly diversified during the early Paleogene; by turtles, crocodiles, lizards, snakes, and other reptiles little affected by the extinction; and by birds and flowering plants, groups not seriously impaired by the extinctions. In the sea, the most conspicuous Mesozoic denizens—ammonoids, mosasaurs, and plesiosaurs—were gone, as were some types of clams, especially the reef-building rudists and the platelike inoceramids. However, many other clams survived, as did representatives of the other hard-hit invertebrate groups. The plankton and bony fish recovered, and sharks remained unscathed by the extinctions.

Problems of Extinction Criterion

The Cretaceous-Paleogene boundary is almost always identified by the extinctions that took place at that time. Thus, in the sequence of strata, certain fossil groups (for example, dinosaurs) are present in Cretaceous rocks but are absent in Paleogene rocks. Using the criterion of extinction, however, to identify the Cretaceous-Paleogene boundary produces two significant problems.

The first of these problems stems from the inherent diachrony of extinction—in other words, an extinction almost always does not occur simultaneously across the geographic range of an organism. Thus, hippopotamuses have been undergoing extinction for thousands of years and disappeared from Europe and Asia a few thousand years ago. They are primarily restricted to Africa, where they will probably suffer extinction within the next few thousand years unless human intervention saves them. Except for a possible pervasive global catastrophe at the Cretaceous-Paleogene boundary, why should not the extinction of many Cretaceous organisms have occurred in the same diachronous fashion as the ongoing extinction of the hippopotamus? Indeed, some paleontologists believe that there is evidence that dinosaurs became extinct in South America after their extinction in North America. If this is correct, then what is identified as the Cretaceous-Paleogene boundary in North America is older than what is identified as the boundary in South America. This presents a serious problem when placing the Cretaceous-Paleogene boundary, which should ideally represent the same point in time everywhere.

The second problem faced when using extinctions to identify the Cretaceous-Paleogene boundary is the circularity of reasoning that can result. That is, if one identifies the Cretaceous-Paleogene boundary by the extinction of dinosaurs, one must be careful in saying that dinosaurs became extinct at the Cretaceous-Paleogene boundary. What if, as some believe, dinosaurs survived longer in some parts of the world than in others? To determine if this was the case, another criterion (usually another group of fossils) must be used to determine the age of the youngest dinosaur fossils.

Extinction Theories

One of the most intriguing aspects of the Cretaceous-Paleogene boundary is what caused the extinctions. To answer this question, the timing of these extinctions must be determined. Did they occur simultaneously and suddenly? If so, a major catastrophe of global proportions apparently caused them. If the extinctions were not simultaneous, and if some groups of organisms were already in decline before the Cretaceous-Paleogene boundary, then a single catastrophe alone cannot explain the extinctions.

In 1979, Nobel physics laureate Luis Alvarez, his geologist son Walter Alvarez, and two nuclear chemists, Frank Asaro and Helen Michel, proposed that a bolide (a comet or meteorite) ten kilometers in diameter collided with Earth sixty-six million years ago and caused the extinction of the dinosaurs and other groups of organisms that died out at the end of the Cretaceous. They initially based this proposition on a chemical analysis of a clay layer at Gubbio in northern Italy. This clay layer was deposited at the bottom of the sea sixty-six million years ago, and the chemical analysis revealed that it contains an unusually large concentration of the platinum-group metal iridium. Such a high concentration of iridium, reasoned Alvarez and his colleagues, could not be produced by known terrestrial mechanisms and thus must have settled in the dust produced by a huge bolide impact. They later identified a site at Chicxulub on the Yucatán Peninsula in southern Mexico as the point of the bolide impact.

Geological studies at other localities worldwide where sixty-six-million-year-old rocks are preserved have confirmed the Alvarez team's proposition of a bolide collision with Earth sixty-six million years ago. Their claim that the bolide impact is linked directly to the Cretaceous-Paleogene-boundary extinctions has not fared as well. Indeed, the fossil evidence indicates that many groups of organisms in the sea (for example, the ammonoids and inoceramid clams) and on the land (dinosaurs) were declining millions of years before the Cretaceous-Paleogene boundary. Furthermore, some groups of organisms (rudist clams are an example) became extinct one million or more years before the boundary. Also, there is some evidence, hotly debated, that a few types of dinosaurs may have survived into the earliest Paleogene. Dinosaurs that share similarities with modern-day birds are suggested to have lived the longest, though amphibians also outlived many other animals of the time. Nevertheless, the fossil evidence has its detractors since many fossils remain to be discovered, and the suddenness and synchrony or diachrony of some extinctions still is subject to debate.

A dispassionate reading of the existing fossil evidence does not support a single, mass extinction at the Cretaceous-Paleogene boundary. Instead, it suggests that, as a result of changing climates and sea levels, a period of extinction beginning three to five million years before the Cretaceous-Paleogene boundary was culminated by the final disappearance of several groups of organisms at (or perhaps just after) the end of the Cretaceous. Perhaps the bolide impact at the Cretaceous-Paleogene boundary is best interpreted as the last piece of bad luck encountered by a Mesozoic biota already doomed to extinction.

Study of the Cretaceous-Paleogene Boundary

Research on the Cretaceous-Paleogene boundary must first focus on locating the boundary in the strata of a given region. There are two places—Stevns Klint in Denmark and Gubbio in Italy—where, by international agreement, the position of the Cretaceous-Paleogene boundary is fixed in the strata. Identifying the boundary elsewhere on Earth has thus been reduced to a problem of stratigraphic correlation, the method by which the equivalence in age or position of strata in disparate areas is determined. The goal of the fieldworker then has to be identifying criteria (usually fossils) by which correlation with the Cretaceous-Paleogene boundary in Denmark or Italy can be demonstrated.

Since the Cretaceous-Paleogene-boundary rocks in Denmark and Italy were deposited at the bottom of the sea sixty-six million years ago, it is sometimes difficult to identify good criteria for stratigraphic correlation in sixty-six-million-year-old rocks that were deposited on land. In these rocks, the youngest dinosaur fossils usually are believed to mark the Cretaceous-Paleogene boundary until other evidence demonstrates otherwise. This other evidence sometimes comes from fossil pollen grains, numerical ages, or other geophysical techniques, such as studying the magnetic properties of the rocks to determine their age.

Once the boundary has been placed with confidence, other aspects of studying the Cretaceous-Paleogene boundary are even more complex. They focus on the extinctions themselves and their potential causes. Data and techniques from many fields are brought to bear here, including paleontology (the study of fossils), sedimentology (the study of how sediment is transported and deposited), and geochemistry (the study of rock chemistry). At its simplest, in a given sequence of strata that encompasses the Cretaceous-Paleogene boundary, the goal of research is to collect and document the vertical ranges of all fossils, their relative abundances, and how their ranges and abundances correspond to environmental changes indicated by the sediments and rock chemistry.

One of the problems these studies face is the incompleteness of the fossil record. For example, when paleontologists think they have found the youngest dinosaur fossil in a local sequence of strata, how can they be sure? Maybe younger dinosaurs lived in the area, and their fossils were not preserved, or if they were preserved, the fossils may not have yet been found. This caveat makes it difficult, especially in rocks deposited on land, where fossil occurrence often is very spotty, not only to be certain of the position of the Cretaceous-Paleogene boundary but also to be confident of the correspondence between fossil range, fossil abundance, and environmental changes indicated by sediments and rock chemistry. The potential for new fossil discoveries always exists. This is only one reason that research on the Cretaceous-Paleogene boundary continues. The cause of the extinctions at and around this boundary remains a subject of heated debate.

Dinosaur Mystery

One of the most interesting aspects of the extinctions at the Cretaceous-Paleogene boundary is the disappearance of the dinosaurs. Dinosaurs included the largest land animals of all time and dominated the Earth's surface for 150 million years. Why such large and seemingly successful reptiles died out has captured the imagination of scientists and laypersons alike for more than a century. More importantly, understanding extinctions in the past, such as those that occurred at the Cretaceous-Paleogene boundary, may allow humankind to understand the causes and effects of massive extinctions. This understanding may help humans avoid future extinctions and provide some basis for understanding the potential effects of the ongoing extinction of species in the globe's tropical regions.

Finally, there is seemingly incontrovertible evidence that a large bolide impacted the Earth sixty-six million years ago. The effects of this impact have been likened to the “nuclear winter” that might result from a global thermonuclear war. Such a nuclear winter would be a period of intense cold when all incoming sunlight is blocked by the smoke accumulated in the atmosphere from continent-wide forest fires. Analogous conditions may have existed on Earth during the first ten to one hundred years that followed the bolide impact at the Cretaceous-Paleogene boundary. Studying the effects of this impact thus provides insight into a global disaster of horrific proportions and, if nothing else, is an inducement to the human species to avoid such a cataclysm.

Principal Terms

bolide: a meteorite or comet that explodes upon striking Earth

Cenozoic era: the youngest of the three Phanerozoic eras, from 66 million years ago to the present; it encompasses two geologic periods, the Paleogene (older) and the Quaternary

Cretaceous period: the third, last, and longest period of the Mesozoic era, 144 to 66 million years ago

era: a large division of geologic time composed of more than one geologic period

extinction: the disappearance of a species or large group of animals or plants

family: a grouping of types of organisms above the level of a genus

Mesozoic era: the middle of the three eras that constitute the Phanerozoic eon (the last 544 million years), which encompasses three geologic periods—the Triassic, the Jurassic, and the Cretaceous—and represents Earth's history between about 250 and 66 million years ago

Paleogene period: the earlier and much longer of the two geologic periods encompassed by the Cenozoic era, from 66 to 23 million years ago

stratum (plural, strata): a single bed or layer of sedimentary rock

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