K/T Boundary

Toward the end of the Cretaceous period, many species of marine organisms became extinct. Land dinosaurs completely disappeared, along with flying reptiles, sea reptiles, and ichthyosaurs. Other land reptiles were little affected. Most species of turtles, crocodilians, lizards, and snakes survived. Amphibians and mammals were only mildly affected.

Overall there was a major, worldwide decrease in the number of species of plants and animals. This drop in the number of species is one of the events that signaled the end of the Cretaceous period and the beginning of the Tertiary period. The transition between the two periods is known as the K/T boundary ("Cretaceous" in German is die Kreidezeit ).

Determining whether all the species died out in a few years or over millions of years has proved to be a difficult problem for geologists and pale-ontologists. Attempts to pinpoint the time of the K/T boundary event result in a margin of error of at least one million years, which means that it could have taken place one million years earlier or one million years later. Although one million years is a very short time on the geologic time scale, such a margin of error indicates that all the species may not have died out at the same time. In any case, instantaneous events are very rare and usually do not affect more than a small region. For these reasons, most geologists assumed the K/T event resulted from a gradual process, such as global cooling.

An alternate to the hypothesis of extinction by a gradual process has been suggested. A group of paleontologists has proposed that the extinction may have been due to a single, catastrophic event. The American geologist Walter Alvarez first discovered evidence for this event. While conducting research near Gubbio, Italy, in the late 1970s, Alvarez discovered an abnormally high concentration of the rare element iridium in a layer of rock at the K/T boundary. The iridium anomaly, or spike, has been found all over the world in layers of rock dating to the same time. It is called an iridium spike because on a graph of iridium concentration versus time, the concentration near the time of the K/T boundary is sharply higher than in adjacent rock layers. The iridium concentration is at least twenty times more than normal and is even greater at some locations. The iridium spike seems to mark one of those rare, catastrophic events that took place worldwide.

Because meteorites often contain high concentrations of iridium, Alvarez and his father, American physicist Luis Alvarez, suggested an extraterrestrial origin for the iridium. If the iridium concentration at the K/T boundary resulted from a collision at that time between Earth and an asteroid, the dust from the collision would have substantially reduced the amount of sunlight available for plants to carry out photosynthesis . The plants would eventually die, the large plant-eating dinosaurs would starve, and the meat-eating dinosaurs that preyed on the plant-eaters would also starve.

However, the fossil record does not show a sharp decrease in the number of large land dinosaurs, as would be expected after an impact large enough to produce global cooling. Instead, the fossil record indicates a gradual decrease in the number of species of large land dinosaurs over millions of years. There is also no marked decrease in the number of land plants. Therefore, the asteroid impact theory may be inadequate in explaining the decrease in large land dinosaurs.

The asteroid theory is generally accepted as the most likely explanation of the iridium spike and may be the best explanation for the extinction of marine organisms. The foraminifera, ammonites, coccolithophores, and other species did disappear suddenly at precisely the right time. However, there are several competing hypotheses and it is not certain that the asteroid impact alone can account for dinosaur extinction. A major difficulty of all hypotheses is the selectivity of the extinctions. Why were dinosaurs wiped out while other land reptiles were little affected?

It is possible that the mass extinctions at the K/T boundary may be due to a combination of asteroid impact and other factors. The land animals may have died out as a result of seafloor spreading, continental drift , and volcanism occurring around the same time; while the marine organisms were affected by the impact. Shifting oceanic circulation patterns due to continental drift may have caused climatic changes and changes in sea level.

see also Cretaceous; Geological Time Scale; Tertiary.

Elliot Richmond

Bibliography

Alvarez, Luis W., et al. "Extraterrestrial Cause for the Cretaceous-Tertiary Extinction: Experimental Results and Theoretical Interpretation." Science 208 (1980):1095-1108.

Gore, Rick. "Extinctions." National Geographic 175 (1989):662-699.

Russell, Dale A. "The Mass Extinctions of the Late Mesozoic." Scientific American 246 (1982):58-65.

Stanley, Steven M. "Mass Extinctions in the Ocean." Scientific American 250 (1984):64-72.