The foraminifera, ammonites, coccolithophores, and other species did disappear suddenly at precisely the right time. 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. Therefore, the asteroid impact theory may be inadequate in explaining the decrease in large land dinosaurs. There is also no marked decrease in the number of land plants. Instead, the fossil record indicates a gradual decrease in the number of species of large land dinosaurs over millions of years. 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. 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. If the iridium concentration at the K/T boundary resulted from a collision at that time between Earth and anĪsteroid, the dust from the collision would have substantially reduced the amount of sunlight available for plants to carry out photosynthesis. The iridium spike seems to mark one of those rare, catastrophic events that took place worldwide.īecause meteorites often contain high concentrations of iridium, Alvarez and his father, American physicist Luis Alvarez, suggested an extraterrestrial origin for the iridium. The iridium concentration is at least twenty times more than normal and is even greater at some locations. 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 anomaly, or spike, has been found all over the world in layers of rock dating to the same time. 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 American geologist Walter Alvarez first discovered evidence for this event. A group of paleontologists has proposed that the extinction may have been due to a single, catastrophic event. For these reasons, most geologists assumed the K/T event resulted from a gradual process, such as global cooling.Īn alternate to the hypothesis of extinction by a gradual process has been suggested. In any case, instantaneous events are very rare and usually do not affect more than a small region. 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. 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. The transition between the two periods is known as the K/T boundary ("Cretaceous" in German is die Kreidezeit ).ĭetermining 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. 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. Overall there was a major, worldwide decrease in the number of species of plants and animals. Amphibians and mammals were only mildly affected. Most species of turtles, crocodilians, lizards, and snakes survived. Other land reptiles were little affected. Land dinosaurs completely disappeared, along with flying reptiles, sea reptiles, and ichthyosaurs. Toward the end of the Cretaceous period, many species of marine organisms became extinct.
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