Dinosaurs aren't the only ones; more than 90 percent of all organisms that have ever lived are extinct.
During The Mesozoic Era, which began 245 million years ago, dinosaurs were the planet's dominant terrestrial species. This "Age of Dinosaurs" lasted for 163 million years until dinosaurs, along with 70 percent of the Earth's species, perished in the Cretaceous-Paleogene extinction event. The nature of this event has been studied extensively and as of 2011, paleontologists support several related dinosaur extinction theories. But as no one knows for sure what killed the dinosaurs 66 million years ago, it remains one of science's great mysteries.
Types of Extinction
A mass extinction is when a large number of species all over the world die out in a short period of time. Mass extinctions are rare; scientists believe that in the last 500 million years only five have occurred. Another type of extinction is a background extinction. As opposed to mass extinction, background extinctions are caused by small changes in habitat and not by catastrophic events, happen over a long period of time and are a normal part of evolution.
Though most scientists agree upon when dinosaurs became extinct, whether they were killed off by a catastrophe or whether they simply faded away through background extinction remains an issue of debate.
Paleontology
In the search for what killed the dinosaurs, paleontologists study prehistoric life by gathering clues from fossils and geological evidence. The most common method used to study terrestrial events that occurred millions of years ago is "statigraphy," the study of layered rock beds.
The Earth's crust is comprised of stacked layers of rocks. This is called the geologic column, and it works as a vertical timeline with the oldest fossils found in the deepest layers and the most recent fossils found in the top layers. By analyzing the position and chemical content of a rock layer, scientists glean information about the climate, volcanic activity, flooding and cosmic collisions that occurred during the time of its formation. Scientists call the rock layer formed at the time dinosaurs became extinct the Cretaceous--Paleogene boundary.
The Facts
The information scientists have learned from studying the Cretaceous-Paleogene boundary has created common ground between proponents of different dinosaur extinction theories. This common ground is found through basic facts that the most prevalent hypotheses agree upon.
It is generally agreed that the Cretaceous-Paleogene boundary is a thin layer of rock, containing clay with 30 times the normal level of iridium. Because it is extremely rare to find iridium in any layer of the Earth's crust, it is thought to be related to the dinosaurs' extinction.
It is generally accepted that there was a global climate change around the time dinosaurs became extinct. Though the cause and rate of change are debated, it is agreed that towards the end of the Mesozoic Era the climate cooled significantly.
It is also widely accepted that 66 million yeas ago, at the same time as the dinosaurs, many organisms, both marine and terrestrial, vertebrate and invertebrate, became extinct.
Though scientists agree on these facts, the major sides of the extinction debate can be classified in two camps: the "intrinsic gradualists" and the "extrinsic catastrophists."
Intrinsic Gradualists
Intrinsic gradualists believe that earthly events caused the extinction of the dinosaurs and that it occurred gradually, over a span of several million years. As of 2011, there are two prominent hypotheses: volcanism and continental drift.
The volcanism theory purports that global-scale volcanic activity, sustained for several million years, spewed immense amounts of ash and dust into the atmosphere. Eventually, the soot was enough to block sunlight from reaching the Earth's surface. This lack of sunlight caused significant atmospheric and climactic changes and dinosaurs, along with many other organisms, were unable to adapt to the harsh conditions and perished.
Some believe that continental drift, or the repositioning of the world's landmasses, made the planet uninhabitable for dinosaurs. As shifting tectonic plates pushed continents upward, oceans were redirected and global sea levels fell. According to this hypothesis, the world climate became drier and cooler. Dinosaurs were faced with ecosystems that no longer provided enough food, and gradually died out.
Extrinsic Catastrophists
The extrinsic catastrophists believe that the extinction of the dinosaurs was fairly sudden and the result of an outside, or unearthly, source.
The most widely-accepted hypothesis is the impact theory. In 1980, Luis and Walter Alverez at the University of California, Berkeley, proposed that roughly 66 million years ago, one or more asteroids hit the Earth, wreaking havoc on dinosaurs and their habitats. They maintained that the impact would have caused tsunamis, wide-spread fires, acid rain and seismic activity. They believed the heat from the asteroid's impact would have incinerated all living creatures in close proximity. The theory also claimed the impact caused atmospheric chemical changes, increases in sulfuric and nitric acid, making conditions uninhabitable for dinosaurs and many other living things.
The father and son team based this theory on the levels of iridium found in the Cretaceous--Paleogene boundary. They concluded that no terrestrial source could produce so much iridium, and so the source had to be extraterrestrial. The Cretaceous-Paleogene boundary also contained other rare minerals formed only through great temperature and pressure such as shocked quartz crystals and microscopic diamonds. After ruling out nearby supernovas and giant nebular clouds, they settled on the only source that could account for all components: an asteroid.
Other Theories
Theories about dinosaurs have passed in and out of favor since Sir Richard Owen first coined the name "Dinosauria" in 1842. Some of the less popular theories are that dinosaurs died out as a result of dinosaur-on-dinosaur wars, slaughter at the hands of extra-terrestrial beings and "paläoweltschmertz," the idea that dinosaurs just grew tired of it all, and through boredom, went extinct.
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