Sixty-five million years ago the dinosaurs died out along with more than 50% of other life forms on the planet. This mass extinction is so dramatic that for many years it was used to mark the boundary between the Cretaceous Period, when the last dinosaurs lived, and the Tertiary Period, when no dinosaurs remained. This is called the Cretaceous/Tertiary (or K/T) boundary, and the associated extinction is often termed the K/T extinction event.The name "Tertiary" is a holdover from the early days of geology, and many geologists now prefer the term "Paleogene" for the time period that immediately follows the Cretaceous. These scientists refer to the Cretaceous/Paleogene or K/P boundary, which represents the same moment in time as the K/T event. Since their discovery in the nineteenth century, the reason for the dinosaurs' demise has been a matter of speculation and debate. Early
paleontologists, working prior to Darwin's theory of evolution by natural selection, suggested that dinosaurs represented the remains of animals that had perished in the Biblical Flood. This explained both the fact and speed of their disappearance. But as other extinctions came to light, and Darwin's theory gained acceptance, this explanation fell out of favor.
For many decades, the fossil record of dinosaurs was poorly known. During that time it was clear that dinosaurs had gone extinct, but it was not yet understood that this extinction was relatively sudden and simultaneous with those of many other species. Only at the end of the nineteenth century did paleontologists realize that nearly all dinosaurs had gone extinct within a brief period of time at the end of the Cretaceous Period.
For most of the next century, scientists focused on explanations for how the extinction might have occurred. Most theories focused on climate change, perhaps brought on by volcanism, lowering sea level, and shifting continents. But hundreds of other theories were developed, some reasonable but others rather far-fetched (including decimation by visiting aliens, widespread dinosaur "wars", and
"paläoweltschmertz"the idea that dinosaurs just got tired and went extinct). It was often popularly thought that the evolving mammals simply ate enough of the dinosaurs' eggs to drive them to extinction.
Regardless of the details, most of these theories shared the common thought that dinosaurs were a group of animals that had reached the end of their evolutionary life. Their extinction was seen as inevitable, the product of having evolved for too long. In most extinction scenarios, the dinosaurs were simply unable to cope with competition from mammals and the changing climate, and so they all went extinct.
As dinosaur science began to alter this hypothesis, producing a new view of dinosaurs as successful and viable organisms, many of these extinction theories became less tenable. New information from fossil localities suggested that many other organisms, most unrelated to dinosaurs, had also gone extinct at the same time. New theories were required to explain these new discoveries and newly understood facts. A favored theory was that tectonically induced climate change interfered with food chains, disrupting them enough to cause widespread extinction among many different organisms.