While it is known that a major event would have been required to bring about the extinction of dinosaurs, there are several theories about what completely killed them off. Although most scientists agree that a large asteroid did in fact hit the earth around the time of the extinction, there is some disagreement as to whether or not that is the only factor to consider regarding the dissapearance of dinosaurs. Throughout the Paleozoic and Mesozoic era's there were multiple extinction periods, but none were as devastating to the earth's population as the one that occurred sixty-five million years ago. This global extinction, referred to as the Cretaceous-Tertiary event, or the K-T event for short, gave rise to mammals and man, and changed the earth forever. The K-T event spelled the end of the dinosaur era and ushered in the age of mammals, although theories regarding the details of the widespread extinction itself are relatively new. Currently the most popular theory is that a major asteroid impact caused the extinction of the dinosaurs, but there are many other theories regarding their fate.
There are many known factors that could and have caused extinctions in the past. One of the most plausible theories regarding the extinction of dinosaurs was that there could have been a massive epidemic that spread amongst the animals. This could have been a viral or bacterial epidemic that sickened the creatures slowly, or perhaps it was terminal from the onset. It is also possible that migration brought creatures from other areas into contact with one another after long periods of having been isolated. During the cretaceous period many of the remaining dinosaurs were coming into contact with one another for the first time after having evolved in their own confined regions. Renewed contact among isolated species could have exposed all of them to illnesses and parasites that their immune systems were not equipped to handle.
Another theory is that the extinction of the dinosaurs was caused by ozone depletion. This doesn't mean that oxygen dissipated from the planet all together, but that enough of the ozone layer was effected as to eliminate the protection it offers life on earth against ultraviolet radiation from the sun. The massive tectonic activity of the ancient world, combined with the widespread volcanic activity could have reduced the ozone layer quite significantly by releasing gases into the atmosphere. Dinosaurs would have been highly susceptible to this danger, whereas mammals would be at least partially protected by their fur or hair. It is also known that many species that existed toward the end of the cretaceous period survived the K-T event initially, most notably ocean based lifeforms, though they would later be wiped out by a flood. This lends some credibility to the ozone depletion theory since marine lifeforms would not have been directly effected.
There are many legitimate threats to the earth from space, whether it be an asteroid impact whose size jeopardizes life on this planet, or an asteroid that brings with it substances that can cause illness. Even a floating cloud of gas that the earth encounters during its orbit of the sun could spell trouble. In the case of gaseous encounters, it is known that there are clouds of gas that float through space, if one of these clouds was hydrogen for example, and if that gas were ignited by the sun, it could result in temperature increases on earth. An encounter of this type could also result in ozone depletion or contamination depending upon the nature of the gaseous anomaly. There is no evidence to support this in the fossil record however, and according to most evidence the extinction was a relatively sudden event.
There is evidence to support the asteroid impact theory however, but it is most likely the size of an asteroid that would have caused extinction, not necessarily what it brought with it. Recently a crater was discovered on the Yucatan peninsula that could be the impact crater of the asteroid responsible for the K-T event. Rock found at the crater site have been dated as being about 65 million years old, correlating with the time of the dinosaurs extinction. Rare elements have also been found at the site, including alloys that could only have gotten here by way of an asteroid. Additionally, rock samples dated for the same time period contain high levels of soot and dust particles, evidence that supports an asteroid impact large enough to have caused atmospheric disturbances across the globe.
Most scientists agree that an asteroid did strike the earth 65 million years ago, but not all agree that it brought about the extinction of dinosaurs. Many argue that an asteroid impact alone couldn't account for the death of all dinosaurs, and in support of that argument is the fact that not all life disappeared. The crater found on the Yucatan peninsula is large enough to account for an asteroid six miles across. Such an impact would have sent dust into the atmosphere for years to come, which would have led to the death of much of the plant life on earth. While the impact was quite violent and would have caused instant destruction on a massive scale, animals in other places of the world far from the impact zone would later starve to death because of the secondary damage that would have been caused. Due to the large size of the dinosaurs and the amount of food that they required, the herbivores would have perished first, to be followed by the creatures that fed on them. The fact that mammals survived could be due in part to their size and their dietary requirements, though it is certain that many mammals would have been lost as well.