What killed the biggest beasts that ever walked this land? How could something live for so many millions of years (more than 160 million years), then suddenly vanish forever? What really killed the dinosaurs?
These are questions science is still trying to solve. Although we have pieces, and hunches, we truly do not have all the answers. For one thing, it was not one single event that resulted in the death of all the dinosaurs. There were several mass extinctions (at least five) that occurred at different times, marking the end of one period of time, and ushering in a new one. The final extinction event was 65 million years ago, and killed not only the dinosaurs, but most other life on the planet at that time.
There are many theories as to what caused these extinctions, below are the six most likely scenarios.
Asteroid
The most common theory is that the last extinction was caused by an Asteroid that impacted the earth in the area that is now the Gulf of Mexico. This is often referred to as the KT Asteroid. The result of a roughly 6 mile wide (10 km) asteroid hitting the earth would have caused massive problems for life on the planet. The sun would have been obscured by dust particles for months, killing plants and the animals that ate them, in turn animals who fed on those animals would have perished. The impact would have also resulted in tidal waves, acid rain, and probably forest fires. Water would have been tainted with falling particles, and survival of many animals would have come to a halt. Evidence for this has been found in the form of a layer of sediment (called the K-T layer)which has turned up all over the globe from the same time period. Perhaps other asteroids caused other earlier periods of extinction.
Climate Change
As the Cretaceous period came to a close 65 million years ago, we saw mostly tropical plants, with the later Tertiary period plant life had changed. As such it is believed that global temperature fluctuations were beginning to occur and some animals just could not keep up. Certain plants would have died, and plant eaters would have been left without food, then too, the meat eaters. Indeed plankton have been shown to have decreased in numbers around this time, clearly cutting the food supply of marine animals. One interesting note about the climate change theory is that it not only allowed for the survival of the furry mammals, but possibly the survival of a few species of dinosaurs whose bodies had been experimenting with the development of feathers. Perhaps this was enough to keep them warm enough to survive the temperature fluctuations and enable them to continue evolving possibly into birds.
Disease
If some sort of disease, or plague, were to take hold it could wipe out a species in a few years. In the case of the dinosaurs the disease would have had to be one that was not species specific, or it could have been one that destroyed plants. Fewer plants means less food for the plant eaters, with them gone, meat eaters would also die. The disease theory may be held responsible for some smaller die offs, but probably did not cause any of the major extinctions.
Oxygen Levels
During the reign of the dinosaurs oxygen levels were higher than they are today. We know this because air bubbles have been found in fossilized amber. These higher oxygen levels have been recently cited as a contributing factor for allowing the dinosaurs to grow to their massive sizes. As the continental plates broke up volcanic activity was probably responsible for changing the levels of oxygen to what they are today.
Mammals
One theory is that early mammals were responsible for eating dinosaur eggs, thus ending their ability to reproduce effectively. This theory has been all but discounted as we know some dinosaurs guarded their nests, and it does not account for the extinction of the marine life that also occurred at the time.
Volcanic Activity
Volcanic activity was at a high and surely there would have been noticeable consequences for life at the time, but could volcanoes cause enough damage to wipe out entire species? They would have to create enough debris in the air to block the sun from reaching the plants thus interrupting the food chain. This would have had to occur on a global scale and would require several volcanoes erupting within a few months of each other. Although volcanoes might have played a role this way, it is more likely they are linked to the changes in the oxygen level as mentioned above.
-Summary-
There were survivors of every mass extinction, only certain species were entirely wiped out. These surviving animals either are known today as “living fossils” or they evolved into new animals. Some examples of the survivors are turtles and snails, others who have evolved since, are sharks, and even more so, mammals. Birds are likely the result of some surviving dinosaurs in combination with natural evolution since the last big extinction.
