The most common theory for the demise of the dinosaurs is that a large asteroid struck Chicxulub in Mexico, forming a 240 kilometre wide crater. The resulting atmospheric debris blocked out the sun creating a 'nuclear winter', which killed plants, then plant-eaters and, finally, meat-eaters.
Those dismissing the theory claim that smaller reptiles like turtles, alligators and crocodiles would also have been harmed. Others suggest that a freshwater, aquatic lifestyle reduced the threat. There are also arguments that the crater simply wasn't large enough to have caused such devastation. Either way, the crater-like shape of the area, the layer rich in iridium and the age of the rocks all lend weight to the theory.
It has been put forward that Western India saw huge volcanic eruptions which unleashed enormous quantities of atmospheric ash. This blocked out the sun, caused widespread climatic change and wiped out the dinosaurs.
Deadly iridium in the Earth's K-T (Cretaceous-Tertiary) boundary could conceivably have been caused by lava eruptions. However, scientists have widely agreed that 65 million years ago there was a global fall in temperature and that a volcanic eruption would have had the opposite effect.
Flowering plants known as angiosperms arose around 65 million years ago, which could have created a mass allergic reaction among dinosaurs.
This isn't very likely. Flowering plants were around for millions of years before the dinosaurs disappeared and we have no evidence whatsoever that their pollen was capable of killing the dinosaurs. Also, hay fever wouldn't explain the massive marine extinction as there have never been any marine angiosperms.
Could the dinosaurs have grown so big and heavy that they couldn't move, defend themselves or withstand their own weight?
Simply put, it is an evolutionary impossibility. Besides, most dinosaurs around by the end of the Cretaceous period were of medium or small size.
Perhaps newly emerging mammals began to out-compete the dinosaurs for food and space, ultimately driving them to extinction?
Perhaps not. Dinosaurs and mammals evolved during a similar time period and may have sometimes come into direct competition, but it is unlikely. There were also no marine mammals at the time, so they couldn't be blamed for wiping out marine reptiles. The belief that mammals ate dinosaur eggs, driving them to extinction is also unsupported. Predators very rarely wipe out their prey. It is unwise to eradicate your food source - you wouldn't last very long once its gone.
Towards the end of the Cretaceous period, land bridges opened up between places such as Asia and North America, allowing species to migrate. Could this have led to a spread of disease to which other animals were not resistant?
In humans, Native Americans were nearly wiped out by smallpox carrying European invaders. But this wouldn't have wiped out every single species of dinosaur, and the argument fails to explain the disappearance of a multitude of species that lived in the sea.
An Ice Age?
Ice ages have occurred throughout Earth's history, with the last one ending about 10,000 years ago. Could this be what happened? A very severe ice age could have altered climates and froze waters to the extent that dinosaurs were unable to weather the conditions, and slowly died out.
This theory has been largely discarded for one simple reason: scientists have not found any evidence of an ice age occurring during the life of the dinosaurs. Climate change could have come about because Earth somehow tilted over about 23.5 degrees around 65 million years ago. But what could have caused that? An asteroid impact? Debris from a passing comet? Scientists have yet to unequivocally answer this question.
This one is a little bit more radical. Could a massive comet travels close to Earth every 26 million years, bringing with it radioactive debris that collides with our planet, disrupts weather patterns and wipes out entire ecosystems even taking the dinosaurs with it 65 million years ago?
We have no concrete evidence of extra-terrestrial events occurring at the point of dinosaur extinction. Furthermore, dinosaur remains show no evidence of radiation damage, pretty much ruling out this suggestion.
Maybe dinosaurs were never wiped out at all? Could they still be alive and amongst us today? Not Dale Russell's slightly ridiculous hypothesis, but as birds!
The birds we see flapping, running and swimming around us evolved from an ancestral theropod dinosaur. Palaeontologists have uncovered a vast array of evidence supporting this, with feathers and other anatomical features of fossils confirming this.
However, this does not explain the disappearance of all the non-avian theropod and sauropod dinosaurs. It may have been that the avian dinosaurs were just better equipped to survive the extinction event.