Dinosaurs - Chronology - 65 million years ago

At the end of the Cretaceous Period, 65 million years ago, all the dinosaurs died out. The question of why the dinosaurs went extinct is one of the most frequently asked questions to all dinosaur palaeontologists. There have been many different ideas put forward as an explanation from the serious to the ridiculous. Probably the two most likely are: the regression and habitat loss hypothesis (gradualistic) or, the extraterrestrial impact hypothesis (catastrophic). See FAQ section.

Gradualist Theory
The gradualist hypothesis points to declines in the numbers and diversity of different groups of land and marine animals.

It suggests that the extinction of these groups was due to climate change.The climate at the end of the Cretaceous was cooling - and a fall in sea level reducing dinosaurian and shallow water marine animal habitats

Impact Theory
The impact hypothesis gets a lot of press coverage because it is spectacular. There is good geophysical evidence for the occurrence of an asteroid impact at the end of the Cretaceous.

A band of clay rich in the mineral iridium and deposited at the end of the Cretaceous has been found at many places in the world; this mineral is rare on earth but more common in meteorites.

It has been suggested that the impact would have triggered a nuclear winter scenario that would have caused the death of the dinosaurs as well as the pterosaurs, several families of birds and mammals and also marine animals such as the plesiosaurs and ammonites.

Volcano
In addition at the end of the Cretaceous there were a lot of volcanic eruptions, at least in some parts of the world.

The Deccan Traps, huge flood basalts, were deposited at this time, and the dust and gases erupted at the same time would have had caused environmental changes over a wide area.

Will we ever know?
Unfortunately while both these hypotheses are plausible and they can both explain how many animals went extinct, neither can explain why certain animals died out while others survived. Why did the dinosaurs which were so successful die out while other animals, which we know are environmentally sensitive, such as frogs, survive.

Although it is usually assumed that the dinosaurs all went extinct all at the same time all over the world, the truth of the matter is that we only have high resolution data for North America. In other parts of the world there is either no terrestrial record (as in the UK) or we do not have good enough age resolution (as in Mongolia). It is likely that as China, and other countries outside of Europe and the US are studied more intensively that we will be able to gather more data and build up a more comprehensive picture of what was going on in the world at the end of the Cretaceous period.