Global palaeogeographic reconstruction of the Earth at the time of the Cretaceous-Tertiary mass extinction. Credit: Dr Ron Blakey, NAU Geology.
Death of a dynasty - the end of the Cretaceaous
At the end of the Cretaceous Period, 65 million years ago, all the dinosaurs died out. Why this happened is one of the most frequently asked - and intriguing - questions in palaeontology.
There have been many different ideas put forward to explain why the dinosuars died out. The two most likely are that their habitat slowly changed, and that a meteor impact triggered their extinction.
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 reduced dinosaur and shallow water marine animal habitats.
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 was deposited at the end of the Cretaceous and 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.
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 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 such as frogs, which we know are environmentally sensitive, 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 or we do not have good enough age resolution. It is likely that as China and other countries outside of Europe and the US are studied more intensively 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.