Extinction survivors were bird brains

20 January 2009

Birds survived the mass extinction that wiped out the dinosaurs because of their larger brains.

The Cretaceous–Tertiary mass extinction 65 million years ago may have wiped out the dinosaurs, but those that survived – the ancestors of today’s birds – may have done so because of their bird brains.

Analysis of computer tomography (CT) scans of fossilised bird skulls shows they had a more developed, larger brain than previously thought.

‘Birds today are the direct descendents of the Cretaceous extinction survivors, and they went on to become one of the most successful and diverse groups on the planet,’ says Natural History Museum palaeontologist (fossil expert), Dr Stig Walsh.

‘There were other flying animals around, such as pterosaurs and older groups of birds,’ says Dr Walsh, ‘but we’ve not really known why the ancestors of the birds we see today survived the extinction event and the others did not. It has been a great puzzle for us – until now.’

A larger and more complex brain may have given them a competitive advantage over the other more ancient birds and pterosaurs, helping them to better adapt when the environment changed after the mass extinction event.

Larger brains in living birds

Species of living birds that have larger brains are more likely to live in more socially complex groups and exhibit more complex and flexible behaviour than those with smaller brains.

For instance, members of the crow family have large brains, and some make and use tools, inventing cunning ways to find food.

Previous research has suggested birds with larger brains are more likely to survive if introduced to new environments than those with smaller brains.

These results suggest that this kind of behavioural flexibility was already a characteristic of the ancestors of modern birds before the Cretaceous–Tertiary extinction event.

Surviving the aftermath

‘In the aftermath of the extinction event, life must have been especially challenging,’ says Dr Walsh. 

‘Birds that were not able to adapt to rapidly changing environments and food availability did not survive, whereas the flexible behaviour of the large-brained individuals would have allowed them to think their way around the problem.’

Fossil skulls from Isle of Sheppey

The team studied 2 fossil seabirds, found in the London Clay Formation on the Isle of Sheppey in England.

These deposits are famous for the huge range of preserved fossils, from molluscs, fishes, birds and even mammals.

These ancient creatures lived in the warm, semi-tropical conditions of southern England 55 million years ago.

CT scans of seabirds
CT scan of skull of ancient seabird

CT scan of skull of ancient seabird - the brain is shown in blue and the inner ear in red

Natural History Museum palaeontologist and team leader, Dr Angela Milner explains how they worked out the brain size. ‘The shape of the brain is imprinted on the inside of the braincase of birds and represents a reasonably accurate approximation of the original shape and volume of the brain’.

‘Using CT analysis, we were able to create a virtual cast of this cavity so that the shape and detail of the brain and its nerves could be analysed.’ You can see a virtual cast of a crow at the top of the page.

Ancient seabirds

The 2 specimens used in the study are from the Museum’s vast collection.

Fossil of ancient seabird Odontopteryx toliapica

Fossil of ancient seabird Odontopteryx toliapica used in the bird brain study

Odontopteryx toliapica belonged to an extinct group of large, gliding seabirds called Odontopterygiformes, or bony-toothed birds. The largest had a wing span of up to 7m.

Prophaethon shrubsolei was an extinct relative of the modern tropic birds that live in tropical areas around the world.

Unexpected results

‘We did not expect the brains of Odontopteryx toliapica and Prophaethon shrubsolei to be so much like those of living birds,’ says Dr Walsh.

‘The brain of the oldest-known flying bird, Archaeopteryx, is very bird-like, but not as large as in the fossil seabirds or living bird species.’

‘The parts of the brain that control sight, flight and high-level functions, including the ability to learn and remember information, turned out to be every bit as expanded in the 55-million-year-old fossils as they are in living species.’

‘This proves that the avian brain was already rather modern in appearance and organisation.’

 

This research is published in the Linnean Society’s Zoological Journal of the Linnean Society