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Engineers have revealed that the size of pterosaurs such as the pterodactyl (pictured) was limited by their ability to get airborne

Think of a pterodactyl and you may imagine a fierce flying predator soaring over volcanoes and prehistoric palms.

But the reality for some larger creatures may have been less successful.

Engineers have revealed that the size of pterosaurs was limited by their ability to get airborne, because if they grew too large and heavy, they would not have had the strength to take off and would likely have been eaten by other predators. 

The discovery of large numbers of fossil species indicates that pterosaurs were initially highly successful, but creatures with a wingspan of over 39 ft (12 metres) may have had trouble taking off, according to researchers.

The flying reptiles lived from the late Triassic to the early Cretaceaous period between 228 million and 66 million years ago.

With wingspans of around 32ft (10 metres) the largest species may have weighed as much as a quarter of a ton. They were the first vertebrates to evolve powered flight. 

A transatlantic team of experts used 3D imaging of fossils to create a computer model of a pterosaur with a 19 ft (6 metre) wingspan. This model was then scaled up to create enlarged models with 29 ft (9 metre) and 39 ft (12 metre) wingspans.

They were used to estimate the wing strength, flexibility, flying speed and power required for flight in massive pterosaurs to see if textbook images of enormous beasts soaring through the skies were scientifically possible.

The results showed that even the largest pterosaur would have been able to sustain flight by using air currents to glide.

It would also have been able to slow down sufficiently to make a safe landing because its wings were formed from a flexible membrane.

With wingspans of around 32ft (10 metres) the largest species may have weighed as much as a quarter of a ton. They were the first vertebrates to evolve powered flight. This illustration shows how large the animals were, compared to a giraffe and human

Take-off, on the other hand, proved an entirely greater challenge in the computer experiments.

Unlike modern birds, pterosaur anatomy suggests that they used both their arms and legs to push themselves off the ground during take-off, in a manoeuvre known as the 'quadrupedal launch'.

However, the results suggest that larger-than-usual specimens, weighing towards half a ton, would have struggled to get off the ground.

Once wingspans approached 39ft (12 metres), the push-off force required to get the model off the ground was too great. 

Bristol University mechanical engineer Colin Palmer said: 'Getting into the air ultimately limited pterosaur size.

'Even with their unique four legged launch technique, the iron laws of physics eventually caught up with these all time giants of the cretaceous skies.'

The team will present their results at the upcoming Society of Vertebrate Paleontology meeting in Berlin.

Mr Palmer, who has worked on sailing boats and windmills, said: 'It has been fascinating to apply an engineering approach to understanding biological systems.'

...AND VULTURES HAVE THE SAME PROBLEM 

California condors (pictured) can become too heavy to take off from the ground and have to find a ledge

Vultures have weak legs and feet along with bulky bodies.

While this is no disadvantage in the air, it makes it difficult for the birds to get airborne from the ground.

To overcome this problem, the heavy birds run downhill to pick up speed in order to launch, or jump off cliffs and high ledges to take off.

Once they are airborne, their large wings enable them to glide and soar to spot prey.

Vultures soar slowly and stably. They average 30 mph in flight and can get up to 40mph. They take about 16 seconds to complete a circle in soaring flight.

When they spot a carcass, the large birds sometimes eat too much, which increases their weight further, meaning they can't take off.

The California condor, for example, can survive up to two weeks without a meal. So when it feasts, it eats until it's full, storing up to three lbs of meat in a part of its oesophagus, not to mention its stomach. 

If it overdoes it, the bird makes itself sick to lighten the load and get airborne again.

The flying reptiles lived from the late Triassic to the early Cretaceaous period between 228 million and 66 million years ago. Pictured is a fossil of one of the best preserved and most complete pterodactyls discovered