By Jonathan Amos
BBC News science reporter, Dublin
Scientists are only now starting to recognise the astonishing size reached by pterosaurs, the flying reptiles that lived at the time of the dinosaurs.
New discoveries in the Americas suggest some had wingspans of 18m (60ft).
But there was nothing ugly about the way they moved through the air, according to expert Dr David Martill, of the University of Portsmouth.
Their ability to utilise air currents, thermals and ground effects would astonish aeroplane designers, he said.
"Pterosaurs were beautifully engineered," he told BBC News.
"Their skeletons were exceedingly light: their bones were very thin and hollow, and those hollows were filled with an air-sack system. They'd also got rid of their reptilian scales and their wing membrane was very, very thin.
"All this meant there wasn't that much weight to get off the ground, and so they probably flew really rather well," the researcher said.
The oldest pterosaur fossils date back 220 million years and scientists have now identified several different forms - some with teeth, some without; and some sporting elaborate head crests.
With their membranous wings attached to their legs, there was something bat-like about them, and their long beaks look like some bird species - but scientists stress they have no line to any living creatures.
Indeed, there is still great debate about where exactly they should be placed in the evolution of life forms on Earth. Dr Martill told the British Association's Festival of Science in Dublin that new discoveries would help solve this riddle - and perhaps reveal just how big these beasts managed to grow.
Pterosaur trackways recently found in Mexico suggest the animals could achieve a wingspan of 18m. There are also Romanian and Brazilian fossils from creatures that reached 13 or 14m (42-45ft) across.
Compare this to today's biggest flying bird, the wandering albatross, which has a wingspan of about 3.5m (11.5ft).
"One of the reasons they were so big may have been because they just kept on growing," speculated Dr Martill.
"We get to teenage years and we stop; but if a pterosaur kept on growing then the older it got, the bigger it got. They would be rare as big ones, though, because the older you get, the more chance you have of being eaten or being involved in an accident."
There is evidence from rare fossil eggs containing pterosaur embryos which suggests the creatures could fly soon after hatching.
If this was the case, scientists say, it was a remarkable achievement because the wings would have had to have grown from just a few tens of centimetres in length to several metres without interrupting the animals flying capability.
"The equivalent of an aircraft engineer trying to convert a Eurofighter into a jumbo jet while it was still flying," enthused Dr Marthill.