The flying cousins of the dinosaurs, one of which seized Raquel Welch in the 1966 film One Million Years BC, have long been known to have dwarfed all modern birds. Fossils discovered recently in Mexico, however, suggest that the very largest were even bigger than had been thought possible.

The size of a set of newly identified pterosaur footprints indicates that some of the airborne behemoths had wings that stretched at least 59ft (18m) from tip to tip, and its true dimensions may have been larger still.

Previous estimates had put the biggest pterosaurs, of a species known as Quetzalcoatlus after an Aztec god, at a span of 38ft — as wide as the wings of a Spitfire, but a mere pigeon in comparison with the new beast. Among modern fighter aircraft, the F15’s wingspan is 42ft and the Typhoon’s 34ft.

Fossilised bones found in Israel, Jordan, Brazil and Romania have also provided evidence of pterosaurs measuring 42ft to 46ft across, suggesting that truly huge individuals may not have been altogether rare.

David Martill, of the University of Portsmouth, a leading authority on pterosaurs, told the BA Science Festival in Dublin that the footprints belonged to the largest flying animal yet discovered.

“Quetzalcoatlus was an animal the size of an aeroplane, with each wing five metres long, but we now have a lot of evidence that these things were even bigger than that,” he said. “The largest may have been nearly twice that size. Experts have come up with estimates that are in excess of 18 metres.”

The fossils were discovered in Mexico by Eberhard “Dino” Frey, of the State Natural History Museum in Karlsruhe, Germany, but details have yet to be published. It is not yet known whether they belong to a new species or a large example of a known one.

Pterosaurs were not dinosaurs, but they lived during the same Triassic, Jurassic and Cretaceous periods from about 220 million years ago to the mass extinction that took place 65 million years ago. The vast flying lizards, of which the most famous are the pterodactyl and the crested pteranodon, lived all over the world.

Dr Martill’s research has shed light on how pterosaurs managed to fly, despite their great size. Their secret was a remarkable, almost paper-thin membrane stretched tightly between light, hollow fore and hind limb bones. It gave them highly aerodynamic wings more similar to those of a modern bat than of a bird.

“This membrane was a very, very sophisticated structure,” Dr Martill said. “It was not just a piece of skin. It was a tissue that enabled thermoregulation by acting as a heat exchanger, and was very, very thin — it was not thick and scaly like a crocodile’s skin.”

He said that the bone was phenomenally structured, giving very low weight.

Unlike those of modern bats or birds, pterosaur wings were not attached to the upper body but to its lower part, rather as the wings of a jumbo jet protrude from its belly. This would have improved their manoevrability, as would the reptiles’ joint configuration.

The creatures were probably capable of powered flight and not just gliding, but would have exploited thermal currents in the same way as large, modern birds such as albatrosses and condors. They may have taken off by jumping — their pelvises show structural similarities to those of frogs — and most probably spent most of their lives in the air.

Three fossilised embryos have shown that pterosaur young were born with wings in the same proportions as adults, indicating that they started flying soon after they hatched. Modern birds do not fly until almost fully grown.

Dr Martill said: “If you were an aviation designer looking at it, you would say how can you build up from the equivalent of a Cessna to a jumbo jet while flying all the time?”

Courtship, he said, would have been conducted “with gusto”. Many pterosaurs had crests that would have impaired aerodynamic performance, making it probable that these were used for sexual signalling.

Pterosaurs are known to have been carnivorous, but no stomach contents have ever been discovered. Most species are thought to have eaten mainly fish, though some show adaptations for catching insects and skimming the surface of water for grubs.

Dr Martill said one possible explanation for their vast dimensions is that they simply failed to stop growing at adulthood. “If they just grew and grew, the oldest individuals would get very large indeed,” he said. “It might be quite rare to find these very large ones, as few animals would have lived for the very long periods needed to get that big.”

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