According to a new computer model, pterosaurs (more commonly known as pterodactyls) had no trouble flying despite their massive size. Taking off, however, posed a big problem — and it probably kept these ancient reptiles from growing any bigger.

Pterosaurs had wingspans of up to 35 feet, and the largest of them may have weighed a quarter of a ton. That's a good 10 feet larger across than the largest known flying bird to have ever lived. Those massive birds couldn't even fly by flapping their wings — their bodies were so heavy they had to glide. Some researchers have argued that the largest Pterosaurs must have used similar hang-glider-like tricks, or not flown at all.

But now we know that pterosaurs simply flew differently than birds do. Instead of running and flapping their wings, the pterosaurs used all four limbs to launch themselves up into the air.

In a study presented today at the Society of Vertebrate Paleontology meeting in Berlin, Colin Palmer (a mechanical engineer and doctoral student at Bristol University) and Mike Habib (a paleontologist at the University of Southern California) combined their areas of expertise to put this unique method of flight to the test.

The researchers worked with a pterosaur model created using 3-D scans of available fossils, scaling the creature to different sizes. They found that the creature could take off using the proposed four-limbed method, and that its flexible membrane-covered wings allowed it to modulate speed, slowing down enough to land safely.

But while a model with a wingspan of nearly 40 feet (larger than any known real-life pterosaur) could still stay airborne, it couldn't actually get itself off the ground. That four-limbed takeoff may have done some serious heavy lifting, but it had limits.

"Getting into the air ultimately limited pterosaur size," Palmer said in a statement. "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."