How Did Pterosaurs Take Off? A Problem of Getting Very Large Animals Airborne
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The largest flying animals had wingspans comparable to a small aircraft, and how they left the ground was genuinely unclear for a long time. The favoured answer has them launching from four limbs rather than two, which changes the numbers entirely.
The problem
A flying animal must generate enough speed and lift to become airborne, and the power required rises steeply with size while the power available from muscle rises more slowly, which sets an upper limit on flying animals. The largest pterosaurs had wingspans of ten metres or more and estimated masses well beyond any flying bird, which put them past the limit calculated on the assumption that they took off like birds, by running or by leaping with the hind legs and then flapping. Those calculations suggested the animals could not have flown at all, which is a conclusion that should prompt examining the assumptions rather than the fossils, since the animals plainly existed and were plainly built for flight.
The quadrupedal answer
The alternative uses the forelimbs to launch and the evidence for it is several:
- •Pterosaurs walked on all fours, which trackways establish directly, so the forelimbs were load-bearing on the ground
- •The forelimb muscles powering flight were far larger than the hind limb muscles, so the power is in the front
- •Launching by pushing off with the forelimbs uses those muscles for the initial leap as well as for flapping
- •Modelling the vault produces enough velocity and height to get a very large animal airborne
- •The shoulder and forelimb bones are built to take the compressive load such a launch requires
- •Bird launch is bipedal because birds walk on two legs and their forelimbs are wings only, which is a different arrangement
Why it changes the limit
The size limit on flight is set by launch rather than by sustained flying, since an animal that cannot get airborne is grounded regardless of how efficiently it could soar. Using the larger forelimb muscles for the leap raises the available power substantially over a bipedal launch, which moves the limit upward and brings the largest known forms inside it. That reasoning also explains why the largest flying birds are far smaller, since a bird launching bipedally is constrained by its legs, and it predicts that very large birds should have difficulty taking off, which is observed in the heaviest flying species. The argument is not universally accepted, with alternative reconstructions proposing lower masses for the animals or different launch mechanics, and the mass estimates themselves remain uncertain enough to matter.
What they were made of
The skeleton was built for lightness to an extreme degree and the details explain how such large animals flew at all. The bones were hollow with walls a fraction of a millimetre thick in places, braced internally by fine struts arranged along the lines of stress, which produces enormous strength for very little material and is the arrangement engineers use in aircraft structures. Air sacs from the respiratory system invaded those bones as they do in birds. The skull was largely open framework rather than solid bone. Estimates of total skeletal mass for the largest forms are startlingly low relative to the wingspan. The consequence is that mass estimates for these animals have varied widely between studies, since the relationship between size and mass is unlike anything living, and the launch argument depends on which estimates are used.
How they flew once up
Sustained flight for the largest forms was probably mostly soaring rather than flapping. The wing shape and loading of the giant species resemble those of modern soaring birds, and the energetic cost of continuous flapping at that size would be prohibitive. That implies dependence on rising air, either thermals over land or the updraughts along coastlines and waves that modern seabirds use, which constrains where such animals could live and travel. Smaller pterosaurs were capable flappers and the group covered an enormous range of sizes and lifestyles over its long history, from forms the size of a small bird to the giants. The wing membrane itself was more sophisticated than early reconstructions assumed, containing stiffening fibres and muscle that allowed its shape to be controlled actively rather than being a passive sheet.
The takeaway
Power required for flight rises faster with size than muscle can supply, and calculations assuming bird-style bipedal launch put the largest pterosaurs beyond the limit. Launching by vaulting off the forelimbs uses the far larger flight muscles for the leap, which raises the limit enough to include them. Trackways confirm they walked on all fours, which is what makes the arrangement available.