A strange little bone called the pteroid protruded from the wrists of pterosaurs, the flying reptiles of the Mesozoic. It was sort of like a thumb,but extended from the base of the wrist and pointed in the opposite direction- towards the body. Or at least that's what most palaeontologists believed,because that was the orientation it always took in fossilised animals. Now,Matthew Wilkinson, David Unwin and Charles Ellington argue that it pointed straight forward - and may have been instrumental in allowing the largest pterosaurs to fly at all.
The main part of the pterosaur wing was a thin membrane stretching between the fore- and hindlimbs and supported in front by the arm with a hugely elongated fourth finger. In front of the arm, though, was a smaller forewing called the propatagium, supported by the pteroid bone. If the pteroid pointed forward, the propatagium would have formed a broad leading edge flap, with potentially dramatic aerodynamic consequences, Wilkinson thought.
First, the researchers examined fossilised pterosaurs to make sure that it was possible for the pteroid bone to swing into a forward-pointing orientation. Facets in the joint where the small bone articulated made it clear that it could swing from pointing towards the body to pointing forwards,furling and extending the forewing.
Then they built three models of the wing with different pteroid positions -fully extended forward, partially furled, and completely absent - and covered them with ripstop nylon to approximate the wing membrane. When they tested the wing models in the University of Cambridge wind tunnel, they found that the extended pteriod conferred a dramatic advantage. The broad forewing increased maximum lift forces by about 60% and, when the wing was nearly parallel with the flow, lift forces jumped from five times the drag force with the forewing furled to about 18 times the drag with forewing extended. As the wing angled up to become more perpendicular to the flow, the pteroid continued to help -it prevented the wing from stalling out, which would cause the lift to drop to zero. Surprisingly, the forewing only seemed to help when it was fully extended; the model with the partially furled forewing performed no better than the model with no forewing at all.
In many ways, the forward-pointing pteroid and broad forewing increased the pterosaur wing's performance in the same way that a leading edge flap augments an airplane wing. So the results weren't unexpected - although the amount the forewing helped was impressive. But some researchers had previously rejected the idea of a forward-pointing pteroid, arguing that the puny bone couldn't have supported the stresses that a broad forewing would have imposed on it. Wilkinson waves this objection away, noting that the tip of the fourth digit -which supports the main part of the wing - is as slender as the pteroid and must have supported much higher forces.
The pteroid's remarkable aerodynamic performance sheds light on how the largest pterosaurs took off. These giants had wingspans exceeding 10 m - as big as a small plane. But without propellers or jet engines, how did they take off? Some clearly jumped off cliffs. But other specimens have been found with fossilised footprints, suggesting that they took off from flat ground. Wilkinson's results suggest that giant pterosaurs might have merely needed to spread their wings while facing into a moderate breeze to take off.