In the past, pterosaurs were usually pictured hanging from cliffs (remember Pteranodon in Fantasia?). Smaller pterosaurs were usually illustrated hanging beneath branches (Wellnhofer 1991) by their feet alone or inverted quadrupedal. Today we’ll start a short series on pterosaurs and trees demonstrating how some pterosaurs (not all) could interact with a tree trunk. If anyone finds this convergent with Archaeopteryx and kin, you’re probably right.
Basal pterosaurs had long fingers with trenchant claws pointed medially when the wings were folded (Fig. 1), as in basal birds. These claws were likely used to grapple tree trunks, clinging to them in the manner of lemurs and other primates (Peters 2002), arms on both sides of the trunk. In that configuration the feet were planted side-by-side beneath the pelvis, toes pointing anteriorly to antero-laterally with the dorsal surface of digit 5 putting extension pressure on the trunk, which enabled the toe claws to dig just a little deeper, much like a church key can opener.
A Chance to Show Off
In that configuration the wing finger would have been free to fully open as it would have been set at a tangent to the circumference of the tree trunk, no matter the diameter (Peters 2002, Fig. 2). Opening and closing the large wing fingers would have created a large display device, much larger than any anole dewlap, but serving the same purpose — finding a mate (Peters 2002), which was the original purpose of wings on predecessors like Longiquama.
The large pectoral muscles would have provided sufficient adduction power to enable clinging and vertical walking.
Landing on tree trunks likely drove the first major change in pterosaurs, once they became volant. That forged the evolution of a longer forelimb. The most primitive pterosaur, MPUM 6009, had relatively longer legs and shorter arms (like its phylogenetic predecessor, Longisquama), but virtually all later pterosaurs had longer forelimbs (see Raeticodactylus for an example).
While some pterosaurs, perhaps anurognathids, were able to find insects on trees, it appears that most pterosaurs found trees a safe haven and a good take-off point. Unless facing down (which appears somewhat doubtful for the larger ones because they were unable to turn their feet backward in the manner of lizards, squirrels and bats, but they were able to point their feet laterally), tree-climbing pterosaurs would have taken off by launching themselves backwards, aided by gravity, twisting quickly to a normal flight configuration, like a woodpecker might do.
Due to a relatively stiff neck, a pterosaur would likely be taking a very close look at the tree trunk while grappling it. Certainly some left or right was possible, but more than 90 degrees would have been unlikely. Pterosaurs could have slept in this position, their forelimbs prevented from overextending by their joints and the propatagium. Since not all trees are vertical, many pterosaurs could have adopted this walking configuration on unobstructed horizontal to diagonal branches and fallen trunks.
As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.
Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.
Peters D 2002. A New Model for the Evolution of the Pterosaur Wing – with a twist. Historical Biology 15: 277-301.
Wellnhofer P 1991. The Illustrated Encyclopedia of Pterosaurs. London, Salamander Books, Limited: 1-192.