Pterosaur Tree Clingers and Non-Clingers

While we know of bipedal and quadrupedal pterosaur tracks left in soft mud and sand, we will never have traces of pterosaurs clinging to trees. Nevertheless, certain pterosaurs have provided clues that they did so, or could have done so, while others could not.

Four pterosaur hands with fingers of various sizes.

Figure 1. From left to right the pterosaurs MPUM 6009, Dorygnathus, Pterodaustro and Nyctosaurus, all to the same relative metacarpus length demonstrating the relative size of the free fingers. Those with larger free fingers were more adapted to cling to trees.

Tree Clinging in Pterosaurs
Tree clinging goes back to LongisquamaCosesaurus and perhaps even Huehuecuetzpalli and Lacertulus, basal tritosaur lizards that had tendril-like hind toes, like a modern Iguana. Unlike Iguana, Huehuecuetzpalli and Lacertulus had relatively smaller hands, fingers and finger claws. A quick look at the modern glider, Draco, can be instructive on this point. Giant claws may not be necessary, but long tendril-like fingers seem to help.

The pterosaur ancestor Longisquama had relatively enormous fingers tipped with trenchant claws and a bipedal body plan. So it clung to trees in a different fashion than a typical lizard — and more like a telephone lineman (or a lemur) with feet planted beneath the hips, the belly elevated off the trunk, the arms extended and the fingers wrapped around the trunk, claws dug in. Finger 4 was rotated axially and posteriorly at the carpus. Despite its great length, finger 4 was no longer involved with tree clinging, as demonstrated by the discontinuous PILs (described here).

Like their Lizard Forebearers
Early pterosaurs, like MPUM 6009 (Fig. 1) and Dorygnathus (Fig. 1) had relatively long fingers tipped with trenchant, bark-stabbing claws.  Primitively their metacarpals and fingers increased in length from 1 to 4. In Dorygnathus metacarpal 2 and 3 were subequal.

Derived Pterosaur Hands
In certain later pterosaurs the metacarpals would appear in different proportions.  Pterodaustro (Fig. 1) had relatively tiny fingers on metacarpals in which mc 1 was longer than mc2, which was longer than mc3. Digit 2 was subequal to 3 and no manual ungual was deeper than its penultimate phalanx.

A series of Nyctosaurus specimens demonstrate the reduction of all three fingers in that genus, culminating in the UNSM93000 specimen which had useless wire-like vestiges.

Needlessly Controversial
In the present supposedly “heretical” configuration the forearm was unable to pronate or supinate, thus the palmar side of the fingers faced ventrally in flight and medially with wings folded (Peters 2002). This controversy was blogged about earlier. The key here is the palmar sides of the fingers facing medially with wings folded so pterosaurs could grapple parasagittal tree trunks between their opposing hands, much like the early bird, Archaeopteryx, which had similarly elongated fingers and a forearm similarly unable to pronate and supinate. Other scientists proposed finger configurations that were permanently supinated (i.e. Bennett 2008), but these did not allow tree clinging.

Questions
Azhdarchids, even large ones, had robust fingers with deep claws. Were they found in trees? Or only on the ground?

Jeholopterus had extremely long, curved hand claws, built like surgeon’s needles. These look to be ideal for stabbing and clinging to dinosaur hide. Were these clues to its vampire lifestyle?

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.

References
Bennett SC 2008. Morphological evolution of the forelimb of pterosaurs: myology and function. Pp. 127–141 in E Buffetaut and DWE Hone eds., Flugsaurier: pterosaur papers in honour of Peter Wellnhofer. Zitteliana, B28.
Peters D 2002. A New Model for the Evolution of the Pterosaur Wing – with a twist. – Historical Biology 15: 277–301.

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