Earlier here and elsewhere in this blog we looked at the various mistakes and oversights promoted by Mark Witton (2013) in his new book, “Pterosaurs.” Today we’ll look at the configuration of the three free fingers and the many problems with Witton’s proposed configuration (Fig. 1).
Witton (2013) promotes the traditional idea that the dorsal surfaces of the three free fingers were appressed to the anterior surface (the former dorsal surface) of the axially rotated metacarpal 4. This means the free fingers would be oriented palmar side anterior during flight (Fig. 1) and extended palmar side ventro-posterior during terrestrial locomotion. Such a configuration allows no room for the extensor tendons, especially the big one for the wing finger (Fig. 2). Moreover, posterior fingers do not match known pterosaur hand ichnites.
The actual orientation and configuration of the three fee fingers is shown below (Fig. 2).
The actual configuration is supported by evidence.
The present configuration provides room for the extensor tendons and permits the extensor tendon process to move unimpeded whenever fully extended in flight. With the free fingers facing palmar side down, like all other tetrapods, during terrestrial locomotion the fingers would extend laterally, matching ichnites. During tree clinging the fingers would grapple medially rather than, as Witton proposes, opening anteriorly, in a “begging” configuration.
The metacarpal “drawbridge”
The connection between metacarpals 3 and 4 produces a hinge. That’s why the metacarpus sometimes swings up during taphonomy to give the impression of the configuration that Witton proposes, as if the “drawbridge” was raised.
So, where did Witton get his configuration?
From Bennett (2008), and this Santanadactylus specimen (Fig. 4, ignoring, apparently all others).
The Santandactylus specimen preserves metacarpals 1-3 raised, like a drawbridge to about 45 degrees, but this is a taphonomic artifact. Bennett’s (2008) configuration further raised the metacarpus to appress against metacarpal 4, palmar side anterior. Peters’s (2002) configuration lowers the metacarpus on its hinge, palmar side down. This provides room for the large extensor tendon and the extensor tendon process of the wing finger when it is fully extended in flight. This also matches all undisturbed pterosaur fossils (Fig. 2).
I’m hearing that my notes hear are not swaying the experts. Please see the conversations below and check out this link to pterosaur finger flexion.
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.
Witton M. 2013. Pterosaurs. Princeton University Press. 291 pages.