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Let's fix those pterosaur hands!

Thanks to Mike Habib, I finally got a chance to read Chris Bennett's paper on pterosaur myology, based on two large pterodactyloids, Anhanguera and Pteranodon.

Morphological evolution of the wing of pterosaurs:
myology and function  Zitteliana B 28 127 - 141

Yes, I have comments, but today they have nothing to do with myology. It's the architecture, the organization of the skeleton according to Bennett, that draws criticism.

In Section 3. Reconstruction of the forelimb skeleton, Bennett described the problems with the Anhanguera specimens AMNH 22552 and 22555 upon which his reconstructions are based. He reports, "Unfortunately, the preserved positions of the metacarpals and digits in the concretions were neither described or illustrated, so there is no evidence which digit articulated with the longest metacarpal." Bennett assumes, correctly IMHO, that it was III, but he also assumes it was ventral to Mc I and II. He notes McI-III are usually found closely appressed, or "bound" to the anterior surface of the big Mc IV in large pterodactyloids and interprets exceptions as resulting from gravity pulling the upper metacarpals away from IV.

Bennett makes no mention of the orientation of these elements in smaller pterosaurs, so I will. In smaller taxa, with much less size disparity between the metacarpals, they all line up like they do in all other tetrapods, palms down, with the exception that the palmar side of mc IV is rotated or torsioned (not sure what is happening proximally) to the posterior for folding, where the wing membrane emerges. So what is happening in the big pterosaurs? When the small metacarpals are back to back with mc IV they get there by drifting there. Bound side by side, together Mc I-III get pushed by currents back against an immovable object, the wall of mcIV. As a unit Mc I-III rotate on the axis hinge between Mc III and Mc IV. The natural orientation is still palms down for McI-III. Gravity doesn't dislodge them from Mc IV. In examples I've seen, any "bonding" can be fixed by removing matrix. In another example metacarpals I-III were found elevated to about 45º, frozen in time at that angle.

In the standard tetrapod configuration, with neither supination nor pronation, the palms and claws can face each other for tree grappling. When pressed down against the substrate the fingers splay laterally. I haven't studied why digit III is oriented 30 degrees from directly posterior, but it could be that the medial side is pressing the substrate. Lizards do that.

In all tetrapod hands and fingers, a set of dorsal extensors pulls them back and a set of ventral flexors curls them under. Now let's imagine Bennett is right and Mc I-III are "bound," to use Bennett's term, to Mc IV. If the metacarpals are back-to-back, where do the extensors go? If there is a small channel for them to operate in? If so, then how can they be "bound?" Bennett is asking for the top of the pterosaur's hand to fold in half and glue itself together back-to- back.

A more parsimonious answer that lets extensors be extensors and do what extensors do is to build larger pterosaur hands like small pterosaur hands are built, without supinating the metacarpals.

Under section 5.1 "Brachial muscles," Bennett illustrates in figure 3 an Anhanguera proximal forelimb in dorsal and ventral views. He makes one inadvertent error when he illustrates digit III dorsal to II and I. Well, that happens. Elsewhere Bennett illustrates and describes digit III ventral to II and I.

The error that has broader ramifications is in the antebrachium. Bennett reports the antebrachium of pterosaurs was supinated, and with it metacarpals I-III were oriented anteriorly in flight. We humans can duplicate this configuration by lying belly down on a flat surface, elbows out in the flight position and rotating our hands to the thumbs up position. In pterosaurs the thumb is not rotated, but would have flexed like fingers II and III, toward the anterior. Note the position of the distal radius, dorsal to the ulna. That's the supinated position. In Bennett's figure 3, he draws the distal radius anterior to the ulna. That's the neutral position. Pronated is thumbs down, palms back, or the position nearly all quadrupedal tetrapods use for walking. So Bennett doesn't supinate the antebrachium, as he reports. He supinates the metacarpals. He bases all of his myological connections on a 90º error.

I compared Romer's myology of a lizard with Bennett's myology of a pterosaur and noted several inconsistencies all due to the supination problem. With everything out of whack by 90º, a new understanding of the skeletal system is needed before a discussion of the myology can begin.

In Bennett's "Discussion" section he illustrates in figure 6 a hypothetical protopterosaur in various stages of developing a wing. Each example has the limb at full abduction, unable to locomote except by gliding. In A his protopterosaur has the standard palms down, non- supinated configuration of other tetrapods. In B the antebrachium supinates and rotates the hand palm forward. Here Bennett neglects to inform the reader that his little tetrapod is now walking on the lateral edge of its hand (no wonder digit V disappears!). Bennett also neglects to inform the reader that his little tetrapod can no longer grip medial objects by humeral adduction, only by rotating the elbows in front of the shoulders. If it tries to adduct the humerus it can only beg with palms lifted upward (supinated). In Bennett's configuration he reports the range of motion (ROM) is radically influenced by a multigenerational problem with banging (or the fear of banging) into tree trunks that, over time, changes the ROM from flexing to hyperextending and losing the ungual. I'm not exaggerating. Quoting Bennett: "3) permitting digit IV to swing posteriorly in flight would reduce the chance of damage to the wing skeleton and undesirable yaw when the wingtip struck an immoveable object."

Is that how evolution works? Really?

I know there are several pterosaur workers who think Chris is right on. Hopefully someone will come to his defense.

Anyway, a better solution to the origin of pterosaur flight can't be far off.

David Peters
St. Louis

cc: SCB