When fingers bend backwards while walking

Primitively tetrapod fingers formed an anterior arc, extending forwards, supporting the body and helping to provide thrust to each step. In the majority of terrestrial tetrapods, this remains true, even if the fingers shorten, hyperextend and flex to become, for instance, complex cat paws.

Chalicothere220

Figure 1. Anisodon (formerly Chalicotherium). A knuckle-walker. Click for enlargement and attribution.

In only a few tetrapods do the fingers point backwards.
In most this occurs because the tetrapod has adopted the configuration of “knuckle-walking,” turning the hand dorsal side down, flexing the fingers, often to protect giant claws. Anisodon (formerly Chalicotherium), the platypus (Ornithorhynchus anatinus) and giant anteaters (Myrmecophaga tridactyla) are examples of these. Typically weight is put upon the second phalanges with the proximal phalanges acting as columns.

Apes
In ape-like primates the nails/claws are not so long, but the fingers are, so knuckle-walking helps them walk quadrupedally. Apes can even carry small objects this way. Chimps tend to extend the wrist while knuckle-walking. Apes do not, but form a column with the arm, wrist and metacarpals (hand bones) all aligned. This form is no doubt derived from brachiation in which the fingers support the weight while flexed around branches and the legs are secondarily used in locomotion.

Gorilla knuckle-walking.

Figure 2. Gorilla knuckle-walking. This enables the gorilla to carry things, and follows the natural curl of all brachiators. Click to enlarge and check attribution.

Then there are the pterosaurs…
Earlier we talked about the strange toe #5 of basal pterosaurs and fenestrasaurs, like Cosesaurus, and how it bent dorsal side down to create a small  impression in the substrate far behind the anterior four toes. Rotodactylus tracks preserve this form of toe knuckle-walking.

Finger #3 also created backwards impressions of their entire length in all quadrupdal pterosaur tracks by extension (palmar side down), no flexion. Sometimes finger #2 was angled as much laterally as posteriorly. Finger #1 often pointed laterally, which also makes it distinct among tetrapods. Rarely #1 pointed anteriorly. These we’ll take a closer look at now.

How does a forward pointing finger work?
We’ve all seen how forward-pointing toes work. They help extend the length of the push-off phase of the step-cycle. They help support the torso, shoulders, head and neck that arc out ahead. During the recovery phase of the step cycle the anterior digits flex to provide clearance, extending only at the last moment when the entire paw or hand plops down almost at once. Some forelimbs are digitigrade, so only the distal phalanges and sometimes only the unguals are in contact with the substrate.

Bipedalism brings on changes
Pterosaurs ancestral cousins, like Sharovipteryx, were bipeds. Based on the shapes of their locked down coracoids, their forelimbs transformed into flapping limbs before they began to elongate into wings. So when pterosaurs re-elongated their forelimbs after the basalmost pterosaur, they did so with the forearms locked into a neutral configuration (unable to supinate or pronate) so their fingers extended laterally (like yours do while clapping or flapping), not anteriorly (when you’re on all fours with a child on your back).

Like lizards
Pterosaurs were derived from tritosaur lizards. In arboreal lizards with elongated fingers we can see the joints don’t move exactly like hinges, but often have more spherical, rather than cylindrical metacarpophalangeal joints. This gives them the ability to cling more readily to any substrate shape. Pterosaurs had the same sort of fingers, especially at the metacarpophalangeal joint on finger #3. Lizards are not known for their grasping abilities, so it is doubtful that pterosaurs could carry a berry or a baseball (depending on size).

Figure 1. Pteraichnus nipponensis, a pterosaur manus and pes trackway, matched to n23, ?Pterodactylus kochi (the holotype), a basal Germanodactylus.

Figure 3. Pteraichnus nipponensis, a pterosaur manus and pes trackway, matched to n23, ?Pterodactylus kochi (the holotype), a basal Germanodactylus.

So, did the claw of #3 touch down first while pterosaurs walked quadrupedally?
It’s hard to say, but appears likely. It was often (but not always) longer than digits 1 and 2 and it would have been unnecessarily awkward to keep the claw elevated until the rest of the fingers had touched down. You can see (Fig. 3) that manual digit 3 was directed posteriorly whenever it touched down. To that point, digit 4 never made an impression, but was kept folded against the forelimb while walking due to an axially rotated metacarpal 4. To THAT point, maybe that’s why digit 3 extended backwards, directly beneath digit 4, which needed protecting.

Beachcombers, like Pterodaustro and Ctenochasma had small, weak manual fingers with small claws. Almost afterthoughts or vestiges by comparison.

Maybe pterosaur fingers didn’t flex (except to dig in the claws while tree perching), but acted like grappling hooks (if the claws were large) on trees. Pterosaurs with the largest fingers and claws, like Dimorphodon and Dorygnathus (Fig. 2), likely did not walk quadruped at all, but used their grappling hook unguals to grapple tree trunks. They have an inturned femoral head enabling parasagittal locomotion and were not far, phylogenetically, from their bipedal ancestors.

Dorygnathus on a tree.

Figure 2. Dorygnathus on a tree, an example of pterosaur with large free fingers. This is the neutral configuration, neither supinated nor pronated. Here the flexibility of the metacarpophalangeal joint enabled the pterosaur to shift its position while grappling and to let the fingers find the right sort of bark to maximize adhesion. We’ll know Dorygnathus footprints when we find them because the manual digit 3 will impress longer than any of the digitigrade toes. 

If we ever find dimorphodontid trackmarks, I’m going to guess the claws will be turned toward the palm in a sort of pterosaurian knuckle walk, if the fingers touch down at all.

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.

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