What Drives the Elongation of the Metacarpus in Pterosaurs?

In several clades of pterosaurs the metacarpus became elongated. We looked at this phylogenetically earlier in an 8-part series on pterosaur fingers starting here. Generally an elongated metacarpus is seen as a synapomorphy of the old “Pterodactyloidea,” but it is more complicated than that. We’ll look at each elongation event clade by clade. I don’t think anyone has tackled this subject yet, hence the lack of references below.

Protoazhdarchids

Figure 1. Protoazhdarchids

Clade 1 – The Proto-azhdarchids
In Pterodactylus? spectabilis, n1, the metacarpus was not elongated. Rather the forearm was shortened with respect to the ancestral sister Dorygnathus (SMNS 50164). In the next taxon, Beipiaopterus, the metacarpus was genuinely elongated. (We looked at this yesterday.) So what happened here?

Beipiaopterus was several times larger overall with a longer tibia, as long as the glenoid-acetabula length.  Beipiaopterus was the first of the stork-like taxa with an elongated neck and more gracile proportions. Still the metacarpus was only half the tibia length. The next taxon after Beipiaopterus, n44, was much smaller overall, but had the proportions of an azhdarchid with a hyper-elongated metacarpus, slightly longer than the tibia. It is likely that Beipiaopterus was a sideline in the lineage between n1 and n44. That means the hyperelongation of the metacarpus occurred in relatively small pterosaurs with very tiny and likely grounded hatchlings.

Protoctenochasmatids

Figure 2. Protoctenochasmatids

Clade 2 – The Proto-ctenochasmatids
In St/Ei I  the pattern of metacarpal elongation was similar. Overall St/Ei I was much smaller than its phylogenetic predecessor, Dorygnathus R156, which also had a much longer forearm. Two intervening taxa, D. purdoni and Angustinaripterus, are known only by skulls. The ctenochasmatid clade achieved its greatest metacarpal length with Gegepterus, which had stork-like proportions overall. The total length of the tibia is unknown. Other ctenochasmatids, like Ctenochasma and Pterodaustro, did not greatly elongate the metacarpus. Neither became subequal to the tibia. The Pterodaustro embryo, interestingly enough, had a relatively shorter metacarpus than the adult with a longer humerus and more robust forearm, but smaller fingers. Is this a clue that forelimb proportions changed slightly during ontogeny?

Tiny scaphognathids

Figure 3. Tiny scaphognathids

Clade 3 – The Post-Scaphognathids
The lineage of cycnorhamphids and ornithocheirids includes several tiny pterosaurs derived from a series of ever smaller Scaphognathus specimens. The first in this lineage to sport an elongated metacarpus was Gmu-10157 with a metacarpus just short of the tibia and ulna length. So, once again metacarpal elongation occurred first with the tiny pterosaurs and their grounded hatchlings. In this clade, Cycnorhamphus had the longest metacarpus and tibia. The ornithocheirids never developed a hyper-elongated metacarpus. Basal ornithocheirids  had a relatively longer metacarpus compared to the ulna. The forearm elongated in derived forms, so the metacarpus appeared to be relatively shorter. The length of the tibia was shorter in succeeding ornithocheirids, then became more elongate, relative to the metacarpus in the most derived ornithocheirds. Matching its adult sisters, the hatchling ornithocheirid had a metacarpus to match its forearm length, but a much longer tibia was present.

Scaphognathians

Figure 4. Click to enlarge. Scaphognathus and its descendants demonstrating the elongation of the metacarpus immediately following the smaller specimens of Scaphognathus.

Clade 4 – More Post-Scaphognathids: Ornithocephalids
The lineage of Pterodactylus and Germanodactylus (with all of its many descendants) originated with a sister to Gmu-10157, the tiny pterosaur, Ornithocephalus. Thus clades 3 and 4 likely had a single common ancestor as yet undiscovered. In this clade the hyperelongation of the metacarpus occurred with Eopteranodon and Eoazhdarcho, two stork-like taxa often mistaken for azhdarchids, and perhaps convergently, Nyctosaurus and Pteranodon. I wonder if these two were derived from long-legged taxa, like Eopteranodon, that reduced their hind limb length? Or did they never have long legs and simply developed elongated metacarpals from the likes of Muzquizopteryx and the SMNK-PAL specimen of Germanodactylus? The skulls seem to point to the latter hypothesis, which would make Eopteranodon and Eoazhdarcho offshoot cousins of these two lineages. The juvenile Pteranodon had a hyperelongated metacarpus.

The Evidence in Summary
Apparently there are two stages to metacarpal elongation. The first occurs among tiny pterosaurs in which the metacarpus elongates to 3/4 or so of the tibial length. The second sometimes, but not always, occurs in larger pterosaurs, in which the metacarpus elongates to at least the tibial length, which is sometimes, but not always also elongated.

That’s evolution for ya.

Guesswork
An elongated metacarpus and tibia impart a stork-like appearance to the pterosaurs that have these traits and the best guess is a stork-like lifestyle was their niche. In Pteranodon and Nyctosaurus this was not the case as these albatross-like soarers developed their long wings by elongating their metacarpus. The first appearance of an elongated metacarpus in tiny pterosaurs and presumably their hatchlings might have provided limbs that had more similar proportions, perhaps for added efficiency during terrestrial locomotion. This efficiency would have been lost in Pteranodon and Nyctosaurus in which the metacarpus was much longer than the tibia.

Nothing about metacarpal elongation in the literature, hence no references.

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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