Chilesaurus and Avian Arm Folding

Chilesaurus diegosuarezi by the impeccable Jaime Headden

Ever since the publication of supreme oddball Chilesaurus (eat your heart out, Halszkaraptor), I’ve been dying for somebody out there to do a full description of the critter’s unusual skeleton. As Chilesaurus may hold the key to our understanding of early ornithischians (or not), this is an animal in dire need of detailed study. This past Sunday, a paper was published in a journal that I can’t pronounce—Ameghiniana—and it is not that description. However, it is very interesting, and since I imagine many of you don’t have access to, uh, Ameg-HEE-ana (?), I thought I might summarize the juicy parts here.

Authored by Chimento et al. (2017), this paper discusses the forelimb posture of Chilesaurus. I was not aware of this, probably because I didn’t read the supplementary information in the initial description of Chilesaurus, but out of the six specimens found, four of them have good forelimb material. In all cases, the forelimbs are strongly flexed—the elbow is tightly folded and, surprisingly, the hand is flexed to a degree we usually associate with maniraptors. This is especially evident in specimen SNGM-1937.

The articulated arm of SNGM-1937

And it turns out that tightly-folding arms may be the norm, not the exception, among theropods. Avian arm-folding is, to some degree, present in most, if not all, coelurosaurs. Does it go farther down the tree than that? Chilesaurus, here considered a basal tetanurine, could apparently adopt this tight forelimb posture, and recall that the best specimen of basal, um, saurischian Guaibasaurus is preserved in an “avian resting posture,” complete with folded limbs (though not to the degree seen in Chilesaurus). Folded limbs are also present in Saltopus and Scleromochlus, both of which are not even dinosaurs.

SNGM-1938, with an articulated arm on one side, and a folded-back hand on the other

Now, in birds, the limb-folding system is essentially automatic—a series of tendons within the propatagium controls the extension and flexion. If the elbow is extended, the wrist is too (and vice versa). This is usually seen as a system that evolved in concert with the feathered wing as a way to protect the primary feathers. However, the fact that increasingly basal theropods seem to have this same system in place suggests that (1) the propatagium appeared very early in theropod (or even dinosauriform) evolution; and (2) its appearance is not connected to flight. I was shocked to discover that Hutson & Hutson (2014) demonstrated that automatic wrist-folding guided by soft tissues is also present in modern crocodilians. Automatic limb-folding, to one degree or another, may be plesiomorphic for Archosauria.

*headcannon*

Now, after initially skimming this paper, I got the mistaken impression that automatic limb-folding supports a tetanurine identity for Chilesaurus because the animals around it (Guaibasaurus, coelurosaurs) also have automatic limb-folding. However, a closer read says that automatic limb-folding is widely distributed among, well, archosaurs as a whole, so its presence or absence in a given taxon may not be a valid phylogenetic signal. Thus, my hope that Chilesaurus has something to do with the origin of Ornithischia remains intact!

It also brings up some interesting questions. Did ornithischians retain this ability or lose it in the transition from bipedality to quadrupedality? Certainly, the change in forelimb posture from medially-facing to rear-facing palms would have had an effect. Did bipedal ornithischians, like heterodontosaurs, basal ceratopsians, and basal ornithopods retain automatic wrist-folding? How about basal sauropodomorphs versus proper sauropods? What’s going on with Aardonyx?

I never gave much thought to arm-folding in dinosaurs but it turns out it’s super interesting.

Oh, I almost forgot: this paper also includes a restoration of the folded forelimb of Chilesaurus. The second finger only has one squat little phalanx, and the third finger is nothing but a greatly-reduced metacarpal and a little nubbin of a phalanx. So that’s something.

But I’m still waiting for that full description.