Mike Keesey  

Whirling Chest Dervishes

January 23rd, 2007 by Mike Keesey :: see related comic

(Sorry for last week, everyone. The “real” job, you know….)

A study was recently done by Senter and Parrish (2006) on forelimb mobility in Carnotaurus. We all know the arms are ridiculously small, but what’s really interesting is their bizarre combination of flexibility and inflexibility. Most of the arm was all but locked into place: barely mobile digits, rigid wrists, and ulnae and radii so short that the forearm almost looks like two extra wrist bones, with the hand sprouting directly from a locked elbow. Yet the humeri still had a bit of length to them, and, what’s more, they each had a hemispherical head, meaning that the shoulder had a larger-than-average range of motion. (For a theropod, anyway—we primates still have them beat in that category.)

Basically, Carnotaurus arms were like two rigid little sticks with claws on the end poking out of the chest and capable of swiveling around quite a bit. (Sort of like a stripper’s nipple tassels—except for the claws.)

What possible purpose could this kind of arm serve? Senter and Parrish look to the phylogeny and note that hemispherical humeral heads occur in all (neo)ceratosaurs (except for Ceratosaurus), so the mobile shoulder seems to have developed before the rigidity (which is only know to occur in carnotaurines). They suggest apprehension of tall prey, grappling contests, or display (like the tassels!) as possible uses in the longer-armed ceratosaurs, while noting that it’s (currently) impossible to know with any certainty.

Once thing’s for sure, though—they are funny-looking.

Reference:

  • Senter, P. and J. M. Parrish. 2006 (Dec. 22). Forelimb function in Carnotaurus sastrei, and its behavioral implications. PalaeoBios 26(3):7–17.

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