Helicoprion had saws for jaws. That’s really all there was to the 270 million year old ratfish’s dental cutlery. No upper teeth or anything else to slice against – just an ever-growing whorl of spiky teeth anchored to the lower jaw.
This new, definitive image of Helicoprion debuted last year thanks to the efforts of artist Ray Troll and a team of researchers led by Idaho State University paleontologist Leif Tapanila. A very special fossil – IMNH 37899 – preserved both the upper and lower jaws in a closed position, finally solving the mystery of what the ratfish’s head actually looked like. But determining the exact placement of that vexing spiral was just an initial step.
Paleontologists and artists had often supposed that Helicoprion had upper teeth to pierce slippery cephalopods and squirming fish, but the fossils Tapanila and colleagues examined showed that Helicoprion only had a buzzsaw embedded in the lower jaw. How did this long-lived and prolific genus of Permian fish eat with a saw for a jaw?
Part of the original Helicoprion project involved creating a virtual model of the fish’s skull from CT scans of IMNH 37899. Now, in a Journal of Morphology paper, University of Rhode Island biologist Jason Ramsay and the rest of the team from last year’s Helicoprion study have gone back to those models to outline how the freaky ratfish fed.
Helicoprion was a biter. The ratfish’s jaws were too narrow to suction feed, Ramsay and colleagues point out, and so old buzzsaw jaw had to actively chomp prey. And chomp it could. The researchers calculated that when IMNH 37899 opened wide and bit down hard, the points of highest bite force along the ratfish’s tooth row ranged between 267 and 538 pounds. Not the greatest bite force of all time, but not too shabby for a 20-foot-long carnivorous fish.
[A model of how Helicoprion bit into prey, on display in the Whorl Tooth Sharks of Idaho traveling exhibit.]
Despite such a powerful bite, though, Helicoprion probably didn’t eat highly-armored prey. The ratfish’s jaws were so slender that trying to balance hard-shelled brachiopods or bivalves in its mouth would have been an exercise in futility, and broken Helicoprion teeth are so rare that it seems the predaceous fish preferred soft fare.
Prehistoric cephalopods – the kin of today’s squid and octopus – were probably the main Helicoprion menu items. Some would have looked squid-like, albeit supported by robust hard parts inside, and been easy enough to snaffle up. Others, however, jetted around in coiled shells and would have presented Helicoprion with the challenge of drawing out the squishier parts from their protective casing. The unique bite of Helicoprion was able to do just that.
The Helicoprion buzzsaw didn’t only slice. It also conveyed food back into the mouth during the bite. The overall effect, Ramsay and colleagues conclude, was like a biological miter saw.
The process went something like this. Teeth at the front snagged the prey and, as the jaws closed, moved the flesh backward. Here, the middle teeth speared the food, securing it in the mouth, before the back teeth bit in and sent the morsel down the hatch.
If Helicoprion bit one of the coil-shelled ammonoids or nautiloids just right, this same process would have automatically shelled the cephalopod. In the case of a head-on bite, Ramsay and coauthors write, the back teeth of Helicoprion could have gripped the cephalopod’s body while the front teeth pushed the shell out of the mouth. The overall effect, Ramsay and colleagues propose, “may have formed a novel mollusk shucking system.” Helicoprion – it slices, it dices, and makes Julienne ammonoids.
Ramsay, J. Wilga, C., Tapanila, L., Pruitt, J., Pradel, A., Schlader, R., Didier, D. 2014. Eating with a saw for a jaw: Functional morphology of the jaws and tooth-whorl in Helicoprion davisii. Journal of Morphology. doi: 10.1002/jmor.20319