Imagine an ancient shark with a single spiral tooth, shaped like a buzzsaw, in its lower jaw.
That’s what’s reported in a new paper in Biology Letters by Leif Tapanila et al. (free download). The spiral-like structure of this fossil, Helicoprion, had been known for some time, but it was curious: what seemed to be a single serrated tooth in the shape of a logarithmic spiral. Biologists had wondered how it was placed in the jaw, how it was used, and even if it was a tooth rather than some other part of the body.
This is what the structure, now known from the paper to indeed be a tooth, looks like (all photos and drawings from the paper):
The authors did CT scanning of a fossil found in 1950 in Idaho, dated to the early Permian—about 270 million years ago. The tooth, fortuitously, was embedded in the remains of the skull, something that’s rare because sharks have cartilage instead of bones in their skeleton. (That’s why shark teeth are so much more common in the fossil record than sharks themselves.) Using scans, they were able to show the placement of the teeth in the jaw.
Here’s a model, based on the CT scan, of the tooth placed in the mouth, taken from the side (lateral position):
And an oblique view from the side with the position of tooth interpolated from the scan:
(For you readers who know anatomy, here’s the key: bp, basal process; c, cup-shaped portion oflabial cartilage; ep, ethmoid process; lj, labial joint with base of root; pf, lateral palatine fossa; pp, process limiting jaw closure; qf, lateral quadrate fossa;qmf, quadratomandibular fossa; qp, quadrate process.)
The tooth apparently grew continuously, and, as the shark’s mouth closed on its prey, could be rotated up and back, cutting the prey and forcing it into the back of the mouth. BBC Nature interviewed the first author and gives more information:
Using the computer images, the team could build a 3D model of the jaw, to reveal how the tooth spiral worked.
“As the mouth closes, the teeth spin backwards… so they slash through the meat that they are biting into,” Dr Tapanila told BBC Nature.
“The teeth themselves are very narrow: nice long, pointy, triangular teeth with serrations like a steak knife.
“As the jaw is closing and the teeth are spinning past whatever it’s eating, it’s making a very nice clean cut.”
What could those teeth be used to eat? The answer is rich (P. Z. take note):
Dr Tapanila said that this evidence, combined with the “rolling and slicing” mechanism, provided clues to what the ancient fish ate.
“If this animal were eating other animals that were very hard or [had] hard armour plating or dense shells, you would expect more damage to their teeth.
“This leads us to believe that our animal was probably eating soft, squishy things like calamari. It was probably eating squid or its relatives that were swimming in the ocean at the time.”
As the paper reports, the spiral teeth are involved with a suite of other skull adaptations. Anatomy buffs take note:
Retention of teeth in a continuously growing whorl necessitates specialized morphologies, including the buttressing labial cartilages to maintain rigidity and alignment of the whorl, as it occludes between the upper jaws. With the jaw articulation next to the whorl, closure of the lower jaw rotates the teeth dorsoposteriorly, providing an effective slicing mechanism for the blade-like serrated teeth and forcing food to the back of the oral cavity.
Accommodating the continuous growth of the logarithmic whorl required commensurate anterior and dorsal expansion of the mandibular arch to house the symphyseal structure. Based on the largest diameter whorls in the IMNH collections, Helicoprion jaw length and height could exceed 50 cm, nearly double the size of IMNH 37899. Pre-mortal tooth wear or breakage is rare in Helicoprion [5,6]. This may be a result of rapid tooth production—some whorls exceed 150 [cm.!]—along with prey selection of soft-bodied animals, such as cephalopods  or poorly armoured fish.
I’m sure you’re probably wondering what the animal looked life in life. Here’s a reconstruction from the paper:
UPDATE: Reader gb james, in the comments below, points out a longer National Geographic article on the species with more pictures. Here’s one:
Tapanila, L., J. Pruitt, A. Pradel, C. D. Wilga, J. B. Ramsay, R. Schlader, and D. A. Didier. 2013. Jaws for a spiral-tooth whorl: CT images reveal novel adaptation and phylogeny in fossil Helicoprion. Biology Letters 9:10.1098/rsbl.2013.0057.