Helicoprion sp. - fossil shark tooth whorl from the Permian of Idaho, USA. (IMNH, Idaho Museum of Natural History, Idaho State University, Pocatello, Idaho, USA)

This remarkable fossil is a symphyseal tooth whorl from the lower jaw of an edestoid shark. It is in a brown concretion from the Permian-aged Phosphoria Formation in Idaho. Sharks have a cartilaginous skeleton and mineralized, phosphatic teeth (as are all vertebrate teeth). Helicoprion is undoubtedly the strangest shark in geologic history (see reconstructions elsewhere in this photo album and at the links given below).

Helicoprion tooth whorls are almost always in large concretions, which are subspherical to rounded discoidal masses of relatively hard, fine-grained material (e.g., siderite, calcite, iron oxides).

Some paleontologists have interpreted the tooth whorl of Helicoprion sharks as part of a externally coiled lower jaw that may have been whipped outward and back to capture fish prey. Although intriguing, this type of reconstruction is incorrect. Instead, the tooth whorl was internal (inside the lower jaw tissues), and occupied the entire length of the lower jaw. The latter interpretation is based on an Idaho specimen with preserved soft-parts (see links below).

The spiral has been interpreted as a single tooth with numerous cusps. The oldest cusps are the smallest and occur at the center of the whorl. New, larger cusps were generated near the articulation joint between the shark's lower jaw (mandible) and the rest of the head. Helicoprion was an experiment in tooth retention - this shark could not eject teeth, unlike modern sharks.

Helicoprion sharks had modern-style scales on the body surface. Such scales result in no ripples and no sound while swimming - useful features in a marine predator. The classic interpretation of this shark having an external, open whorl as its lower jaw defeats the no-ripples-no-sound advantage of modern-style shark scales. An open, external tooth whorl is not hydrodynamic.

Classification: Animalia, Chordata, Vertebrata, Chondrichthyes, Elasmobranchii, Eugeneodontida, Edestoidea, Agassizodontidae/Helicoprionidae

Stratigraphy: Phosphoria Formation, Roadian Stage to Wordian Stage, mid-Permian

Locality: Gay Mine, Southeastern Idaho Phosphate Mining District, Bingham County, southeastern Idaho, USA

Thanks to Jesse Pruitt who provided info. and access to Helicoprion museum specimens.

See info. at: <a href="http://web.uri.edu/celsnews/two-uri-biologists-solve-mystery-of-a-strange-ancient-shark/" rel="nofollow">web.uri.edu/celsnews/two-uri-biologists-solve-mystery-of-...</a> and