Saturday, March 2, 2013

We Are the Whorled

Helicoprion was one of a myriad of bizarre cartilaginous fish which swam the earth’s oceans from the late Carboniferous to the early Triassic. This fish is known from puzzling, yet beautifully preserved, teeth, as many cartilaginous fish are. That’s the problem with fossilized sharks and rays: while bones are easily fossilized under proper conditions, cartilage is very rarely preserved. Unlike most sharks and rays, Helicoprion is not merely known from isolated teeth or even jaws. All we know about this animal comes from curiously coiled whorls of teeth.
A Helicoprion whorl. The teeth line the outer edge of the spiral. Photograph by Chip Clark.
Ever since its discovery, the animal has been shrouded in mystery. Being known only by these whorls, no one had the slightest idea of how the rest of Helicoprion looked. Some even proposed that the coiled teeth may in fact be the shells of ammonites.  However, when Alexander Petrovich Karpinsky coined the name in 1899, he demonstrated that Helicoprion was a sharklike fish rather than a mollusk. His idea about the function and placement of the whorl on this animal, however, was completely off.

Former depictions of Helicoprion range from the reasonable to just plain goofy. Artwork by the brilliant artist, a personal favorite of mine, Ray Troll.
Karpinsky proposed that the whorl was actually located on Helicoprion’s nose. While this idea is ridiculous compared to what we now know about this animal, it was nowhere near the most outlandish, or creative, idea. The whorl has been speculatively placed everywhere from within the fish’s throat to the tips of its fins and tail. More recent reconstructions place the whorl in a sort of buzzsaw-like placement at the end of the lower jaw, which is usually depicted as being elongate. A new study by Lief Tanapila and Jesse Pruitt agrees with the buzzsaw placement of the whorl, but it reveals much more about the evolution and overall appearance of Helicoprion.
Most modern reconstructions display Helicoprion with a buzzsaw-like, elongated jaw. Reconstruction by Dmitri Bogdanov.
Using CT scans, Tanapila and Pruitt discovered a number of intriguing features of this fish. While comparing the lower jaw of Helicoprion to a buzzsaw may seem like a stretch, both the shape and the function of this dentition was very similar to such a tool. As the jaw closed, the whorl slightly rotated. Another puzzling discovery was that Helicoprion lacked teeth on its upper jaw altogether; the whorl was the fish’s only set of teeth. Such adaptations are ideal for a very specialized niche, which we will examine in a couple of paragraphs.
The spotted ratfish, rather than true sharks, is the closest living relative of Helicoprion. Photograph by Dan Hershman.
Perhaps even more surprising than Helicoprion’s physical features is its newfound phylogenetic placement. While commonly referred to as a shark (you don’t know how hard it was not to refer to it as such in this post; I’ve always known it to be a shark), Helicoprion is actually more closely related to ratfish than it is to true sharks. Ratfish are part of the class Chondrichthyes (cartilaginous fish), but they are not sharks; they are part of a different subclass. Helicoprion was found to be part of this separate subclass, very close to the point at which sharks and ratfish diverged. (Tanapila et al., 2013)

A brand-new look for Helicoprion, and it only took over 114 years to discover it. Reconstructions by Ray Troll.
The fact that such a large fish, sometimes reaching 8m (~25ft) long, was so common and yet only distantly related to true sharks reveals a lot about the environment in which it lived. With no large marine reptiles yet ruling the seas, and certainly no whales, all of the major niches were filled by fish. Helicoprion was able to grow to great sizes as it filled a niche which, it seems, no other animals were exploiting: hunting cephalopods and other soft-bodied creatures. (Lebedev, 2009) Helicoprion was unlikely to be the only specialized holocephalan at this time; the diversity of such fish was far greater back then. Eventually, such strange fish did go extinct, and in their wake, sharks and rays evolved to fill their niches.

It is a wonder that over 100 years after its discovery, we are only now discovering new information about Helicoprion. It has had a confusing, misfortunate, and pretty hilarious history, but it seems that all the speculations and hypotheses of the function and position of its characteristic whorl can be put to rest. While not as outlandish as some depictions, the real Helicoprion was no less unusual or unique.

Lebedev, O. A. (2009). A new specimen of Helicoprion Karpinsky, 1899 from Kazakhstanian Cisurals and a new reconstruction of its tooth whorl position and function. Acta Zoologica, 90, 171–182. doi:10.1111/j.1463-6395.2008.00353.x

Tapanila, L., Pruitt, J., Pradel, A., Wilga, C. D., Ramsay, J. B., Schlader, R., & Didier, D. A. (2013). Jaws for a spiral-tooth whorl: CT images reveal novel adaptation and phylogeny in fossil Helicoprion. Biology Letters, 9(2). doi:10.1098/rsbl.2013.0057

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