The Tooth Whorl of a Strange Shark

A typical tooth whorl of Helicoprion. For a long time, these strange whorls were the only thing known about them.

Take a look at the fossil in the picture above. This is a fossil of Helicoprion. Can you tell what it is? If not, that is okay. When it was first described in 1899, Helicoprion was a bit of a mystery. It took paleontologists about half a century to figure out what is the whorl structure you see above. Then, it took another half a century to get its reconstruction correct. Let us take a little bit of a walk through history as we learn about Helicoprion and how its reconstructions have changed over time.

To be fair, paleontologists had a pretty good idea what type of animal was Helicoprion: it was a cartilaginous fish, something that we would call a shark. That posed the first big problem in identifying Helicoprion. Since cartilaginous fish have skeletons made of cartilage, their skeletons do not fossilize very well. That means that the most commonly found fossils of sharks are their teeth. Indeed, the Helicoprion whorl looks like a spiraling set of teeth. Unfortunately, for a long time, this spiral of teeth was the only thing known about Helicoprion.

As an aside, it is interesting to note that, even though paleontologists did not know what the rest of Helicoprion looked like, nor even how these teeth fit on the shark, but one thing was clear: the entire whorl represented a growth series. You may be familiar with the notion that sharks’ teeth are constantly replaced. The same thing goes for Helicoprion: new teeth were constantly being produced. However, while the used teeth of most sharks are pushed out of their mouths, the teeth of Helicoprion do not fall out: they just stack up in a whorl. The tooth on the innermost part of the spiral is the first tooth whole the last and largest tooth on the outside of the whole is the youngest tooth. Paleontology can be interesting that way. Because paleontologists build ideas based on preserved remains, and preserved remains can be fragmentary and scattered, they can know about the actual remains in detail, like knowing how Helicoprion teeth grow, but no nothing about what the whole animal looked like, like how the teeth were actually positioned on Helicoprion.

Now, I have been calling them teeth for a whole now, but the truth is that at first, the toothy spiral was thought to be an exterior feature of the body. Early reconstructions showed the toothy spiral emerging from the snout of the shark, from the tip of the dorsal fin, or even off of the caudal fins. Why would paleontologists put the teeth on the outside of the shark like this? There are a couple reasons. First of all, shark skin is covered with denticles. These denticles are very tooth-like. They are usually pretty smaller, but they do give a shark’s skin a rough texture, especially if one rub’s his hand against the grain of the denticles. Since denticles can resemble teeth, it is not a stretch to think that the toothy whorl is actually a set of large, tooth-like denticles. Second, many sharks have spikes that lie directly in front of the dorsal fins. While many sharks have simple spikes, other have more elaborate, even brush-like structures. Thus, a specialized whole of large denticles emerging from the fin of a shark is not an outlandish idea.

Helicoprion reconstructed with its whorl outside of the mouth. This is actually one of the oldest reconstructions of Helicoprion. Illustration from Lebedev (2009).

While the idea that the whorl was an external feature was common from the time of Helicoprion‘s discovery, ideas began to change in the 1950’s and 1960’s. Around that time, paleontologists began to accept that the whorl contains true teeth and that it was specifically found in the lower jaw. I am not sure what sparked this transition, but I think some of it had to do with the discovery of other sharks, such as Ornithoprion, which was named in 1966. These sharks actually had some preserved cartilage showing what the skulls and jaws looked like. While these sharks did not have a complete whorl like Helicoprion, they did have an arc of teeth found directly down the middle of the lower jaw. These teeth are called symphyseal teeth.

In anatomy, the word symphysis refers to a union of two mirror halves that meet in the middle. For example, the front part of the human pelvis is called the pubic symphysis, because two projections, called the pubes, meet right in the middle. Another symphysis commonly found in animals is the mandibular symphysis. While humans have a single bone in the lower jaw, many animals have at least two. These two halves are mirror images of each other and meet in the front of the jaws at a joint called the mandibular symphysis. Symphysial teeth are teeth that lie along the mandibular symphysis.

Just think about how strange are these symphysial teeth. Almost every vertebrate we know of has teeth that line the edges of the lower jaw. These sharks, however, have teeth that run right down the middle of the lower jaw. It is such a bizarre and strange arrangement, it is no wonder it took so long to figure out where the teeth were placed. Even then, how the teeth were placed in the mouth of Helicoprion and how it used those teeth remained a mystery for several more years.

Reconstruction of Helicoprion where the tooth whorl are actually teeth. Illustration from Lebedev (2009).

Probably the most common image of Helicoprion that shows its tooth whorl in the lower jaw portrays the shark with a ridiculous lower jaw that spirals back underneath itself, and this entire spiral is studded with teeth. There are a couple of accompanying illustrations to show what this restoration of Helicoprion looked like. Notice that the line drawing, as it is drawn from the side, makes it ambiguous whether or not the whorl existed by itself as symphyseal teeth whereas the other drawing clearly shows two parallel whorls as well as upper teeth. As mentioned a moment ago, other sharks like Ornithoprion suggested that Helicoprion had a single row of teeth down the middle of the lower jaw, rather than rows of teeth lining the jaws. Nevertheless, teeth along the edges of the jaw is such a pervasive thing that illustrators seem to want to draw Helicoprion with rows of teeth along the edges of the mouth. Notice also that both of these illustrations have a common theme: all of the teeth on the whorl are exposed. Again, this is something that we intuitively expect. After all, what animal can we think of that has teeth hidden inside its jaws? It just does not make sense, so intuitively, all of the teeth must be exposed.

This is still probably the most common style of reconstruction of Helicoprion. Note that there are two, parallel tooth rows and blade-like teeth in the upper jaw. Illustration by James St. John.

However, evidence began to mount that the tooth whorl was partially embedded in the lower jaw, that some of the teeth are actually hidden and not used (technically, not used any more, but more on that later). One such reconstruction shows Helicoprion with its whorl at the tip of rather long jaws. The research accompanying this illustration made another important point: there are more teeth in the mouth of Helicoprion. Now, it is not another tooth whorl, nor are they even pointed, blade-like teeth. Rather, they are flat tooth plates. In this reconstruction, tooth plates lined the upper jaw opposite the tooth whorl. That way, when the mouth closed, the tooth whorl struck against the plates, rather than cutting the mouth itself.

One of the earlier reconstruction of Helicoprion where the tooth whorl is embedded in the lower jaw. Even though they cannot be seen here, there are tooth plates in the upper jaw opposite the tooth whorl. Illustration from Lebedev (2009).

Along with these new reconstructions of Helicoprion came more plausible explanations of their diets. Many large carnivorous whales, such as the sperm whale and the beaked whales, have few teeth in their mouths (some beaked whales have as few as four) or teeth confined to the lower jaw (like the sperm whale). These whales typically eat fish or squids, which they grab with their teeth and then swallow. There is little processing (that is, chewing) of their food before it is swallowed. A similar habit was proposed for Helicoprion. Indeed, in the illustration, you can see ammonites around it, which may have been its source of food.

It is also worth pointing out at this point that remains of Helicoprion indicated that some of them got to be pretty big. In fact, some estimates of their size put them along along as 8 meters. That is nearly thirty feet long. Among living sharks, that would put it close to the size of basking sharks and exceeded in length only by the whale shark.

Reconstructions of Helicoprion changed one more time. Newer material of the skull and lower jaw itself showed that the toothy whorl not only was embedded in the lower jaw, it nearly occupied the entire length of the lower jaw. In this reconstruction, the mouth is rather short and the lower jaw is deep. The top of the whorl, where the new teeth are produced, are exposed, and as the old teeth are replaced, they eventually are rotated into the jaw. What these teeth do in the jaw is not certain, since as far as we know, the only usable teeth are the few exposed on the top.

Modern reconstruction of the head of Helicoprion. Note that the tooth whorl basically occupies the entire lower jaw. Tooth plates are still reconstructed in the upper jaw. Also, the tip of the snout is pointed, a bit like many chimera and ratfish today. Illustration from Tapanila et al. (2013).

The upper jaw still has tooth plates that lie opposite the tooth whorl. In fact, the upper jaw probably acted as a platform that held the prey while the lower jaw cut into the body of the prey. See, with a short jaw and an arching tooth row, as the jaw closed, the tips of the teeth would rotate into the body of the prey. Now, the teeth themselves do not move, but the jaw is short enough and the teeth long enough that as the jaw rotates up, the tips of the teeth have to move up and backwards. This motion effectively allows the teeth to act like a saw. In addition, it has been found that the lower jaw is flexible relative to the skull, meaning that the lower jaw can be pulled back while it closes. This allows the teeth to move against the prey even more. In fact, it is estimated that when it closes its mouth, as many as three teeth would be dragged across a single point on the body of its prey. Helicoprion literally had a buzz-saw for a mouth!

Now, because of the nature of its teeth, it is thought that Helicoprion ate soft bodied animals. Creatures like cephalopods, which includes squids and ammonites. Squids, which are just about entirely soft bodies, could be caught, sliced in half, and then the parts swallowed. Ammonites have a shell, but the head is exposed. The bit of Helicoprion would cut the head right off the shell of the ammonite, allowing the shark to eat that.

There is one more thing to note about Helicoprion. I have been calling it a shark this entire time. Indeed, most people will call it a shark because that is what it looks like. However, it actually belongs to a group of cartilaginous fish called Euchondrocephali. This group includes fish called chimeras and ratfish. These are deep sea cartilaginous fish with large pectoral fins. They typically eat bottom dwelling creatures with shells, as they have large tooth plates that allow them to crush their prey. While chimeras and ratfish are deep sea fish, Helicoprion probably swam in the water column and was a more active swimmer. In other words, even though it was not related to them, it would have looked and behaved more like a typical shark.

A compilation of various reconstructions of Helicoprion. Note that these reconstructions span a hundred years and are based on varying amounts of data. In general, the older reconstructions are at the top and new reconstructions are at the bottom. Illustration from Tapanila et al. (2013).

Looking back at the various reconstructions of Helicoprion, it is amazing how much has changed. Keep in mind, those earliest reconstructions had little to go on. As time progressed, and more fossils were found, additional details were added. The whorl went from an external structure to actual teeth. The whorl went from existing in a pair to a single, symphyseal structure. The teeth used to be all exposed, now they are embedded in the jaw. These transitions illustrate how reconstructions from fossils, especially from scrappy material, are often subject to change as new information is gathered. Is the current reconstruction of Helicoprion it? Have we arrived at its final, accurate reconstruction? Frankly, we never know. More data may change it once again. We will just have to wait and see. That is part of the fun of paleontology: there is always more that can be found that could completely change our understanding of extinct creatures. It is like a long, drawn-out, potentially never ending mystery.

Thoughts from Steven

References:

O. A. Lebedev (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(Suppl. 1):171-182

Leif Tapanila, Jesse Pruitt, Alan Pradel, Cheryl D. Wilga, Jason B. Ramsay, Robert Schlader, and Dominique A. Didier (2013) “Jaws for a spiral-tooth whorl: CT images reveal novel adaptation and phylogeny in fossil HelicoprionBiol. Lett. 9: 20130057

Rainer Zangerl (1966) “A New Shark of the Family Edestidae, Ornithoprion hertwigi from the Pennsylvanian Mecca and Logan Quarry Shales of Indiana” Fieldiana: Geology 16:1-44