Sharks are an incredibly diverse group of fish. Most live in the ocean, some live in freshwater, and some move back a forth between both. "Jaws" perpetuated the image of the shark as an ambush predator, tearing big pieces out of large prey and chewing it up. Many sharks do eat like this, but more of them swim up onto a school of fish, open their mouths and swallow whatever goes in whole. Some strain algae and other plankton out of the water, while others eat hard shelled critters like mussels, clams, lobsters, crabs...etc.
The Cretaceous Western Interior Seaway was inhabited by many different kinds of shark. One of the most peculiar was the durophagous (eats hard-shelled animals) shark
Ptychodus. These sharks had jaws with robust teeth with low roots and massive crowns that could apply three point forces to hard material to break it. The crowns have transverse ridges and the margin of the crowns are decorated with a number of ridges and bumps (tubercles). Their mouths were filled with pavement dentitions composed of hundreds of teeth. Collections of these teeth is seen at right and below (pictures by Mike Everhart). Note the flattened surfaces caused by wear of the teeth from grinding hard materials.
Although teeth from these sharks are relatively common in the Cretaceous Greenhorn and Niobrara Formations of Kansas, little is truly known about the shark. It is estimated some species of this shark were up to 11 meters in length. Since there was abundant hard-shelled prey and little competition, this is entirely possible. Included in this diet were likely mollusks such as these small inoceramid clams (left). Nautiloids (think squid with shells) and small fish would have also been important food sources. The general body shape has been inferred to be fusiform, since the vertebral centra are round. The fact that these centra are calcified suggest that these are modern sharks (neoselachians). The only semi-well known skeletal elements of these sharks are the jaws. No well articulated skeleton of
Ptychodus has ever been found, so all attempts at classification of this fish are based on circumstantial evidence.
While I was examining the enameloid of teeth of a 305 million year old shark that I had collected from the Farley Limestone as Park University, I decided that I should examine the enameloid of a more recent shark to understand the difference between primitive and modern sharks. I did a couple of quick surface digests of
Ptychodus teeth with 10% HCl. After 30 seconds, I was able to see the single crystallite enameloid (SCE) on the surface (figure at right: Panels 1,3, 5 are before digestion and 2, 4, 6 are of single crystallites). After 3 minutes, I could easily see parallel-bundled enameloid (PBE) crystals on the surface (figure below). After a couple of days sectioning a tooth, I could see a triple-layered enameloid. A pretty good week's work I thought. Then I made the mistake of searching the literature for what was known about
Ptychodus tooth ultrastructure. Turns out, the answer is very little. But what is accepted says that these teeth do not exhibit a triple-layered enameloid, but rather an SCE. Based on this observation and ignoring a lot of other evidence, the experts placed this shark among the hybodonts, an ancestral group of modern sharks.
I puzzled over this for quite a while, because my results had seemed so clear-cut. I repeated these observations on several teeth and in several planes of section, but kept coming up with the same result: the enameloid of these teeth had a triple-layered structure. There was a superficial SCE/SLE, PBE on the crown, especially at the level of the transverse ridge, and tangled-bundled enameloid (TBE) next to the dentine. More careful examination of the literature revealed a couple of other studies that documented a triple-layered enameloid in
Ptychodus. One of the reports was in an obscure journal and the
Ptychodus teeth were a side study and only shown in a couple of pictures. The other report was a Masters thesis which was unpublished. What had started as an attempt to gain a proper control for one study turned into the main focus of another. I would have to prove that what I was seeing was a real phenomenon.
The figure at the right shows the PBE adjoining the TBE, and the TBE next to the dentine in sectioned teeth. Getting just the right images with the correct brightness and contrast took about 6 months. The enameloid of
Ptychodus had a lot of similarities to that of
Squalicorax curvatus, including having a TBE that became single crystallite in structure at the enameloid-dentine junction. Dentinal tubules rise high into the crown, penetrating into the enameloid, much like those seen earlier in my post on the hybodontiform shark. Preservation of the teeth is amazing and casts of the odontoblasts (tooth-building cells) can be seen below.
The figure at right is my recreation of the one experiment that is cited the most often. A whole Ptychodus tooth (1, 2, 3 below) was soaked in 10% HCl for 23 minutes, 35 seconds (4, 5, 6) it is easy to see the great degree of erosion in the surface decoration of the tooth. In 7 and 8 you can see that the enameloid has been eroded to the level of the dentinal tubules, which show up as divots in the surface of the tooth. The enameloid in this area (9) is single crystallite enameloid in appearance. The previous studies are correct in interpretation of the results of the experience. The problem is in the preparation. The tooth was soaked in acid way too long, and the bundled enameloid layers were destroyed.
This study solidified (for me, at least) the idea that "If it isn't published, it isn't known. If it is published, ask if it is truly good science in technique and interpretation." The work done here shows that Ptychodus is not a hybodont (primitive shark or proto-shark), but rather is a selachimorph neoselachian fish (modern shark). Reviews of the work have been very positive and the paper is cited in the second edition of "Oceans of Kansas" by Mike Everhart, which should be published this Fall.
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