Ancient shark was a shell-crushing giant

The study of prehistoric sharks is no easy task. Specialists in other branches of vertebrate paleontology at least have the reasonable hope of discovering complete skeletons of their subjects; except in instances of exceptional preservation the scientists who study sharks typically only have teeth and a few vertebrae to work with. Still, you can tell a lot about a shark by its teeth, and a new study published in Cretaceous Research suggests that one peculiar form was a shell-crushing giant.

Thanks to Jaws, "Shark Week", and other sensationalist films the word "shark" most immediately conjures up images of streamlined predators with triangular, razor-sharp teeth. For much of the public the great white shark (Carcharodon carcharias) is the epitome of "sharkiness", but there is a much wider variety of shark types. The largest fish in the sea, the whale shark (Rhincodon typus) is a filter feeder with teeth smaller than your fingernails, while the much smaller Port Jackson shark (Heterodontus portusjacksoni) has differentiated teeth adapted for crushing mollusks. And, just like today, there was a diversity of shark types in the past. One of the most enigmatic was the Late Cretaceous form Ptychodus mortoni.

Like many other kinds of prehistoric sharks Ptychodus is primarily known from bits and pieces. A tooth here, a vertebrae there, but altogether the scraps add up. In fact, last year a team led by Kenshu Shimada used what was known about the shark to suggest that it was enormous, perhaps in excess of 11 meters in length, and the new paper builds upon this hypothesis.

The new specimen comes from Jewell Country, Kansas, and is little more than a portion of the upper jaw associated with a total of 19 teeth. Smack in the middle of the continental United States the county is about as land-locked as it is possible to get, but in the time of Ptychodus (about 89 million years ago) there was a great sea that stretched from Canada down to the modern Gulf of Mexico. This was the Western Interior Seaway, the home of plesiosaurs, mosasaurs, and, of course, enormous sharks.

But the teeth of Ptychodus did not look very much like those of its living relatives. The largest tooth from the jaw fragment, measuring 33.5 by 41 mm, looks like a flat garbage-can lid. It is not a tooth for stabbing or slicing but for crushing, as were the rest of the recovered teeth. In life these would have been arrayed in a set of opposing plates in the upper and lower jaws containing about 500 teeth each; vast, knobbly surfaces perfect for crushing large clam-like organisms and barnacles. This made Ptychodus a "durophage", or an organism that primarily consumed hard-shelled prey.

But what did Ptychodus look like? Several interpretations have been put forward. Some think that it had a flattened body like a stingray, while others have suggested that it had a more streamlined shark shape. The authors behind the new study split the difference by suggesting that it was something like a nurse shark (Ginglymostoma cirratum), a shark well-adapted to cruising over the bottom in search of hard-shelled prey to consume.

Ptychodus would have been much larger than any nurse shark, though. Based upon the fragments of the upper and lower jaws recently recovered and what is known about the relationship between jaw size and body length the researchers proposed that Ptychodus mortoni was over 11 meters long. This is in the range of modern basking sharks (Cetorhinus maximus) and whale sharks, and a few isolated teeth hint that there may have been individuals (or even a different species) even larger in size. Along with the shell-crunching mosasaur Globidens and the even more ancient placodonts, Ptychodus mortoni was one of the largest shellfish-eating animals ever.

[This post was inspired by Carl Zimmer's post on the enormous filter-feeding fish Bonnerichthys published last week.]

Shimada, K., Everhart, M., Decker, R., & Decker, P. (2010). A new skeletal remain of the durophagous shark, Ptychodus mortoni, from the Upper Cretaceous of North America: an indication of gigantic body size Cretaceous Research, 31 (2), 249-254 DOI: 10.1016/j.cretres.2009.11.005