Laelaps

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A restoration of the giant, durophagous shark Ptychodus, courtesy paleo-artist Matt Celeskey.

ResearchBlogging.org

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.

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The largest tooth recovered for the new Ptychodus mortoni specimen. (From Shimada et al., 2010)

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.

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Four views of the jaws of two species of Ptychodus. A) Lower jaw of Ptychodus decurrens as viewed from above, B) Upper tooth plate of Ptychodus occidentalis as viewed from below, C) Lower jaw of Ptychodus occidentalis as viewed from above, D) Lower jaw of Ptychodus occidentalis as viewed from below. (From Shimada et al., 2009)

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.

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An outline of the upper jaw of Ptychodus mortoni showing the position of the new fragment, and a comparison of the size of the shark next to an adult human. (From Shimada et al., 2010)

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

Comments

  1. #1 kevin Z
    February 22, 2010

    Any evidence of its diet? Was it a benthic scrounger or a pelagic predator (shelled cephalopods for instance).

  2. #2 Laelaps
    February 22, 2010

    Kevin; There seems to be some evidence of predation by Ptychodus on the clam-like inoceramids (huge, flat bivalves that lived on the seafloor – sorry, can’t find a link to the paper on the web but it is E.G. Kauffman, Ptychodus predation upon a Cretaceous Inoceramus, Journal of Paleontology 15 (1972), pp. 439–444.), and fossilized shark puke attributed to Ptychodus has also suggested that it often fed on cirripeds. Overall it seems to have been more of a “benthic scrounger.”

  3. #3 220mya
    February 22, 2010

    Brian,

    The citation you listed has the incorrect journal name, hence why you couldn’t find it online. The correct reference is:

    Kauffman, E.G. 1972. Ptychodus predation upon a Cretaceous Inoceramus. Palaeontology 15(3): 439-444.

    It is available online and free to all here:

    http://palaeontology.palass-pubs.org/pdf/Vol%2015/Pages%20439-444.pdf

  4. #4 Laelaps
    February 22, 2010

    Thanks! Just realized that after I hit “post.” Citation was wrong in the ref. section of the new paper so I did not catch it immediately when I hit “copy-paste.”

  5. #5 Blake Stacey
    February 22, 2010

    Don’t you hate it when people screw up their bibliographies? Just yesterday evening I was reading an ecology paper in PNAS which got the volume number of an article it cited off by 64.

  6. #6 kevin Z
    February 23, 2010

    I would demand a retraction. How can you believe their results if their citations are so sloppy?

  7. #7 Mike Everhart
    February 24, 2010

    Brian,
    Thanks for the nice review of our article. I certainly regret the mistake in the journal citation. The authors, of course, are responsible for such errors, and we are human. But before your readers “throw the baby out with the bathwater”, I would remind them that one mistake in the name of a journal does not negate the value of the discovery or the description. If someone wants to find the article, the journal name is not essential. As an editor and reviewer, I have seen far more far more serious errors by authors.
    Regards,
    Mike

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