A photograph and line drawing (left side) of the fossil dolphin Astadelphis gastaldii. The crescent-shaped line in the line drawing represents the bite of a large shark, with the red portions representing damage done directly to the bone. From Bianucci et al, 2010.



Shark attacks are events of speed and violence. When they have locked on to a prey item sharks seem to come out of nowhere, and though they can be quite gentle with their jaws (as on occasions when they are unsure about whether something is food or not) their ranks of serrated teeth can inflict a devastating amount of damage. They are not the cruel, vicious, or bloodthirsty villains they have often been portrayed as, but instead are exquisitely-adapted predators which rely on their ability to catch and consume a variety of prey. And, just as it is among present day sharks, so it was among their prehistoric relatives.

Between 19 and 8 million years ago Maryland's Calvert Cliffs were covered by the ocean. Those shallow waters were inhabited by at least fifteen different genera of sharks, and their teeth (typically all that is left of them) are scattered everywhere along the beaches. Indeed, they are abundant enough that paleontologists Christy Visaggi and Stephen Godfrey recently cataloged of 26,000 of them to determine what kinds of sharks lived off the shores of ancient Maryland and in what numbers.

Their findings, printed in the Journal of Vertebrate Paleontology, reveal that this habitat was home to a mix of both living and extinct shark genera. There were fossils from Hemipristis (snaggletooth sharks), Galeocerdo (tiger sharks), Carcharias (sand tiger sharks), Carcharhinus (a subset of requiem sharks), and Isurus (mako sharks) in addition to the famous superpredator Carcharocles megalodon, most of which came from the time interval between 19 and 14 million years ago. (Teeth from many other genera, such as those related to whale sharks and great white sharks, were also found, but were so rare that they did not constitute a significantly significant sample.) While not exactly the same as their living relatives, these Miocene sharks would have looked very familiar to us, and clearly the area that would become the Calvert Cliffs was a very productive marine ecosystem which could support such a wide array of predators. Not surprisingly, there was plenty of prey in the water, too. Although not explicitly considered in their study, Visaggi and Godfrey noted that fish, sea turtles, crocodiles, birds, seals, sea cows, and numerous whale species all lived in the same place, and every now and then a specimen of one of these animals is found showing evidence of shark attack.

In a second new paper published by Godfrey and Joshua Smith in Naturwissenschaften, the paleontologists report on one such trace. In this case the evidence is two coprolites (fossil feces) that had been washed out of the Miocene fossil deposits and found on the beach. Exactly what species produced the coprolites is unknown, but after analyzing a third specimen of the same composition found nearby the scientists determined that it had been produced by a carnivorous vertebrate other than a shark. Though they could not be absolutely sure, a crocodile seemed to be a likely candidate, but the thing that made the paleontologists undertake this analysis in the first place was that the fossils showed showed characteristic tooth marks. One of the coprolites had been bitten into and the other had been severed. (You don't often see lines like "This tooth penetrated the feces to a depth of about 3 mm." in the literature.) A shark had bitten into these feces, but what kind of shark, and why?.


Photographs of the coprolites CMM-V-2244 (left, lower surface) and CMM-V-3245. The specimen on the left preserves the tooth impressions of the attacking shark while the specimen on the right was severed (the numbers denote where the teeth cut through the feces). From Godfrey and Smith, 2010.

The coprolite that had been severed, given the label CMM-V-3245, was not especially helpful in identifying the biter, but the other coprolite (CMM-V-2244) preserved a row of tooth marks. The scientists made a silicone cast of the impressions to see if the punctures held any clues as to the identity of the biter. What they found was that the animal that had made them had a single row of asymmetrical teeth, and while there were as many as eight shark genera with this characteristic most of these were deemed "innocent" on the basis of anatomical peculiarities. The best fits for the tooth marks were the genera Physogaleus and Galeocerdo (which, in fact, might be synonymous), sharks that, like their living relative the tiger shark (Galeocerdo cuvier) possessesed asymmetrical teeth in the shape of a bent A. (...)