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?.
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.
With the list of potential culprits successfully narrowed down Godfrey and Smith were left with the question of how the bite marks had been made. Even though coprolites are relatively common at the Calvert Cliffs site no one had ever found a shark-bitten piece of shit before, so they had no other reference to go by. They ultimately settled on several possible scenarios.
The simplest explanation was that the shark (or sharks) which left the marks had been intentionally trying to eat the feces. “From the curvature of the toothmarks and their positions on the specimens,” Godfrey and Smith write, “we reason that
the majority of the fecal masses were in the sharks’ mouths.” Yet, strangely, the coprolites were not ingested. Even though tiger sharks have often been cast as indiscriminate when it comes to food there has been no indication that they have ever deliberately eaten feces, and so the authors looked for a different explanation.
Another possibility was that the shark bit the coprolites to see if they were palatable. Sharks have been known to tentatively bite objects for this reason, yet if the shark in question did so, the authors noted, the bite marks would have been deeper on both sides of the coprolites (particularly CMM-V-2244). Hence the authors favored a different scenario. The pattern of the bite marks and the fact that the feces were not ingested is consistent with a reconstruction in which, during an attack on another animal, the shark either bit through the body wall and guts to leave the tooth impressions or bit the intestines after disemboweling its prey. Such an attack would have left tooth marks on the feces, which probably fell out of the intestine shortly afterward, hence “In this scenario, the shark chose not to eat the feces, which drifted away, settled out of sight, or otherwise avoided attention.”
Unfortunately there is not enough information to know for certain how the coprolites from the Calvert Cliffs came to be bitten, but another discovery made on another continent is a little more straightforward. As reported in the latest issue of Palaeontology, scientists Giovanni Bianucci, Barbara Sorce, Tiziano Storai, and Walter Landini took another look at the exceptionally-preserved remains of a 3.8-3.1 million year old dolphin Astadelphis gastaldii which had been discovered in Italy during the late 19th century. Though long-forgotten, this particular specimen was significant as its bones were lacerated by the teeth of a large shark (thought to be a great white by the naturalists who originally examined it), and the team of researchers went back to these bones to see if they could reconstruct what had happened to the dolphin.
Like the scientists working with the geologically older Calvert Cliffs material, one of the first steps in reconstructing the events was determining what kind of shark had bitten the skeleton. There was a diversity of large genera, both living and extinct, to choose from, but the marks seemed most consistent with those of a large shark with pointy, unserrated teeth, with the top contenders being Cosmopolitodus hastalis and its still-living relative Isurus oxyrinchus (the shortfin mako). To test this idea the researchers used teeth from both these sharks to make cutmarks on plasticine, but while the marks seemed to be consistent with the damage seen to the dolphin skeleton it was difficult to distinguish between the damage caused by each type of tooth. Likewise, even though the maximum height of Cosmopolitodus hastalis teeth is three millimeters higher than the tallest shortfin mako teeth, this alone is not enough to distinguish between the marks the two species might have left on the bone. The apparent size of the shark involved makes Cosmopolitodus hastalis a seemingly better candidate, but there is no way to tell for sure.
Nevertheless, the numerous toothmarks on the jaw, vertebrae, and ribs of the Astadelphis specimen confirm that it had been bitten by a large shark with smooth-sided, sharp teeth. Now the question was whether the bones recorded an actual hunting event or were the result of a shark scavenging an already dead dolphin. As the scientists discovered, there were traces of both types of feeding.
Based upon observations of damage done to large prey by living sharks, the authors of the study proposed that a large shark killed the dolphin. As indicated by the deep cuts on the dolphin’s rib bones it appears that the shark would have swam up to the dolphin from behind and to the right. As the shark delivered its powerful bite, however, the dolphin struggled to get away, causing further trauma to the flesh and bone, and there is little doubt that at this point the dolphin would be suffering catastrophic blood loss. As it died it appears that it may have rolled over onto its back, and as it did so the shark bit again just behind its dorsal fin (leaving a second set of bite marks along the vertebrae). At this point the shark probably began to feed on the dolphin’s soft tissues, and the array of other small scrapes and marks on the ribs and jaws, then, would have been inflicted by smaller scavengers who picked at the remains after the attacking shark had finished. In the ocean, bodies do not go to waste.
Together the discoveries from Maryland and Italy provide scientists with narrow, but very informative, windows into the distant past. They remind us that fossils are not just inert remains. They are the last vestiges of living creatures and every single fossil, from the most common shell to rare treasures like shark-bitten croc poop, tell us about what life was like back then. We cannot answer all the questions we have, but discoveries such as these allow us to reconstruct the past in way usually only possible in our imaginations.
CHRISTY C. VISAGGI and STEPHEN J. GODFREY (2010). VARIATION IN COMPOSITION AND ABUNDANCE OF MIOCENE SHARK TEETH FROM
CALVERT CLIFFS, MARYLAND Journal of Verterbrate Paleontology, 30 (1), 26-35
Godfrey, S., & Smith, J. (2010). Shark-bitten vertebrate coprolites from the Miocene of Maryland Naturwissenschaften DOI: 10.1007/s00114-010-0659-x
BIANUCCI, G., SORCE, B., STORAI, T., & LANDINI, W. (2010). Killing in the Pliocene: shark attack on a dolphin from Italy Palaeontology, 53 (2), 457-470 DOI: 10.1111/j.1475-4983.2010.00945.x