When Sharks Ate Dinosaurs

Once upon a time, roundabout 86 million years ago, a dead dinosaur drifted out to sea. The shovel-beaked hadrosaur expired somewhere inland, and, despite the herbivore’s bulk, the gases from decomposition buoyed up the carcass just enough to float the animal out into the warm waters where hungry sharks tucked into the dinosaur’s flesh. The scant details of the feast are recorded in bone.

In 2005, in the Smoky Hill Chalk of western Kansas, amateur fossil hunter Keith Ewell discovered a set of nine dinosaur tail bones. The ancient setting in which the bones were deposited made them remarkable. During the time the sediment of the Smoky Hill Chalk was being laid down, Cretaceous Kansas was blanketed by the Western Interior Seaway. Dinosaurs did not live in this marine environment, but their bodies were sometimes transported by local floods or other water-bound means to the jaws of seagoing carnivores. The quad-paddled plesiosaurs and Komodo dragon-like mosasaurs were the top reptilian predators, and prehistoric sharks took their fare share, too. In the case of the unfortunate hadrosaur that Ewell stumbled upon, we know this because of distinctive tooth marks on the tail bones.

Sternberg Museum of Natural History paleontologist Michael Everhart described the tail with Ewell in a 2006 Transactions of the Kansas Academy of Science paper. Aside being a rare occurrence of a dinosaur washed out to sea – only six had previously been found in the Smoky Hill Chalk – at least four of the hadrosaur’s vertebrae bore tooth marks that could have only been created by a scavenging shark.

But which shark? One serrated, A-shaped tooth of the shark Squalicorax was discovered right beneath the tail bones, yet the proximity does not automatically mean that the tooth was shed by a shark feeding on the dinosaur. Association does not always imply interaction.  In fact, Everhart and Ewell observed, the set of bite marks were apparently made by a shark with non-serrated teeth. The large, approximately 21 foot long shark Cretoxyrhina was the best match, especially because tooth fragments of this shark have been found embedded inside other bones showing similar bite marks. At least one Cretoxyrhina had sliced into the hadrosaur.

While remarkable, Ewell’s shark-bitten dinosaur was not unique. According to Everhart, all but two of the partial dinosaurs found in the Smoky Hill Chalk have bite marks on them. In the case of a heavily-armored Niobrarasaurus, for example, bite marks indicate that the lower portion of one of the dinosaur’s forelimbs was sliced off by a scavenging Cretoxyrhina.

A Squalicorax tooth associated with ankylosaur Aletopelta, hinting that sharks might have fed on the carcass after the dinosaur was washed out to sea. Photo by Brian Switek.
Shark-bitten dinosaurs have been found in strata formed in other places and times, too. While the relatively small shark Squalicorax didn’t create the damage Everhart and Ewell saw on their hadrosaur, in 1997 paleontologist David Schwimmer and coauthors mentioned a hadrosaur foot bone with an embedded Squalicorax tooth that had been found in the 80 million year old rock of Alabama. And at last year’s Society of Vertebrate Paleontology meeting in Raleigh, North Carolina, New Jersey State Museum paleontologist Jason Schein presented a poster on the upper arm bone of a small hadrosaur found in the state’s 66 million year old marl that was badly sliced by scavenging sharks. Without a doubt, sharks ate dinosaurs.

But none of these cases record a shark bite on a living dinosaur. All are cases of scavenging. Despite a recent Discovery News piece that suggested Squalicorax and other sharks might have killed dinosaurs, there’s no reason to think this was true.

There is no solid evidence that any non-avian dinosaurs were adept at swimming in the seas. They were terrestrial animals, and the sites found in the Smoky Hill Chalk, New Jersey’s Hornerstown Formation, and others are marine environments that dinosaurs would have had to swim considerable distances to reach. From what we know about dinosaur biology and the geologic context of known shark-bitten bones, seagoing sharks only dined on dinosaurs when the carcasses of hadrosaurs, ankylosaurs, and their ilk occasionally drifted out to sea.

This doesn’t mean that living dinosaurs and sharks never encountered one another. For one thing, some forms of the extinct hybodontid sharks were swimming through rivers and lakes long before dinosaurs evolved and persisted through the mass extinction that closed the Cretaceous. So far, there’s no indication that these sharks ever attacked or fed upon dinosaurs, but such interactions aren’t outside the realm of possibility. The only way we’ll know, though, is if such long lost sharks left their toothy calling cards in dinosaur bone.
References:

Everhart, M., Hamm, S. 2005. A new nodosaur specimen (Dinosauria: Nodosauridae) from the Smoky Hill Chalk (Upper Cretaceous) of western Kansas. Transactions of the Kansas Academy of Science. 108, 1/2: 15-21

Everhart, M., Ewell, K. 2006. Shark-bitten dinosaur (Hadrosauridae) caudal vertebrae from the Niobrara Chalk (Upper Coniacian) of western Kansas. Transactions of the Kansas Academy of Science. 109, 1/2: 27-35

Schwimmer, D., Stewart, J., Williams, G. 1997. Scavenging by sharks of the genus Squalicorax in the Late Cretaceous of North America. PALAIOS.