When I was a fossil-crazed tyke, I used to spend hours flipping through a set of LIFE Young Readers Nature Library books my parents had purchased. The multicolored collection was one of my earliest introductions to natural history and science, and all the time I spent with those pages seared a few illustrations onto my memory. One of the most frightening depicted the blue outline of a man projected against the black and white grimace of a monstrous marine reptile with a gape as tall as the human silhouette. This dragon, according to the caption, was Kronosaurus – a 42-foot-long, quad-paddled carnivore that ruled the seas of Cretaceous Australia around 100 million years ago.
I’m a little surprised the book’s simple illustration didn’t give me nightmares. To think that the seas once held a sharp-toothed giant that could snap me up in one quick bite… And even though more recent estimates have downsized Kronosaurus to around 30 feet long, that has barely diminished my imagination’s ability to imagine the destruction the toothy reptile was capable of.
Kronosaurus wasn’t an aberration or evolutionary one-off. It was one of many imposing marine reptiles called pliosaurs. Cousins of the famous, long-necked plesiosaurs, the pliosaurs kept the four-flippered body plan but typically bore huge heads with elongated jaws. They spanned much of the Mesozoic – about 150 million years from their Triassic origin to their Cretaceous extinction – and the largest of them were probably in the neighborhood of 30 to 40 feet long. The biggest were the most massive flesh-eating marine reptiles of all time, but despite the ferocity apparent in their bones, an essential aspect of their predatory lives has remained little-studied – just how did pliosaurs employ their powerful jaws?
To answer this mystery, University of Bristol research associate Davide Foffa worked with a team of paleontologists and visualization experts to create a digital model of a Jurassic pliosaur’s jaw. The animal they chose was a well-preserved specimen of the roughly 155 million year old Pliosaurus kevani found in Dorset, England’s Weymouth Bay. While many other species of Pliosaurus are represented by fragments, this particular seagoing reptile is known from a nearly-complete skull that stretches over six feet long. That’s roughly the size of the Kronosaurus skull that entranced my childhood self.
After using CT scans to create a digital model of the Pliosaurus kevani skull, including reconstructing parts of the lower jaw and cranium that are missing, Foffa and coauthors modeled the extent of muscles used to close those jaws and further investigated how the skull would have coped with forces that would have been caused as the predator twisted or shook its skull to rip apart prey. What they researchers found questions the image of pliosaurs as the rapacious big game hunters they’re often portrayed as.
Just how hard Pliosaurus kevani could chomp differed according to what part of the jaw was doing the biting. While bites at the front part of the jaw were in the relatively meager 2,000 to 4,000 pound range, back jaw bites spiked between 6,000 and 11,000 pounds. That’s pretty impressive, and in the same ballpark as estimates for Kronosaurus, the extinct alligatoroid Deinosuchus, and Tyrannosaurus. As far as prehistoric giants go, Pliosaurus kevani was a prominent member of the bite club.
Despite such a strong bite, though, Pliosaurus kevani had a relatively weak skull. Estimates of skull strength using techniques called beam theory and Finite Element Analysis showed that the predator would likely damage its skull if it tried to twist off chunks of flesh or shake prey to death. And despite my childhood awe at the gape of Kronosaurus, the similarly-sized Pliosaurus kevani could have only swallowed prey 30 inches in diameter or less. Anything larger would have to be broken down into smaller bits. With shaking and twisting being so risky, how could Pliosaurus have done so?
Foffa and colleague propose an alternative way for Pliosaurus to process prey. Instead of violently tearing at or throttling prey, the researchers suggest, big pliosaurs probably nabbed prey and hefted it to the back of the jaw. That’s where all their power was. With the prey positioned just so, they chomped and chomped and chomped until their victim died and started to go to tatters. Whatever couldn’t be swallowed whole was repeatedly gnashed until the dismembered parts were small enough to gulp.
Pliosaurus wasn’t a marauder that only targeted the biggest game, though. To the contrary, Foffa and coauthors point out that gut contents show that cephalopods and sharks made up a large part of the pliosaur diet, while turtles, other plesiosaurs, and the odd dinosaur – perhaps washed out to sea – were rarer menu items. Apex predators they were, but pliosaurs were not monsters hellbent on consuming the biggest and most dangerous creatures they swam alongside. These extraordinary reptiles were generalist predators who had a specific way of catching and pulverizing the abundance of prey that flourished alongside them in the Mesozoic seas.
Foffa, D., Cuff, A., Sassoon, J., Rayfield, E., Mavrogordato, M., Benton, M. 2014. Functional anatomy and feeding biomechanics of a giant Upper Jurassic pliosaur (Reptilia: Sauropterygia) from Weymouth Bay, Dorset, UK. Journal of Anatomy. DOI: 10.1111/joa.12200