Between 100 and 65 million years ago, at the time when dinosaurs reigned on land, the ocean was home to a remarkable group of lizards—the mosasaurs. With powerful flippers and a tail built for thrust, these marine lizards were a formidable presence in the marine food web. Some were cunning predators akin to the killer whales and sharks of today and had razor sharp teeth that they used to slash and tear chunks of flesh from large prey. Others were proficient foragers and had specialized blunt and bulbous teeth that hammered away at the tough shells of oyster-like shellfish.
Over millions of years many of the traces of this ecosystem have eroded, and so scientists use several clues from the fossil record to make informed hypotheses as to what they ate.
Scant Remnants
The most obvious clues that help determine what a mosasaur ate are fossilized meals. But finding such a specimen is like winning the lottery. First, the mosasaur had to have died immediately after consuming their prey, before digestion obscured all traces of its shape. Second, the animals needed to be buried shortly after death, away from scavengers like sharks and fish. And finally, the fossil needs to be in a place easily visible to scientists but sheltered from the elements.
There are only a handful of instances where this has occurred, and for many, the stomach contents are a jumble of bones that only a trained eye could identify. In cases where the remains can be identified, they have included everything from pieces of fish, squid and octopus relatives, seabirds, turtles, plesiosaurs, and even other mosasaurs.
The best example of clearly identifiable prey (even to the untrained eye) can be found displayed in the Ocean Focus Gallery at the far end of the Sant Ocean Hall’s Sea Monster’s Unearthed: Life in Angola’s Ancient Seas exhibit at the National Museum of Natural History. The featured Prognathodon kianda lived 72 million years ago and has at least three mosasaur skulls within its stomach, including a “Platecarpus” ptychodon, a juvenile Mosasaurus, and even another juvenile Prognathodon kianda— indicating that mosasaurs, at least in this case, were not averse to cannibalism.
Modern Day Analogue
Often in paleontology, understanding animals of the past requires looking to animals that exist today. Specifically, comparing teeth shape can reveal what kinds of prey an animal would target. Blunt teeth, like the molars of walruses, can indicate crushing of shellfish while serrated teeth, like those of a great white shark, can slice through bone and can indicate the consumption of larger prey.
The earliest mosasaurs were most likely fish eaters. They almost exclusively had conical teeth, a simple shape common in present-day dolphins, that could efficiently snag fish and, in some cases, more formidable prey. But by 80 million years ago mosasaurs underwent an explosion of evolution and soon teeth that could crush, puncture, and slice like blades appear. Globidens phosphaticus, on display in the exhibit, is one such mosasaur that crunched on shellfish with teeth the shape of mushrooms, while Prognathodon kianda tore into larger prey.
The “Platecarpus” ptychodon had an especially unique tooth arrangement for mosasaurs. Like the South Asia river dolphin and the gharial, a crocodile from the Ganges, this mosasaur had interlocking teeth that ran the length of its narrow, cylindrical snout. It likely swam alongside its prey and flicked its head sideways to snap up small fish. The shape of the snout helped reduce drag and is a classic example of convergent evolution, where environmental pressure causes animals of completely different lineage to evolve a very similar trait—much like how dolphins and sharks evolved similar body plans to maximize hydrodynamics.
A Tooth’s Story
Examining teeth visually can reveal much about what a mosasaur ate, but hidden within the molecular structure of the tooth yields another clue. Both bone and tooth enamel are built as crystal structures, the atoms spaced in intricate geometric honeycomb patterns. Tooth enamel has ample atoms of phosphorus, but in some instances, the phosphorous is replaced by carbon. It is this carbon that scientists use to help understand the story of mosasaur diets.
Not all carbon is the same, a fact that scientists exploit to their advantage. Some atoms of carbon contain six neutrons to pair with the six protons in the core of the atom— Carbon 12. But there is another naturally occurring and stable form of carbon that has an extra neutron. This is called Carbon 13 and the extra neutron makes it heavier than Carbon 12. Scientists know that the amount of Carbon 12 compared to Carbon 13 also indicates where a creature likes to feed. This is because plants tend to have higher amounts of Carbon 13. Since you are largely what you eat, the farther an animal feeds from shore, the less Carbon 13 it will incorporate into its tooth enamel.
By comparing the carbon profiles of the mosasaurs to known profiles of animals that live today, the scientists are able to make inferences as to where mosasaurs ate. “Platecarpus” ptychodon had similar amounts of Carbon 13 to modern sea otters, indicating that it likely ate in a shallow habitat, close to shore. Mosasaurus was closer to whales and dolphins, meaning they likely ate in slightly deeper waters, but still not too far offshore. Prognathodon kianda, the species found with other mosasaurs in its stomach, appears to have ranged from nearshore to offshore feeding areas.
While this method does not specifically pinpoint what a mosasaur ate, it does help flesh out the story of their life as predators. Oceans of millions of years ago may have had different animals, but the structure of ancient food webs are not as foreign as you might think. An animal with a powerful, hydrodynamic build, strong flippers, and teeth meant to ensnare fish—this could be the description of both a killer whale and a mosasaur. So, as you attempt to wrap your head around what life might have been like millions of years ago under the sea, just think about the creatures of today— you may be surprised by how close your vision comes to reality.
Editor's note: The great mosasaur fossils along with other animals that lived in the ancient Angolan sea will be on display in the Sant Ocean Hall beginning November 9, 2018. Come by the new exhibit to learn more about how geology impacted the lives of these amazing creatures and their marine ecosystem!