Riley Black

Science Correspondent

In 1780, before the word “paleontology” had even been coined, chalk miners in the Netherlands uncovered a strange skull. The jaws were clearly from an animal unlike any roaming the European countryside and represented a creature that had died at some ancient time. One naturalist thought it was a whale. Another saw the smiling jaws of a crocodile. French revolutionaries who’d invaded Maastricht thought it was a fine prize and took it back to France. But when the powdery dust settled, the identity of the “great animal of Maastricht” turned out to be something no one had anticipated. In 1822, English paleontologist William Conybeare dubbed the creature Mosasaurus, a seagoing relative of today’s monitor lizards that hunted in prehistoric seas.

The first fossil was so singular that scientists just called it Mosasaurus to start. The full name Mosasaurus hoffmannii wasn’t added until 1829 as scientific standards evolved. Mosasaurus wasn’t just of interest to researchers, though. The fossil lizard soon became legendary. The story of the fossil’s theft from the Netherlands became well known and even gained some embellishments, such as the myth that the French soldiers stole the bones in exchange for 600 bottles of wine. Studied by the iconic French naturalist Georges Cuvier, Mosasaurus became evidence that the ancient Earth must have witnessed terrible upheavals that drove some species to extinction. As similar fossils turned up, Mosasaurus turned out to be just the first genus of a whole group of marine reptiles—the mosasaurs. More recently, the seagoing reptile has received the Hollywood treatment as one of the central monsters in the Jurassic World franchise.

Despite the reptile’s storied reputation, however, paleontologists have only recently begun to understand the lives and evolution of such creatures. And they’ve found that it’s possible mosasaur-shaped reptiles evolved more than once.

Just as with whales, seals and various other seagoing creatures, the ancestors of mosasaurs were landlubbers. Around 100 million years ago, lizards living along Earth’s waterways began to become more adapted to life in the water. They could swim with sinuous, snake-like motions of their bodies and use their limbs to steer. The most skilled swimmers were better able to nab prey and otherwise survive, beginning an evolutionary trajectory that would result in 50-foot-long, multi-ton ocean giants like the extremely toothy Mosasaurus.

So far, says American Museum of Natural History paleontologist Amelia Zietlow, researchers have yet to uncover the earliest terrestrial ancestors of mosasaurs. Nevertheless, experts have found multiple examples of early mosasaurs that display some common anatomical themes. “The first mosasaurs likely looked like noodle-y monitor lizards, possibly with webbed fingers, that were between two and ten feet long,” Zietlow says. The reptiles had triangular, pointy faces, but they also stood out from other reptiles of their time by giving live birth. From that point, however, what we think of as mosasaurs evolved more than once. Instead of one moment when the lizards slid into the water, researchers think that aquatic mosasaurs evolved at least three times.

“Mosasaurs seem to have committed to marine life multiple times,” Zietlow says, with different lineages independently evolving larger size, flipper-like limbs and broad tails. Such a scenario is not so unusual as it might seem. After all, Zietlow notes, lizards have repeatedly lost their limbs and evolved worm-like forms. Snakes evolved from lizard ancestors, of course, but glass lizards, snake lizards, skinks, amphisbaenians and more are all lizards that lost their legs independently of each other. Lizards might have been doing the same in the Cretaceous, only taking to the water instead of becoming noodles.

Regardless of how many times large mosasaurs independently evolved, the lizards thrived in the seas. And life in the water transformed them into predators even large sharks had to be wary of.

Tylosaurus
The mosasaur Tylosaurus hunts a shark. Mohamad Haghani / Stocktrek Images via Getty Images

In broad strokes, mosasaurs tend to share the same body shape. “Mosasaurs look the same with each other as an evolutionary response to the stringent aquatic environment they lived and evolved in,” says University of Cincinnati paleontologist Takuya Konishi. All mosasaurs hunted prey of one form or another, and doing so required streamlined shapes easily able to navigate water—a fact that precluded them from evolving ornate and showy structures as the dinosaurs did on land. “This is why mosasaurs, over millions of years, modified the claws and limbs of their terrestrial lizard ancestors into flippers, streamlined their torso, bent the tail end downward to support a forked tail fin, and may have even evolved a dorsal fin,” Konishi says.

Some, like Mosasaurus itself, were fearsome apex predators that ambushed their prey much the way large sharks hunt seals. Such formidable carnivores tore their prey into chunks and swallowed the parts without chewing. “No mosasaurs swallowed prey larger than their head, whole,” Konishi says, “so they must have reduced a large food item into a bite size before swallowing.” Other mosasaur species preferred to dine on ammonites. Numerous spiral ammonite shells bear bite marks that exactly match the V-shaped outline of mosasaur jaws. Then again, some of these reptiles were bottom feeders with bulbous teeth. The mosasaur Globidens plucked bivalves and other shelly prey from the seafloor and crushed the invertebrates to the point of bursting. Once mosasaurs had gone out to sea, they evolved into all manner of different forms.

Of all mosasaurs, though, Mosasaurus itself remains the most iconic even more than two centuries after the early finds in Maastricht. The menacing screentime the reptile has received in the Jurassic World movies has certainly helped. “It tends to surprise people that I have a huge soft spot for the Jurassic World Mosasaurus,” Zietlow says, and not only because it’s popularized her favorite fossil subjects. “One of the reasons I personally like the depiction is that the movies, in my opinion, nail its movement and behavior,” she says. Mosasaurus was a large generalist predator that pretty much ate anything it could catch in the prehistoric seas, from sharks and large fish to ammonites and other mosasaurs. Even dead dinosaurs, floating out at sea, were likely mosasaur chow.

All Mosasaurus Scenes In The Jurassic World Trilogy | Science Fiction Station
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Those teeth did more than slice into prey. Zietlow notes that several Mosasaurus fossils have bite marks, both healed and unhealed, that were certainly made by other mosasaurs. One particular fossil held in Canada has a mosasaur tooth stuck into its lower jaw. “We can’t say for sure whether mosasaurs were ever social with one another in ways like traveling in groups or raising young, but, based on living lizards and the multitudes of bite marks, I think it’s safe to say that they were social in an antisocial way,” she says, fighting by biting each other on the face.

Mosasaurs
Mosasaurs likely attacked each other. Mohamad Haghani / Stocktrek Images

The comparisons between living lizards and their ancient mosasaur relatives is a critical aspect of their study. Mosasaurs evolved webbed feet and flippers, Zietlow notes, and so studying the way geckos grow webbed feet can help paleontologists better understand the way mosasaurs did something similar back in the Cretaceous. And there are still so many fossils to find. Late last year, Zietlow and colleagues named a new mosasaur, Jormungandr walhallaensis, that is a connecting form between earlier mosasaurs and the giant Mosasaurus itself, helping experts better outline how such striking creatures evolved.

Mosasaurs were the closest the world ever came to real life sea dragons, and we are still just beginning to follow the sinuous trail of their story.

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Riley Black

Riley Black is the author of The Last Days of the Dinosaurs and many other books. She is a science correspondent for Smithsonian magazine covering fossils and natural history, and she writes about the prehistoric past for a variety of publications.