These Prehistoric Sea Monsters Had a Mean Breast Stroke

A new study shows Mosasaurs not only swam using their tails but used powerful pectoral muscles for short bursts to ambush prey

Dmitry Bogdanov via Wikimedia Commons under CC BY 3.0 license

During the Cretaceous period between 65 million and 145 million years ago, dinosaurs like the Tyrannosaurus Rex ruled the land, while another large reptile—the mosasaur—ruled the seas.

The traditional view of the animal was fearsome enough. Some species of mosasaurs were up to 50 feet in length, sporting long, slender tails that propelled them through the water. As they swam, they gobbled up anything in their way using massive jaws and two rows of teeth. But new research suggests they had another super weapon: Mosasaurs likely had massive pectoral muscles that allowed them to do a version of the breaststroke, giving them a burst of speed during predatory ambushes.

Over the last decade, researchers have been slowly piecing together the evolution of mosasaurs. Wynne Perry at Live Science reports that studies of the sea monsters shows that the species first went from the land into the water. In the beginning, they only had limited swimming ability. But within about 27 million years, they adapted to life in the sea—with their limbs transforming into powerful paddles and their tails turning into into powerful, flexible propulsion devices.

Initially, researchers thought the mosasaurs were “cruisers,” primarily using only their tails for long-distance swimming. But paleontologists also noted that many mosasaur fossils have very large pectoral girdles, the area that supports the muscles of the forelimbs. That’s why anatomists at the University of Southern California decided to take a closer look at how mosasaurs got around.

The team looked at the fossil of Plotosaurus, a species of mosasaur, at the Natural History Museum of Los Angeles County and also collected measurements of other mosasaur species from other studies. Their analysis found that the pectoral girdle was likely the site of large muscle attachments. Asymmetry in the bone structure also indicated that the limbs were used for adduction, the pull down motion used in the human breast stroke.

According to a press release, the team, which recently presented their findings at the Annual Meeting of the Geological Society of America, concluded that mosasaurs had powerful forelimbs that they used for “burst” swimming, likely to catch unsuspecting prey during an ambush.

“We know that mosasaurs most likely used their tails for locomotion. Now we think that they also used their forelimbs, or their tail and forelimbs together,” says lead author Kiersten Formoso, a USC paleontology Ph.D. student.

The combo of cruising and bursting puts mosasaurs in a unique category. “Like anything that swims or flies, the laws of fluid dynamics mean that burst versus cruising is a tradeoff,” says USC paleontologist and study co-author Mike Habib. “Not many animals are good at both.”

That combo probably helped the animals become dominant sea creatures for the final 30 million years of the Cretaceous period before disappearing from the world’s oceans about 65 million years ago, likely victims of the same asteroid that finished off the dinosaurs.

Researchers are slowly but surely uncovering new details about the mosasaur, which are related to the modern-day Komodo dragons and other monitor lizards. Just last year, paleontologists realized that fragments of a baby mosasaur skull were from an infant Tylosaurus, the largest type of mosasaur with a long toothy snout. The baby, however, was snub-nosed, meaning it likely developed its long snout after birth.

Figuring out just how mosasaurs moved and how fast they could swim will require more modeling and finding more fossils. But Formoso says one thing is certain: “Mosasaurs swam unlike anything else.”

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