Could T. rex swim? Kinda

When Michael Crichton released the novel Jurassic Park in 1990, he included a terrifying chase through one of the park’s ponds. Fictional paleontologist Alan Grant, looking after kids Lex and Tim, tries to sneak by a dozing Tyrannosaurus rex and motor across a lake away from the dinosaur. The T. rex follows, swimming after them like “the biggest crocodile in the world.” The scene stood out so much that it’s been revamped for the new movie Jurassic World: Rebirth. But while tyrannosaurs might swim in movies and books, could they swim in real life?

Carnivorous dinosaurs are not generally thought of as good swimmers. During much of the 20^th century, in fact, paleontologists wrongly assumed that herbivorous dinosaurs ran into rivers and lakes to avoid the jaws of T. rex, Allosaurus, and other predators. No one had found any direct evidence that such carnivores could swim. But the discovery of dinosaur swim tracks at fossil sites around the world have indicated that theropod dinosaurs—the group that contains T. rex, birds, and their relatives—were more aquatically adept than suspected and may have even done their own version of the doggy paddle.

At one 200 million-year-old fossil site in southern Utah, paleontologists have found over 2,500 scratches and traces made by small carnivorous dinosaurs swimming across a Jurassic lake. Over 120 million years ago, a larger theropod dinosaur swam through the shallows in what’s now La Rioja, Spain. Another fossil site found in La Rioja has even allowed paleontologists to begin distinguishing between different types of swimming traces left by theropods kicking through the water, indicating that swimming was not unusual for feathery, sharp-toothed dinosaurs.

To date, no one has found swim traces from a tyrannosaur. Paleontologists have found some rare tyrannosaur footprints, but not direct evidence of swimming. Still, University College London paleontologist Cassius Morrison notes, today, “the majority of animals can swim” even without specific aquatic adaptations, and the fossil evidence of other swimming theropods suggests that big tyrannosaurs could, too. The question is how they would have done it.

(Could dinosaurs swim? This fossil revives an age-old debate.)

What did tyrannosaur swimming look like?

Mature T. rex were very large animals. The biggest reached more than 40 feet in length and weighed over nine tons. Hefty as that is, however, nine tons is relatively light for such a big animal. The secret is that tyrannosaurs, like many dinosaurs, had a complex system of air sacs that branched out from its respiratory system and infiltrated the dinosaur’s bones, just like in birds today. The air sacs allowed the dinosaur to be a little lighter without sacrificing strength, allowed the animal to breathe more efficiently, and, in the water, to float a little more easily. 

The effects of dinosaur air sacs on swimming ability has been underscored by the bones of another giant carnivore and Jurassic Park alum—the croc-snouted, sail-backed Spinosaurus. While researchers debate how much time the paddle-tailed dinosaur spent in the water, fossil evidence suggests it had extra-dense bones. These heavier bones helped the dinosaur avoid being too buoyant, so that it could more easily use its muscle power to move through the water rather than actively working to stay submerged like we do when we go under water with lungs full of air.

(Read more about Spinosaurus’ penguin-like bones.)

Spinosaurus had a skeleton adapted to dealing with the buoyancy of the air sacs, but other dinosaurs without such dense bones would have only been capable of a more unstable doggy paddle. Giant long-necked dinosaurs, for example, have been described as “tipsy punters” that were relatively unstable in the water and could kick off the bottom but not swim the way a crocodile does.

For the same reason, T. rex probably wouldn’t be able to completely disappear beneath a lake’s surface and burst out with open jaws, like in the movies. And even though the dinosaur’s arms were too small and lacked the range of motion for a swim stroke, the same is true of many other carnivorous dinosaurs that left swim traces behind. The emerging picture is that T. rex was probably a strong, if unsteady, swimmer. The available evidence hints that a swimming T. rex would float near the surface of a body of water, using its powerful legs to kick along to cross. 

Did T. rex stalk prey in the water?

T. rex’s swimming abilities would have inevitably shaped its hunting strategy. In 2023, University of the Republic of Uruguay paleontologist R. Ernesto Blanco modeled how quickly T. rex could move through the water. He proposed that the tyrant lizard would have been too slow to catch prey like the duckbill Edmontosaurus and the ostrich-like Struthiomimus on dry land but could move faster while wading or swimming in shallow water.

“Depending on the water depth, T. rex would have different ways of propulsion,” Blanco says. In deep enough water, T. rex could have swum with most of its body under the surface, but more often the dinosaur likely waded or “punted” off the bottom as the swim traces of other theropods indicate. Perhaps, Blanco suggested, T. rex preferred to hunt along shorelines where herbivores trying to escape in the water would have been slowed down and more vulnerable.

Other experts are not yet convinced that T. rex preferred hunting and feeding along shorelines. The bulk of the evidence so far points to the reptiles ambushing prey on land and breaking any carcasses it could find down into splinters with its impressive jaw strength. Finding swim traces, tyrannosaur poop with aquatic animal remains, or other fossil evidence could help test the idea further.

Nevertheless, splashing around in the water seems within the range of what T. rex could do during its Cretaceous days. Swimming—even awkwardly—was a useful ability in ancient lowland habitats that likely resembled the wetlands and swamps along the coast of the Gulf of Mexico today. Being able to cross such waterways and wet habitats would have been advantageous to big tyrannosaurs, and it’s likely the dinosaurs did so at times. “With current evidence available to us,” Morrison says, “I would suggest that swimming may have been a way for Tyrannosaurus to navigate its environment.” 

The answer to why T. rex crossed the lake, in other words, may have been that lunch was on the other side.