Spinosaurus was great at staying afloat, but struggled when it came to swimming.

Key Insights

With its massive and striking dorsal sail, the Spinosaurus stands out as one of the most visually memorable dinosaurs.

You might recall Spinosaurus aegyptiacus as the antagonist in "Jurassic Park III." In the 2001 film, this sail-backed predator emerges from the water to ambush the human passengers aboard a barge.

It's an exciting scene, but was this really how Spinosaurus hunted?

A Semi-Aquatic Predator

Since the 1980s, scientists have been speculating whether this enormous predatory dinosaur was adapted to life in the water, primarily inhabiting aquatic environments. The discussion took a major turn in 2014.

For paleontologists, Spinosaurus remains an enigmatic species. Its fossils are scarce, and the ones that are discovered (skull fragments, tail vertebrae, limb bones) are often broken or incomplete, complicating efforts to fully reconstruct its skeleton.

When a flood of new discoveries was revealed in 2014, fans of natural history were thrilled. Paleontologist Nizar Ibrahim and his team published a groundbreaking paper detailing the partial skeletons of at least two Spinosaurus specimens found in Morocco.

Using these bones, Ibrahim and his colleagues rethought the appearance and behavior of Spinosaurus. Previously, scientists believed that, like most carnivorous dinosaurs, the creature's hind legs were much longer than its arms.

However, after examining the new specimens, Ibrahim's team determined that Spinosaurus was actually a short-legged giant — a bizarre creature measuring around 50 feet (15.5 meters) long, with unusually small hind limbs.

This strange anatomy didn’t seem to lend itself to bipedal movement across dry land. Based on its unusual body structure (and several other traits), Ibrahim and his collaborators argued that Spinosaurus was a semi-aquatic predator, likely swimming after fish in its swampy African environment 97 million years ago.

A recent study challenges this view. While Spinosaurus may have been drawn to water, a series of computer simulations suggest it wasn’t much of a swimmer — it was, in fact, a mediocre one at best.

Digital Modeling and Ancient Fossils

The findings from these digital simulations were published in the Aug. 16, 2018 issue of PeerJ, an open-access academic journal. The study was led by Donald M. Henderson, the curator of dinosaurs at the Royal Tyrrell Museum in Drumheller, Alberta, Canada.

Henderson's aim was to evaluate Ibrahim's earlier assertion that Spinosaurus was semi-aquatic. To achieve this, Henderson created a 3D virtual model of the dinosaur based on images and diagrams from Ibrahim's 2014 paper.

He used the same software to digitally reconstruct five other theropod (carnivorous) dinosaurs:

The last one is a close relative of Spinosaurus aegyptiacus, belonging to the same family.

Since every experiment requires a control group, Henderson also created digital models of the American alligator and the emperor penguin—two modern animals with well-documented semiaquatic behaviors.

In his simulations, both of these models floated just as their real-world counterparts do, reinforcing the accuracy of Henderson's methodology.

The Breaking Point

After assembling his digital dinosaurs, Henderson proceeded to drop them into a virtual freshwater lake. The results suggested that Spinosaurus was not particularly adept at swimming. While its body remained buoyant in still water, the model Spinosaurus managed to keep its nostrils safely above the surface—but so did the other five theropod models.

Simply put, the fin-backed predator didn't demonstrate anything exceptional in this test.

Moreover, its massive sail turned out to be a major disadvantage. The Spinosaurus' distinctive back fin likely reached over 6 feet (2 meters) in height at its peak and—according to Ibrahim's team—would have weighed around 738 pounds (335 kilograms) when covered with muscles, skin, and tissue.

Navigating the water with such an enormous structure on its back must have been a struggle. Alligators maintain their balance even in turbulent water, but Henderson’s digital Spinosaurus model keeled over to one side whenever it was disturbed.

He concluded that the dinosaur would have needed to continuously move its limbs to maintain balance and avoid capsizing. A constant doggy paddle is hardly an efficient way to swim.

Spinosaurus Could Stay Afloat but Struggled to Dive

For a semi-aquatic predator, being able to sink on command is just as crucial as staying afloat. After all, to catch prey underwater, you need the ability to dive.

Despite his best efforts, Henderson couldn't get his Spinosaurus to sink.

Birds possess a complex respiratory system: In addition to their lungs, they have a series of air sacs that connect to their hollow, air-filled bones. There's strong evidence suggesting that non-avian theropod dinosaurs shared this same system, something Henderson factored into his digital models.

The recreated Spinosaurus model remained too buoyant to sink because its calculated density was less than that of fresh water.

Even when Henderson reduced its lung volume by 75%, removed the air sacs, and increased the skeleton's density, the Spinosaurus still refused to sink. For comparison, the virtual alligator sank when just 40 to 50% of the air was removed from its lungs.

Spinosaurus on the Waterfront

Ibrahim told National Geographic that he "welcomed" Henderson's research. However, he pointed out that the Royal Tyrrell scientist did not examine the actual Spinosaurus fossils from the 2014 study before running his digital simulations. Had he done so, Ibrahim believes the results might have been different.

Regardless of its swimming abilities, Spinosaurus clearly spent a great deal of time near water. Its conical teeth were perfect for spearing fish, and half-digested fish remains have been discovered inside the stomach cavity of a Baryonyx—a close relative of Spinosaurus.

Henderson’s computer analysis indicated that Spinosaurus’ center of mass was positioned just ahead of its hind legs. If accurate, this would mean the dinosaur was more capable of walking bipedally than Ibrahim’s team had previously suggested.

Instead of actively swimming after fish, Spinosaurus may have hunted by wading in shallow waters, much like a grizzly bear. Alternatively, it could have behaved more like a heron, standing at the water’s edge and snatching up prey.