Spinosaurus Was a Good Floater but Lousy Swimmer

Since the 1980s, experts have been wondering if this giant predatory dinosaur was amphibious, sticking largely to aquatic habitats. The debate thickened in 2014.

For prehistoric scholars, Spinosaurus has been a very elusive dinosaur. Its fossils are rare, and those that are found (Spinosaurus skull bones, tail vertebrae, limb bones) tend to be fragmented or incomplete, making skeletal reconstruction that much more difficult.

So when a load of fresh information was announced in 2014, natural history fans got pretty excited. Paleontologist Nizar Ibrahim and his colleagues wrote a now-famous paper describing the partial skeletons of at least two individual Spinosaurus that were found in Morocco.

Using these bones, Ibrahim's team reinterpreted the way Spinosaurus might have looked and behaved. Scientists used to assume that, like most carnivorous dinosaurs, the animal's hind legs were significantly longer than its arms.

But upon reviewing the new specimens, Ibrahim and his team concluded that Spinosaurus was a short-legged giant — a weird-looking beast with an estimated body length of 50 feet (15.5 meters) and disproportionately small hind limbs.

Such a creature seemed ill-suited for walking around on two legs over dry land. Pointing to its weird body shape (along with some other features), Ibrahim and his co-authors said Spinosaurus was a semiaquatic predator who swam after fish in its marshy African habitat 97 million years ago.

A new paper calls this into question. Our friend Spinosaurus may have had an affinity for waterways, but according to a series of computer simulations, it had relatively poor aquatic locomotion; basically, it was a mediocre swimmer.

The results of these digital trial runs were published in the Aug. 16, 2018 edition of PeerJ, an open-access scientific journal. Donald M. Henderson, who serves as the curator of dinosaurs at the Royal Tyrrell Museum, in Drumheller, Alberta, Canada, led the study.

Henderson's goal was to assess Ibrahim's earlier claims that Spinosaurus was semiaquatic. To do this, Henderson built a 3D virtual model of the dinosaur based on illustrations and photographs from the 2014 paper.

He used the same software to create digital recreations of five other theropod (meat-eating) dinosaurs:

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

Since all experiments need a control group, Henderson also made digital replicas of the American alligator and the emperor penguin, two living animals with well-documented semiaquatic lifestyles.

In his simulations, both of these models floated exactly as their real-life counterparts do, validating Henderson's methods.

Once he'd assembled his digital dinos, Henderson was ready to toss them into a virtual freshwater lake. The results indicated that Spinosaurus was not an especially gifted swimmer. While its body mass floated in tranquil water, the model Spinosaurus was able to keep its nostrils safely above the surface — but so did the other five dinosaur replicas.

In other words, there was nothing unique about the fin-backed carnivore's performance here.

Also, the fin itself turned out to be a serious handicap. The sail on Spinosaurus' back likely stood more than 6 feet (2 meters) tall at its apex and — according to Ibrahim's crew—it would have weighed about 738 pounds (335 kilograms) when covered in muscles, tissues and skin.

Trying to swim with such a large piece of flair on its back may have been challenging for the dinosaur. Alligators can keep their bodies upright even as they move through choppy water, but Henderson's floating Spinosaurus model tipped over to one side whenever it was nudged.

He surmised that the animal would need to be constantly pumping its arms and legs in order to stay on an even keel and avoid rolling over. Doggy paddling is not exactly an energy-efficient solution.

If you're a semiaquatic predator, sinking on command is just as important as staying afloat. After all, you've got to be able to pursue victims underwater when necessary.

Try as he might though, Henderson couldn't sink his Spinosaurus.

Birds have a complicated respiratory system: In addition to their lungs, they've got a network of air sacs that are connected to hollow, air-filled bones. There is ample evidence to suggest that non-avian theropod dinosaurs had this same apparatus, which Henderson took that into account while building his computer models.

The recreated digital Spinosaurus would have been too buoyant to sink because its calculated density was lower than that of fresh water.

This remained true even when Henderson deflated its lungs by 75 percent, got rid of the air sacs and increased the density of its skeleton. None of those alterations did the trick; his Spinosaurus remained afloat. (For the record, the virtual alligator sank when a mere 40 to 50 percent of the air left its lungs.)

Ibrahim told National Geographic that he "welcomed" Henderson's study. However, he notes that the Royal Tyrell scientist did not personally inspect the Spinosaurus fossils mentioned in the 2014 paper before carrying out his computer tests. Had Henderson done so, Ibrahim thinks the digital run-throughs may have gone differently.

Regardless of whether Spinosaurus was a good swimmer or not, it's clear that the dinosaur spent a lot of time around waterways. The creature's cone-shaped teeth were ideal for skewering fish, and half-digested piscine remains have been found in the belly cavity of a Baryonyx (whom, you'll recall, was akin to Spinosaurus).

Henderson's computer experiments suggest that Spinosaurus' center of mass was located just in front of the rear legs. If this is true, then the beast would have had an easier time walking around bipedally (on two legs) than Ibrahim's team previously thought.

Rather than swim after fishes, Spinosaurus might have caught them by wading in shallow water, as grizzly bears do. Or, perhaps it made like a heron and snatched fishy prey from the shoreline.