Sharp-toothed Spinosaurus didn’t just stand in the shallows to snag fish for dinner; this dinosaur may have been an excellent swimmer. Spinosaurus aegyptiacus, a new fossil discovery reveals, had a paddle-shaped tail that may have helped the predator slice through the water with the grace of a crocodile.
The fossilized tail, unearthed from 95-million-year-old rocks in Morocco, is the most complete Spinosaurus tail ever recovered. Its unusual shape suggests that this dinosaur may have been aquatic — contrary to prevailing wisdom that dinosaurs were solely land dwellers, researchers report in a study published online April 29 in Nature.
“It was basically a river monster,” says Nizar Ibrahim, a vertebrate paleontologist at the University of Detroit Mercy who led the study.
“When I first saw the illustrations of the tail, I literally giggled with surprise and delight — and I’m not someone who usually giggles,” says Matthew Lamanna, a vertebrate paleontologist at the Carnegie Museum of Natural History in Pittsburgh, who reviewed the paper for Nature. “The tail was just so awesomely weird-looking for a predatory dinosaur. I’d never seen anything like it.”
Spinosaurus was known to have lived near the water and to have dined on seafood: The animal’s cone-shaped teeth would have been adept at snagging slippery fish. “But for most people, the model they were more comfortable with was a wading dinosaur that waited for the fish to swim by,” the way a grizzly bear may splash into the water to catch a fish, Ibrahim says.
Ibrahim has previously proposed that Spinosaurus was more than an occasional wader. In a 2014 paper in Science, he and colleagues reported that the creature had denser bones than most other theropods, the branch of predatory dinosaurs that includes Tyrannosaurus rex and Allosaurus (SN: 9/11/14). Denser bones could be an evolutionary adaptation to a more aquatic life, allowing for greater buoyancy control.
The 2014 study was based on a fossil discovered in Morocco’s Kem Kem beds in 2008, first partially recovered by an amateur fossil hunter. Ibrahim’s team returned to the site and recovered more of the skeleton. After the 2014 paper was published, he went back again and targeted a large chunk of rock next to the original excavation site.
“We struck gold,” he says. Bone after bone emerged, revealing a tail that was nearly 80 percent complete. That tail contained very long neural spines, bony projections on the vertebrae, that together formed a tall fin shape, making the tail look like a paddle, he says.
On the underside of the tail are a row of v-shaped bones called chevrons, a feature that many dinosaurs have. In most dinosaurs, chevrons “are fairly long at the base of the tail, and then get shorter and shorter” toward the tip, Ibrahim says. But in Spinosaurus, the chevrons remain long until near the tail’s end.
Furthermore, the tail was surprisingly flexible. Many theropods had stiff, inflexible tails, which helped the dinosaurs balance their weight on land. But Spinosaurus’ tail had a wide range of movement that allowed it to swing sideways.
Whether that movement translated to the animal propelling itself through water is another question. Ibrahim and colleagues tested the possible power of Spinosaurus’ tail shape against that of other animals, including two terrestrial theropods and two modern, semiaquatic animals, a crocodile and a newt.
Plastic versions of the different tail shapes were swung back and forth by robotic controllers in a water tank, while sensors measured the tails’ propulsive power and thrust. The Spinosaurus tail performed nearly as well as the tails of the semiaquatic animals and far better than the other dinosaurs’ tails, Ibrahim says.
That finding suggests that Spinosaurus was able to actively pursue prey in the water, he adds. “At this point I think it’s the nail in the coffin of the idea that dinosaurs never invaded the aquatic world.”
It’s also possible, however, that the extended tail vertebrae were meant for display rather than swimming, notes Thomas Holtz, a vertebrate paleontologist at the University of Maryland in College Park who was not involved in the new study. “But to be fair, those aren’t mutually exclusive things,” he adds. And when the tail shape is considered along with other unusual attributes, such as the dinosaur’s dense bones, “there is a consistent pattern which is certainly suggestive of it being an aquatic animal.”
If true, that would mark a paradigm shift in our understanding of dinosaurs, previously thought to exclusively have been land animals, Holtz says. But there are similar types of exceptions. “We think of mammals as a terrestrial group, but we have whales and bats,” he says.
And Spinosaurus’ ancestors had stiffer tails more like those of other theropods, which suggests that the tail shape evolved over time, Holtz says. “It would be interesting to see if there turns out to be a member of Spinosaurus’ branch [that lived later] and was even more aquatic. It’s the sort of thing we want to keep our eyes out for.”