The discovery of a well-preserved Spinosaurus tail fossil is refining what we know about these ferocious carnivores, further affirming their role as aquatic hunters.
Spinosaurs are among the most frightening dinosaurs to have ever lived, rivaling even tyrannosaurs in size. New research published today in Nature describes the discovery of a well-preserved Spinosaurus tail, unearthed at the Kem Kem site southeastern Morocco. The tail’s unique anatomy suggests Spinosaurus, or at least Spinosaurus aegyptiacus, was more adapted to life in the water than previously thought.
This group of dinosaurs lived during the Cretaceous period, but not much is known about these theropods owing to the poor fossil record. The new fossil, found in 95-million-year-old sediment, is the most complete Cretaceous theropod ever to be found in mainland Africa. What’s more, it’s now the most complete—and only—Spinosaurus skeleton in existence, even though it’s just the tail plus some other bones. Other Spinosaurus fossils exist, but these pieces are loose and unassociated with other fossilized bones.
“This is quite the discovery for a variety of reasons,” Eric Gorscak, assistant professor of anatomy at Midwestern University in Downers Grove, Illinois, told Gizmodo. “Even though Spinosaurus was discovered over a century ago, Spinosaurus and many other dinosaurs are only known from a fraction of the skeleton—the hard truth most paleontologists live with when we look for fossils,” said Gorscak, who’s not affiliated with the new research.
As a sad aside, paleontologists used to have a partial Spinosaurus skeleton, which was kept at a museum in Munich, Germany, but it was destroyed during an air raid in the Second World War.
A modeling study conducted by the researchers suggests the tail, built from an array of elongated neural spines, was “unusually flexible,” allowing for “a pretty impressive range of movement,” explained Nizar Ibrahim, the lead author of the new study and an anatomist and paleobiologist at the University of Detroit Mercy, Detroit. He told Gizmodo that the tail was a “highly specialized propulsive structure” that would have allowed this dinosaur to actively pursue prey in the water.
Rocking its tail from side to side, this juvenile Spinosaurus aegyptiacus swam like a modern crocodile. The tail, said Ibrahim, was “built for locomotion,” and was unlikely used for something else, such a sexual display for attracting mates.
“It’s truly remarkable how just the discovery of one part of the body—this beautifully preserved tail, for instance—has the potential to surprise us in many ways,” said Gorscak, who described the creature as an “aquatic weirdo.”
“The discovery of new Spinosaurus material is always newsworthy, and it’s great that we can now pin a tail on the skeleton,” Steven Brusatte, a paleontologist from the University of Edinburgh who wasn’t involved in the new research, told Gizmodo. “It is a very unusual tail for a theropod dinosaur.”
This nearly complete tail corroborates a hypothesis made by the same team in 2014, though with fewer fossils as evidence, explained Brusatte—the claim being that the tail of Spinosaurus was adapted for side-to-side bending to provide propulsion during swimming.
Prior to the new study, indirect and circumstantial evidence pointed to spinosaurs as a semi-aquatic group of dinosaurs, including long narrow jaws (like a crocodile’s), cone-shaped teeth (for catching slippery prey like fish), dense bones to improve buoyancy control, and a retracted nose opening that was farther back on the skull compared to other predatory dinosaurs, explained Ibrahim. The retracted nose would’ve made it easier for Spinosaurus to breathe while surfacing.
Ibrahim, a scientist funded by the National Geographic Society, described this animal as a “river monster” that lived within a massive river system teeming with diverse life, especially fish. Some fish “were the size of cars, or bigger, and Spinosaurus had all the necessary adaptations to hunt these prey animals in the water,” he said. Spinosaurus aegyptiacus was built for life in the water, but it still needed to creep on land to lay its eggs.
“The biomechanical analysis is particularly interesting and welcome,” Brusatte told Gizmodo. “This new discovery therefore adds to the voluminous evidence that Spinosaurus was a dinosaur that fished in shallow water. No doubt Spinosaurus was an able swimmer in shallow waters, but its fossils are also found inland, so it probably was comfortable on land and in water.”
Gorscak said the new discovery will make it harder to argue against Spinosaurus having a more amphibious lifestyle than previously appreciated. He was a bit skeptical prior to this paper, but after seeing the fossils in this study, “I was pleasantly surprised,” said Gorscak.
“However, the aquatic-nature of other Spinosaurus relatives is still a mystery for two reasons: They are also known from incomplete skeletons, and the extent of the proposed aquatic traits are not as well-developed as we see in Spinosaurus,” said Gorscak. “But I would love to be surprised again.”
Brusatte said we shouldn’t infer too much beyond this species and jump to conclusions about how other dinosaurs might have adapted to aquatic lifestyles.
“If there was a larger diversification of aquatic dinosaurs, we would surely find their fossils alongside the many thousands of ichthyosaurs, plesiosaurs, pliosaurs, thalattosuchians, mosasaurs, and other purely aquatic, open-ocean-living reptiles of the Jurassic and Cretaceous,” said Brusatte. “The way it looks now, dinosaurs were never able to go into the open ocean the way whales did. It was their one big limitation over their hundred-plus million years of evolution.”
The primary reason for this, as noted earlier, is the need for dinosaurs to lay their eggs on land. But as the new fossil shows, at least one dinosaur found a niche in the water, where it undoubtedly unleashed its fair share of Mesozoic terror.