Bones that raise foundational questions about Spinosaurus aegyptiacus, one of the weirdest dinosaurs ever discovered.

Longer than an adult Tyrannosaurus rex, the 50-foot-long, seven-ton predator had a large sail on its back and an elongated snout that resembled the maw of a crocodile, bristling with conical teeth. For decades, reconstructions of its bulky body have ended in a long, narrowing tail like the ones on its many theropod cousins.

These bones of Dinosaur assemble into a mostly complete tail, the first yet found for Spinosaurus. It’s so large, five tables are required to support its full length, the appendage resembles a giant bony paddle.
The structure of the bones—along with state-of-the-art robotic modeling of the tail’s movement—add fresh and compelling evidence to an argument that has raged for years among paleontologists: How much time did Spinosaurus actually spend swimming, and, by implication, how close did large predatory dinosaurs ever get toward a life in the water? In 2014, researchers argued that the predator was the first confirmed semiaquatic dinosaur, a hypothesis that generated pushback from peers who questioned whether the team were studying was actually Spinosaurus, or even a single individual.

By the time of Spinosaurus, 95 to 100 million years ago in the Cretaceous period, several groups of reptiles had evolved to live in marine environments, such as the dolphin-like ichthyosaurs and the long-necked plesiosaurs. But those dino-era sea monsters sit on a different branch of the reptile family tree, while true dinosaurs like Spinosaurus have long been believed to be land dwellers.

Now, with evidence from the newly analyzed tail, there’s a strong case that Spinosaurus didn’t merely flirt with the shore but was capable of full-fledged aquatic movement. Collectively, the findings published today suggest the giant Spinosaurus spent plenty of time underwater, perhaps hunting prey like a massive crocodile. “This tail is unambiguous,” says team member ,a paleontologist at the Université . “This dinosaur was swimming.”

Other scientists who have evaluated the new study agree that the tail puts some lingering doubts to rest and strengthens the case of a semiaquatic Spinosaurus.