Exclusive: Bizarre Spinosaurus makes history as first known swimming dinosaur

At the end of a dim hallway in Casablanca’s Université Hassan II, I’ve walked into a dusty room containing a remarkable set of fossils. Bones that raise foundational questions about Spinosaurus aegyptiacus, one of the weirdest dinosaurs ever discovered.

Longer than an adult Tyrannosaurus rex, the 16 metre (50 foot)-long, seven-tonne 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.

The red-brown remains laid before me are altering that picture. These bones 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, and to my shock, the appendage resembles a giant bony paddle.

Described today in the journal Nature, this tail is the most extreme aquatic adaptation ever seen in a large dinosaur. Its discovery in Morocco stretches our understanding of how one of Earth’s most dominant groups of land animals lived and thrived.

Exclusive: Bizarre Spinosaurus makes history as first known swimming dinosaur

Shovels scrape and pickaxes fly as crew members chip away at Morocco’s Zrigat site, where paleontologist Nizar Ibrahim and his colleagues have been excavating a Spinosaurus skeleton.

Delicate struts nearly two feet long jut from many of the vertebrae that make up the tail, giving it the profile of an oar. By the end of the tail, the bony bumps that help adjacent vertebrae interlock practically disappear, letting the tail’s tip undulate back and forth in a way that would propel the animal through water. The adaptation probably helped it move through the vast river ecosystem it called home—or even dart after the huge fish it likely preyed upon.

“This was basically a dinosaur trying to build a fish tail,” says National Geographic Emerging Explorer Nizar Ibrahim, the lead researcher examining the fossil.