When Nizar Ibrahim suggested in 2014 that Spinosaurus was the first known dinosaur built to swim, not everyone was convinced. Skeptics wanted a better explanation of how the giant creature would have propelled itself through the water, notes Live Science. Now, Ibrahim and other researchers say they have the answer: Spinosaurus had a tail perfectly suited for that purpose—"the most extreme aquatic adaptation ever seen in a large dinosaur," reports National Geographic. The 2018 discovery in southeastern Morocco, described Friday in the journal Nature, suggests Spinosaurus not only swam through an ancient river system but may have darted after prey including fish and crocodiles. In other words, it was "the Michael Phelps of the Cretaceous," per Live Science, which describes a tall, flat tail "like a fin."
"Delicate struts nearly two feet long jut from many of the vertebrae that make up the tail, giving it the profile of an oar," per National Geographic. An undulating tip would have created significant thrust. "This was basically a dinosaur trying to build a fish tail," says Ibrahim. The 80% complete tail is thought to come from a single individual, which would've been 50 feet, or longer than a Tyrannosaurus rex, when fully grown. Spinosaurus also had a 6-foot-tall dorsal sail, possibly webbed feet, and a long snout like that of a crocodile, though more fossils and more research are needed to determine how those features might have impacted its swimming ability. (Read more dinosaurs stories.)