Well over 60 years ago, a team of workers building a dam struck bone near the town of Tsuchiyu Onsen, in Fukushima, Japan. It was recognizably a femur, but didn’t look as though it belonged to any species alive today. Locals concluded that it must be a dinosaur bone, and put it on display in the Village Hall. Later, it found its way to the University of Tsukuba, where it languished in a wooden box for decades. Now, newly applied scrutiny has revealed the bone is not dinosaur, but belonged to an extinct hippo-like creature that lived long after the dinosaurs: Paleoparadoxia.

Paleoparadoxia lived in the waters of the northern Pacific coastal region around 16 million years ago. They were large and bulky, measuring over seven feet in length and weighing almost 2000 pounds, and feasted on seaweed and sea grasses from Alaska to Japan, and as far south as Mexico. They spent most of their lives in the water, prowling the bottom of the ocean floor for foliage like underwater hippopotamuses. Modern reconstructions suggest they may have looked like a cross between hippos and manatees, with an appealing rounded snout and webbed flipper-feet.

But it took some time for scientists to link the stray bone fragment with this prehistoric hippo. Their research, published this week in the Royal Society Open Science journal, was guided by a handwritten note left by its original finders and interviews with local people, which led them to where it had been found. When they returned to the original dig site, they took samples of the rock, and tested them to date the fossil. At less than 20 million years old, they could conclusively say it was not a dinosaur―since all dinosaurs died out some 44 million years prior. After further study, they concluded that it was Paleoparadoxia.

The right femur of the Paleoparadoxia. (Credit: Royal Society Open Science)

New technology makes it easier than ever to clear up the misidentifications of the past—of which there are many. One study suggests that as many as a third of all known dinosaur species may never have existed. Sometimes, that’s because juveniles and adult fossils look so different as to be virtually unrecognizable as kin; sometimes, it’s due to a jumble of bones from different animals found at the same site.

The dinosaur Nanotyrannus, for instance, looks a little like the famous Tyrannosaurus rex, though it’s leaner and more graceful, with 17 teeth on its lower jaw to T. rex’s 14. But recent studies have revealed that Nanotyrannus seems to have been little more than a teen T. rex—as it aged, its long skull grew thicker and less elongated, while those small, knife-like teeth fell out and morphed into bone-crushing gnashers.

Perhaps the most famous example of these mix-ups, however, is Brontosaurus. This famous dinosaur has been the victim of decades of wonky paleontology: one early model gave it the head of an entirely different species, Camarasaurus. In 1903, decades after it was first discovered, paleontologists decided that Brontosaurus was the same species as its look-alike Apotosaurus. Apotosaurus became the standard name, and Brontosaurus was canceled. Over a hundred years later, however, another study concluded that the original paleontologists had been right all along. Apotosaurus and Brontosaurus were subtly different, and each warranted their own species.

They aren’t mix-ups quite as egregious as mistaking a hippo for a terrible lizard—but it’s not hard to see how confusion of this sort might occur among early paleontologists and laypeople alike. After all, to the untrained eye, one fossilized bit of bone from millions of years ago looks very much like another.