More than 60 years ago, a rare thighbone was discovered in Japan. At the time it was thought that the fossil belonged to a dinosaur, but only recently did a research team discover that it actually belonged to an ancient hippopotamus creature that lived on Earth long after the extinction of the dinosaurs.
The Femoral Fossil was discovered in the 1950s by workers building a dam near the city of Tsuchiyu Onsen in Fukushima, Japan. While the locals did not pay much attention to the discovery, the researchers classified the bone as belonging to an ancient dinosaur.
The fossil ended on a small village display, but in a few years a part of it The city was destroyed by fire and the remains were donated to the University of Tsukuba, Japan. It remained in a wooden box for decades as part of the university's geological collection until a group of scientists discovered the strange bone and instituted an investigation to investigate its origins.
As the team reported in a newspaper, the fossil found in the box was accompanied by the original investigators with a handwritten note. Usually, the labels provided with museum fossils, such as the ones examined here, contain information about the specimen in question, which allows further investigation, but in this case nothing was mentioned except the name and local address of a person.
The team used the information available to visit the city of Tsuchiyu Onsen. There, they carried out a detailed field work in which the locals living there were interviewed and archived documents were checked. All the effort that revealed the "dinosaur" part of the story took them to the discovery site where they tested rocks with zircons ̵
After combining the results of the work and the comparative English: www.dlr.de/en/DesktopDefault.aspx/t…mage.1.3579/ The team concluded that the fossil in question was from a long – extinct fossil marine herbivore, which belongs to the genus Paleoparadoxia. The mammal, as the researchers have described, looked like a modern hippopotamus and lived at least 15.9 million years ago, ten million years after the extinction of the dinosaurs.
The researchers described the fossil as the best preserved femoral surface of paleoparadoxia and created a 3D reconstruction of the creature to show what it would look like. 
Many fossils and post-cranial remains were found in the past. The extinct genus was discovered by today's Pacific Ocean. It is believed that the herbivorous creature lived 20 to 10 million years ago and combed waters between Japan, Alaska and New Mexico. Initial work indicated that these creatures were amphibians, but recent analyzes have supported the notion that they were fully marinated and walked on the seabed feeding on seaweed and grasses.
The study titled "A Long-Forgotten" Dinosaur Bone from a Museum Cabinet exposed to an iconic extinct mammal, Paleoparadoxia (Desmostylia, Mammalia), was published July 25 in the Royal Society Open Science.