Paleontologists have discovered a new type of fossil turtle that was five feet across and almost perfectly round.
Named Puentemys mushaisaensis, it lived 60 million years ago in what is now northwestern South America, and was discovered by a team from the Smithsonian Tropical Research Institute in Panama, North Carolina State University and the Florida Museum of Natural History.
It was found in La Puente pit in Cerrejón Coal Mine - famous for the discovery of a whole menagerie of ancient creatures, including Titanoboa, the world's biggest snake, and more recently Carbonemys, a freshwater turtle as big as a Smart car.
Indeed, all Cerrejon's fossil reptiles seem to be extremely large, and this latest discovery represents more evidence that following the extinction of the dinosaurs, tropical reptiles were generally much bigger than they are now.
The most peculiar feature of this new turtle is its extremely circular shell - about the size and shape of a big car tire, says Edwin Cadena, post-doctoral fellow at North Carolina State University. Its round shape, he says, could have discouraged predators, as it was probably just too wide for Titanoboa's mouth.
Its circular, low-domed shape would also have helped it stay warm, by increasing its volume-to-surface-area ratio.