A turtle that lived 60 million years ago in what is now northwestern South America was nearly round with a shell the size of a big truck tire, scientists say. Paleontologists at the Smithsonian Tropical Research Institute in Panama and colleagues at North Carolina State University and the Florida Museum of Natural History discovered the new species of fossil turtle in the Cerrejon Coal Mine in Colombia, famous for the discoveries of the extinct Titanoboa, the world\'s biggest snake, and Carbonemys, a freshwater turtle as big as a small car, a Smithsonian release reported Wednesday. The new species, Puentemys mushaisaensis, was about 5 feet in diameter and adds to growing evidence that following the extinction of the dinosaurs, tropical reptiles were much bigger than they are now, researchers said. Its extremely circular shell, about the size and shape of a big truck tire, could have discouraged predators, including Titanoboa -- which would have had a hard time swallowing the turtle -- and may have aided in regulating its body temperature by increasing body area exposed to the sun, study lead author Edwin Cadena of North Carolina State said.