It’s a summer’s day in 2013 and paleontologist Robert DePalma is digging for fossil remains at a site in North Dakota. He hits pay dirt, a fossilized specimen of a five-foot paddlefish. Much to his surprise, beneath the fish is a tooth that belonged to a mososaur, a massive extinct reptile. But the finds don’t make sense since the paddlefish lived in freshwater while the reptile was a sea creature. And DePalma’s explanation of this anomaly will astonish – and divide – the world of paleontology.