The scientists said in a new study, published in the journal Nature Communications on Wednesday, that during the Mesozoic era, about 252 to 66 million years ago, the seas were full of reptiles like the nothosaur. While scientists knew that these creatures were predators with long bodies and paddle-like limbs, exactly how they used their limbs to propel themselves through water was unknown. Now, recently discovered trackways on an ancient seabed in Yunnan province in southwestern China, have helped scientists find an answer.