All tetrapods are believed to have evolved from crocodile-like Elpistostege watsoni fish that inhabited Earth during the Devonian period (Credit: Katrina Kenny/CC-BY-SA 2.0/The Conversation)

Researchers have long believed that all four-limbed animals or tetrapods, a group that includes amphibians, reptiles, and mammals, evolved from an ancient group of fish known as Elpistostege watsoni, which lived between 416 million and 358 million years ago, during the Devonian Period.