"At that time the Peruvian sea was warm," added Salas-Gismondi, who heads the Department of Vertebrate Paleontology at the Natural History Museum in Lima. "Thanks to this type of fossil, we can reconstruct the history of the Peruvian sea."
The first cetaceans, like the basilosaurus, evolved from land animals some 55 million years ago.
By the late Eocene period (between 56 million and 34 million years ago), cetaceans had fully adapted to marine life.
Whales had not yet evolved, and almost all cetaceans were marine macropredators, according to the research team.
The Ocucaje Desert is rich in fossils, the researchers said, providing scientists with 42 million years' worth of evolutionary evidence.
Other fossils found there include four-legged dwarf whales, dolphins, sharks and other species from the Miocene period (between 23 million and five million years ago).