A 165-million-year-old fossil has revealed an extraordinary Jurassic predator that may help explain how mammals developed one of their most important evolutionary advantages. Although only 20 inches long, Megacauda sungei was a heavyweight among early mammal relatives, swimming through ancient waterways and hunting prey with specialized teeth. Its advanced swallowing anatomy reveals an early step toward the ability to safely nurse on milk, a key development in mammalian evolution.