In what period did mammals develop?

The earliest direct evidence for true mammalian life lies in a single fossilized brain case, found in China and described by Zhe-Xi Luo and Ai-Lin Sun in the 2001 issue of Science. The find, named Hadrocodium wui, dates to around 195 million years ago, placing the origin of true mammals sometime before the early Jurassic Period and perhaps even the late Triassic Period.