Mammaliaformes
Castorocauda, a member of Docodonta lived in the Middle Jurassic about 164 million years ago (mya). It was about  long, weighed , had limbs that were adapted for swimming and digging and teeth adapted for eating fish. Another docodont, Haldanodon, also had semi-aquatic habits. Aquatic tendencies were probably common among docodonts, since they lived in wetland environments.