Some of the planet's first mammals have Mesozoic origins, including the castorocauda, an early aquatic mammal similar to the contemporary beaver, and the morganucodon, a critter that was characterized by a strange blend of both reptilian and mammalian characteristics. According to About.com, there were a variety of animals walking and scurrying about the planet during the Mesozoic Era, a period of time when small mammals came into being and dinosaurs reigned supreme.
Perhaps the most significant animals of the period were dinosaurs. The smallest dinosaurs emerged at the beginning of the Mesozoic Era during the Triassic Period, followed by the gigantic dinosaurs during the Jurassic Period. Dinosaurs eventually became extinct at the close of the Cretaceous Period — which marked the end of the Mesozoic Era. There were also small reptiles and birds during the 180-million-year Mesozoic Era.