The early tetrapods split first into two major groups: the amphibians, which laid their eggs in water, and the amniotes, who laid their cleidoic eggs on land. The amniotes gave rise to two groups both of which became dominant at different times. They were the synapsids, which eventually gave rise to the mammals, and the sauropsids, which gave rise to the dinosaurs and other reptiles. These momentous events took place in the lower part of the Carboniferous, the Mississippian.