The amniotic egg represents a critical divergence within the vertebrates, one enabling amniotes to reproduce on dry land—free of the need to return to water for reproduction as required of the amphibians. From this point the amniotes spread around the globe, eventually to become the dominant land vertebrates. Very early in their evolutionary history, basal amniotes diverged into two main lines, the synapsids and the sauropsids, both of which persist into the modern era. The oldest known fossil synapsid is "Protoclepsydrops" from about 312 million years ago, while the oldest known sauropsid is probably "Paleothyris", in the order Captorhinida, from the Middle Pennsylvanian epoch (c. 306–312 million years ago).