<P> 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 the evolutionary history of amniotes , 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 ) . </P>
<P> 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 the evolutionary history of amniotes , 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 ) . </P>
the two major lineages that make up the living amniota are
the two major lineages that make up the living amniota are
synapsids sauropsids
synapsids sauropsids