 All molecular studies have strongly upheld the placement of turtles within diapsids, most commonly as a sister group to extant archosaurs.With the close of the Carboniferous, the amniotes became the dominant tetrapod fauna.While primitive, terrestrial reptiliomorphs still existed, the synapsid amniotes evolved the first truly terrestrial megafauna (giant animals) in the form of pelycosaurs, such as "Edaphosaurus" and the carnivorous "Dimetrodon".