Complex sentence: They replaced the pelycosaurs as the dominant large land animals in the Middle Permian and were replaced, in turn, by the archosauromorphs in the Triassic, although one group of therapsids, the kannemeyeriiforms, remained diverse in the Late Triassic.
Split into simpler sentences: They replaced the pelycosaurs as the dominant large land animals in the Middle Permian. They remained the dominant large vertebrate fauna until replaced by archosaurs in the Middle Triassic, although one group of therapsids, the kannemeyeriiforms, remained diverse in the Late Triassic.