Complex sentence: Flowers are modified leaves possessed only by the angiosperms, which are relatively late to appear in the fossil record; the group originated and diversified during the Early Cretaceous and became ecologically significant thereafter.
Split into simpler sentences: Flowers are modified leaves possessed only by the angiosperms, which are relatively late to appear in the fossil record. The group originated and diversified during the Early Cretaceous and became ecologically significant thereafter.