By the close of the period, therapsids had given rise to early mammals

By the close of the Triassic period, therapsids had given rise to early mammals.

The Triassic period is the era when the therapsid existed.
To be short, Therapsids (which means a group of species related to wild beasts in Latin) are the ancestors of the mammals. They competed in survival with dinosaurs from the Permian era until the Triassic era.

Nucleus is the organelle that contains chromosomes