During the late Cretaceous Period Gymnosperms were replaced Angiosperms. Angiosperms perform clear double fertilization followed by the formation of an endosperm through fertilization. While this characteristic is found only in angiosperms, there are exceptions. For example, where the ovule is encased in an ovary and possesses conductive vessels. Exceptional plants like these offer up clues for learning about the ancestors of angiosperms.