 The third period brought emergence of the cycads, and the fourth by the development of the flowering plants (such as the dicotyledons).The transitions between each of these periods was marked by sharp discontinuities in the fossil record, with more gradual changes within the periods.Brongniart's work is the foundation of paleobotany and reinforced the theory that life on earth had a long and complex history, and different groups of plants and animals made their appearances in successive order.