Breakthroughs in Paleobotany: The Rise of Plants on Land and Origin of Complex Reproductive Systems

The evolutionary conquest of land occurred sometime between the late Cambrian and early Silurian. By the Silurian vascular land plants had evolved life cycles similar to those of bryophytes and ferns. Reproductive adaptations brought about rapid evolutionary changes in the land plant life cycle and decreased dependence on water. The evolution of the seed during the Devonian was one of the most important events in plant evolution and caused profound changes in the earth's landscape. Seed plants began to dominate sites not previously available to plants (homosporous and heterosporous) whose life cycles tied them to water. Although our understanding of the evolution of the seed developed by the 1970's, details of seed plant reproduction have only recently been appreciated. “Hydrasperman reproduction” in the earliest seed plants involved the trapping of wind-borne pollen by the elaborate nucellar apex, free integumentary lobes and surrounding cupules. Modern appearing gymnosperm life cycles arose in the Carboniferous when the total enclosure of the megasporangium was complete, and reproduction was accomplished by pollination droplets, pollen tubes and true pollen. Total enclosure of seeds by a carpel (angiospermy) did not evolve until the Lower Cretaceous. This was the most important innovation in land plants that affected the appearance of today's landscape. Rapid reproductive cycles of flowering plants and their co-evolution with insects caused an explosion in diversity in the Cretaceous. The Lower Cretaceous Apple Bay flora from Vancouver Island provides a unique perspective on the time of transition from primarily gymnosperm and fern dominated floras to angiosperm dominated floras of the later Cretaceous. Both seed ferns and bennettitaleans show seeds with enclosing structures and unique reproductive syndromes. Flowering plants and conifers of the Pinaceae were undergoing rapid evolution during this time, and like their early ancestors, some of the flowering plants returned to the water.