Evolution of the freshwater ecosystem: The fossil record
Abstract The freshwater ecosystem may be as old as most life. Although freshwater life is taxonomically impoverished in comparison with marine and terrestrial biotas and ultimately derived from them, it cannot be assumed that freshwater communities have necessarily been less complex in the geologic past than they are today, although it is clear they have undergone a number of major changes. Freshwaters contain not only their own ancient and peculiar groups, some with ancestries as far back as the Middle Paleozoic, but they have been a theater for adaptive radiation of more recent immigrants from the land or sea, and a way-station for still other groups that passed through, both from sea to land and the reverse. This review summarizes first fossil records in freshwaters for many of these organisms, and examines the question of the changing complexity of the freshwater ecosystem through time by looking at the fossil record of continental aquatic habitats.