"Fossils of Pediastrum, an alga that today lives exclusively in freshwater, occur in marine sediments in Pakistan (lower Cretaceous) and California (upper Cretaceous). Therefore, fossil Pediastrum does not (as has been assumed) establish the freshwater origin of its enclosing sediments. The genus ranges back at least to the lower Cretaceous, and known Cretaceous forms closely resemble some modern ones."