The shells of tiny foraminifera (nickname forams) have left a fossil record dating to the beginning of the Cambrian, about 550 million years ago. The earliest forams, the benthic forams, lived in ocean bottom sediments. Later, during the Jurassic, floating forams evolved (planktic forams). Both benthic and planktic forams underwent many periods of diversification and extinction that show up as changes in the distribution of fossil foram shells in sediments of different ages. Species of forams have tended to persist on Earth for about 0.5-20 million years, a short period relative to geologic time. Scientists can, therefore, determine the age of sediments based on the fossil foram shells found in them. Because some forams are very sensitive to environmental conditions such as temperature, oxygen concentration, salinity, and food availability, their fossils offer evidence of what conditions were like in the ancient past. Changes in foram distributions over time track environmental changes, including global climate change.