to bristles, making them the largest toothed animal to roam the sea. Most species live in shallow areas of tropical and temperate oceans throughout the world. Five species live in the world’s rivers. There are two major aquatic or water biomes, the marine biome and the freshwater biome. The marine biome is primarily made up of the saltwater oceans. It is the largest biome on planet Earth and covers around 70% of the Earth’s surface. Go here to learn more about the world’s different oceans. However, more than 99 percent of all species that ever lived on Earth, amounting to over five billion species, are estimated to be extinct. These extinctions occur at an uneven rate. The dramatic rise in diversity has been marked by periodic, massive losses of diversity classified as mass extinction events. Mass extinction events occur when life undergoes precipitous global declines. Most diversity and biomass on earth is found among the microorganisms, which are difficult to measure. Recorded extinction events are therefore based on the more easily observed changes in the diversity and abundance of larger multicellular organisms, rather than the total diversity and abundance of life. Marine fossils are mostly used to measure extinction rates because of their superior fossil record and stratigraphic range compared to land organisms. Many marine animals begin life as zooplankton in the form of eggs or larvae, before they develop into adults.