From approximately 240 to 66 million years ago, dinosaurs dominated most of the land habitats on Earth. Around 66 million years ago, most dinosaurs went extinct when an asteroid struck the Earth. Mammals, which were much smaller and lived primarily in underground habitats, were able to avoid extinction. Mammals then underwent an adaptive radiation to develop a wide variety of new species.

oxygen is the first element in group 16 (via) of the periodic table. the periodic table is a chart that shows how chemical elements are related to each other. the elements in group 16 are said to belong to the chalcogen family. other elements in this group include  sulfur, selenium, tellurium,  and  polonium.  the name chalcogen comes from the greek word  chalkos,  meaning "ore." the first two members of the family, oxygen and sulfur, are found in most ores.

oxygen is by far the most abundant element in the earth's crust. nearly half of all the atoms in the earth are oxygen atoms. oxygen also makes up about one-fifth of the earth's atmosphere. nearly 90 percent of the weight of the oceans is due to oxygen. in addition, oxygen is thought to be the third most abundant element in the universe and in the solar system.