The Mesozoic Era (250 - 65 million years ago), which includes the Triassic, Jurassic, and Cretaceous periods, is known particularly for the evolution of gigantic reptiles, both in the sea (ichthyosaurs, plesiosaurs, and mosasaurs) and on land ( dinosaurs). Flying reptiles and possibly the first birds, as well as the first primitive mammals, appeared during the later portion of the Triassic Period (250 - 192 million years ago). On land, during the Jurassic Period, forests of conifers and cycads had largely replaced the lycopod- and seed-fern - dominated forests of the Paleozoic Era. In the sea tiny calcite-armored, photosynthetic unicells called coccolithophorids appeared, and massive calcium carbonate (chalk) deposition began in the deeper oceans ( deep-sea ooze). The Cretaceous Period (144 - 65 million years ago) is the time of the wide expansion of two great groups of plants - the flowering plants (angiosperms) on land and the diatoms in water, although the first angiosperms may have appeared as early as the Triassic. Flowering plants also triggered a great wave of evolution among the insects. At the end of Cretaceous time, the extinction of dinosaurs resulted in the spectacular evolution of terrestrial mammals, and giant sharks and marine mammals replaced large reptiles in the sea.
The group of mammals known as the primates - now represented by lemurs, monkeys, apes, and humans - dates back to the beginning of the Cenozoic Era (65 million years ago to the present), but humanlike creatures are known only from the last few million years, the Pliocene and Pleistocene epochs. Homo sapiens, modern humans, appeared in the Old World during the Pleistocene but did not reach the Americas until its latter part.
Fossils record the progressive evolutionary diversification of living things, the progressive colonization of habitats, and the development of increasingly complex organic communities. The development of new species and such larger groups of species as genera and families has gone on throughout time, but so also has the loss of species by extinction. The rate of extinction at some times in Earth history greatly exceeded the rate of speciation, and the faunas and floras of the world became reduced. Notable biotic crises occurred in Cambrian time, near the end of the Devonian time, at the close ( Permian Period) of the Paleozoic Era, and at the end of the Cretaceous Period. Various theories have been advanced to explain the cause of these extinctions.