References Cited

The geological time-period terms Cambrian, Ordovician, ..., Jurassic,..., Cretaceous, and on through the Quaternary, define successive changes in species of animals and plants through time on Earth. Thus, Ordovician trilobites differ from Devonian trilobites, Silurian and Devonian fish differ from Jurassic and Cretaceous fish, Mesozoic mammals differ from Cenozoic mammals, and so forth. In addition to changes occurring in many different species found in different geological time intervals, whole groups of organisms that were once abundant and diverse, such as trilobites, can become extinct. 

The boundaries between the great blocks of geologic time called  Eras are defined by major changes in the types of fossils found in the rocks deposited in those Eras: Paleozoic means “ancient animals,” Mesozoic means “middle animals,” and Cenozoic means “recent animals.” Trilobites and shelled animals called brachiopods are common and typical Paleozoic fossils. Dinosaurs, certain large marine reptiles, such as ichthyosaurs and mosasaurs, and the flying reptiles called pterosaurs are found only in Mesozoic rocks. Fossils of mammals, clams, snails, and bony fishes are typical of Cenozoic fossil assemblages. Some species can be found on both sides of a time boundary; however, the overall assemblage of organisms found in the rocks of a given age is recognizably different from the assemblages found in the rocks above and below.