The Cenozoic Era (pronunciation: /ˌsiːnəˈzoʊɪk, ˌsɛ-/; also Cænozoic, Caenozoic or Cainozoic pronunciation: /ˌkaɪnəˈzoʊɪk, ˌkeɪ-/; meaning "new life", from Greek καινός kainos "new", and ζωή zoe "life") is the current and most recent of the three Phanerozoic geological eras, following the Mesozoic Era and covering the period from 66 million years ago to present day. The Cenozoic is also known as the Age of Mammals and the Age of Birds, because of the large mammals and birds that dominated, such as Entelodont, Paraceratherium, Gastornis and Basilosaurus. The extinction of many large Diapsid groups such as non-avian dinosaurs, Plesiosauria and Pterosauria allowed the mammals and birds to greatly diversify and be the predominant fauna, even to the present day. Early in the Cenozoic, following the K-Pg event, the planet was dominated by relatively small fauna, including small mammals, birds, reptiles, and amphibians. From a geological perspective, it did not take long for mammals and birds to greatly diversify in the absence of the large reptiles that had dominated during the Mesozoic. Some flightless birds grew larger than the average human. These species are sometimes referred to as "terror birds," and were formidable predators. Mammals came to occupy almost every available niche (both marine and terrestrial), and some also grew very large, attaining sizes not seen in most of today's terrestrial mammals. Climate-wise, the Earth had begun a drying and cooling trend, culminating in the glaciations of the Pleistocene Epoch, and partially offset by the Paleocene-Eocene Thermal Maximum. The continents also began looking roughly familiar at this time and moved into their current positions.

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