The Cenozoic Era of the Phanerozoic Eon is the Age of Mammals. Although mammals dominated the Cenozoic, but it was also the age of birds, insects, sharks, bony fish, turtles, snakes, lizards, and mammals. The Cenozoic Era began 66 Ma with the End Cretaceous extinction. It has three periods, Paleogene, Neogene, and Quaternary. The Paleogene has three epochs: Paleocene, Eocene, and Oligocene. The Neogene has two epochs: Miocene, Pliocene. The Quaternary has two epochs, Pleistocene and Holocene.
Cenozoic fossil beds are strewn across the western United States and Florida. The Nacimiento Formation at Angel Peak in New Mexico and the Bug Creek Anthills in Montana contains early Paleocene mammals. Early Eocene fossils are at Fossil Butte National Monument in Wyoming (Figure 12-1). John Day National Monument in Oregon documents the Middle Eocene to the Late Miocene (44 to 7 Ma). Late Eocene plant fossils are at Florissant Fossil Beds National Monument in Colorado and Badlands National Park in South Dakota. Miocene mammals are at Agate Fossil Beds in Nebraska. Pliocene (5.3 – 2.6 Ma) fossils are in the Hagerman Fossil Beds National Monument in Idaho (3.5 Ma), and Pleistocene (2.6 Ma – 10 ka) fossils such as terror bird Titanis walleri are in Florida.
Horses are in the Perissodactyl mammalian order, along with rhinoceri and tapirs. The evolution of horses is one of the more interesting sequences of mammal evolution. They began as small dog-sized animals that lived in the forest. They gradually adapted to grazing in grasslands and defended themselves from predators by running. Gradual changes in the horse brain size, height, leg structure with loss of toes, increasing size of teeth, and shape of teeth are easy to track in the fossil record; however, some of the evolutionary connections have been difficult to sort out because of the numerous species and taxa.
Whales are Cetartiodactyls, which means they are related to cows, pigs, hippos, and deer. Whales evolved from mammals that hunted in lakes and streams and gradually became completely aquatic. They developed ears adapted to hearing underwater and gradually lost their legs. Their front legs became pectoral fins, and they kept only a small remnant of one bone from their rear legs .In one of the greatest fossil discoveries in history, the Thewissen Lab in Pakistan uncovered the story of their evolution from small squirrel-like animals to seagoing whales such as the basilosaurids.
Marsupials include all of your favorite animals from Australia along with the Virginia possum and a few animals in South America. They are the inferior cousins of the placental mammals, and I don’t say this just because I am a placental mammal. They are generally wiped out in almost all ecological niches by placental mammals when the two clades come into contact by the merging of continents.
The Atlantogenata includes the eutherian mammals that evolved in South America and Africa, which split off from the northern continent more than 100 Ma, prior splitting off from each other. The Xenarthrans evolved in South America and the Afrotheria evolved in Africa. Molecular DNA analysis indicates that the Xenarthra are more closely related to the Afrotheria (Figure 12‑47) than to the northern placental mammals in Laurasia. Thus, scientists group them within the Atlantogenata.
Along with all other placental mammals, primates evolved from small Cretaceous insectivores during the Cenozoic Era. Whales evolved in the water, horses evolved in the plains, and primates evolved in the trees. The primate lifestyle led to their advanced color vision, grasping hands, and high intelligence. This section tracks almost the entire evolutionary history of primates but ends with chimpanzee-like primates, prior to upright walking and brain expansion in prehomo and homo species.
This concludes the history of the evolution of the solar system, Earth, and life, both in natural history and in Moses' account of the six ages. All that is left are humans, which are last in the fossil record and at the end of Moses' six ages.
Galloping horse and rider moving at 40 mph. Credit: Eadward Mudbridge (1887). Public domain