12-Introduction

The Cenozoic Era of the Phanerozoic Eon is the Age of Mammals, 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 (Figure 12-1). The Paleogene has three epochs: Paleocene, Eocene, and Oligocene. The Neogene has two epochs: Miocene, Pliocene. The Quaternary has two epochs, Pleistocene and Holocene.

This introduction focuses on the impact of climate on mammal evolution during the Cenozoic Eon. The climate was hot in the Paleocene and early Eocene, but it began to cool approximately 50 Ma and continued to cool until the end of the Eocene. At this point, Antarctica became covered with ice. The temperature remained stable during the Oligocene and early Miocene, but mountains formed in the Miocene, which shifted the weather and much of the landscape to grasslands rather than forests. Grasslands favored grazing animals like cows and horses. The climate then cooled again during the Miocene and Pliocene, and the world entered the Ice Ages, which favored the evolution of large animals such as mammoths as well as the evolution of modern humans.

Carbon dioxide and temperature were high at the beginning of the Cenozoic, but rose even higher, 5 to 8 ⁰C in 20,000 years, in an incident at the end of the Paleocene (56 Ma). This rapid rise is called the Paleocene-Eocene Thermal Maximum (PETM). Carbon dioxide in the atmosphere reached 2,000 ppm, and it was so warm in the early Eocene that North America and Asia were covered by tropical forests (Figure 12‑2). The first definitive ancestors of primates, horses, and whales appear in the Early Eocene fossil record. Giant snakes and turtles evolved in this hot period. Carbon dioxide and temperature began to drop 49 Ma and dropped during the rest of the Eocene until the beginning of the Oligocene (34 Ma). Scientists think that the temperature decline in the Mid and Late Eocene might have been caused by massive blooms of azola in the ocean that dropped to the sea floor in the Arctic Ocean, thus removing carbon from the carbon cycle and dropping the level of carbon dioxide in the atmosphere.

The following video describes the impact of climate on the evolution of mammals during the Cenozoic Eon.

As with the Mesozoic Era and the Colorado Plateau, there are some outstanding geologic formations in the United States that represent the evolution of animals during the Cenozoic Era (Section 12-2). Every sequence of animal evolution in the Cenozoic is fascinating, as animal orders adjusted to changing climates and landscapes. It would take a set of books to document all of them; however, this brief chapter focuses on two of the more well-known and historically controversial sequences: the evolution of horses (Section 12-3) and the evolution of whales (Section 12-4). Horse evolution has been controversial because the sequence in the fossil record was the focus of some early attempts to prove Darwinian evolution. There were some mistakes about the precise sequence of evolution from one epoch to the next due to the incompleteness of the fossil record in the 19th and early 20th century, which only amplified the arguments of the opponents of Darwinian evolution. Whale evolution has been controversial because it is difficult for many people to believe that giant aquatic whales evolved from little squirrel-like land animals; however, the fossil record makes it obvious.

Three of the oddest groups of mammals are the marsupials in Australia (Section 12-5), the Xenarthra in South America (Section 12-6), and the Afrotheria in Africa (Section 12-6). One of the interesting questions is why the marsupials never became placental mammals and why the Xenarthra and Afrotheria became placental mammals even though they diverged from the other eutheria over 100 Ma, before any eutherian fossils exhibited placentalism.

One of the most fascinating topics in mammal evolution is the evolution of the mammalian brain, which built upon the earlier reptilian brain (Section 12-7). Even through they diversified at the beginning of the Cenozoic, most mammal orders experienced an increase in brain size and intelligence during the Cenozoic Era.

The Cenozoic Era went from one of the hottest periods with the least polar ice cover of the Phanerozoic Eon to one of the coldest with the most ice cover (Figure 12-3) at the end of the Phanerozoic Eon. Glaciers covered much of North American and Eurasia during the coldest periods of the recent Ice Ages. These cold temperatures changes had a major impact on the evolution of humans and animals (Section 12-8) during the Pleistocene and Holocene epochs.

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Wooly Mammoth at Royal British Columbia Museum. Photo credit: Thomas Quine. Used here per CC BY 2.0.