Paleontology: A Brief History of Life (Templeton Science and Religion Series) by Ian Tattersall

Author:Ian Tattersall [Tattersall, Ian]
Language: eng
Format: epub, mobi
ISBN: 9781599473680
Publisher: Templeton Press
Published: 2011-06-30T22:00:00+00:00

FIGURE 6.3. The Cretaceous gliding mammal Volaticotherium. Courtesy of Meng Jin.

CHAPTER 7

The Age of Mammals

WITH THE DEMISE of the dinosaurs the mammals proliferated on land, and soon in the seas, too, where fish diversified along with mollusks and corals. As the Cenozoic (“new life”) Era began some 65.5 million years ago, the Earth was relatively warm, though the subsequent tendency was toward cooling and increasing temperature contrast between equator and poles. The most notable break in this trend was the “Paleocene-Eocene Thermal Maximum” around 55 million years ago, when world temperatures sharply peaked.

Oceans are major distributors of heat around the globe, and climates worldwide were greatly influenced by continental repositioning as the remnants of Pangaea continued reconfiguring toward today’s familiar geography. When Australia separated from Antarctica around 34 million years ago there was significant general cooling, as the frigid new Arctic Circumpolar Current brought cold water to the surface and isolated Antarctica from the warmer waters to its north. A major ice sheet developed on the formerly forested Antarctica, and sinking cold water began a northward flow that affected ocean temperatures right into the northern Atlantic and Pacific: an event correlated with a major faunal replacement at higher latitudes that amounted to a minor mass extinction. Radical changes also occurred some 3 million years ago when the formation of the Isthmus of Panama rerouted the world’s major oceanic currents to create the “great conveyor” that dominates oceanic circulation today. Cooling temperatures and high precipitation thereafter combined to produce an ice cap over the northern polar regions, 30 million years after its southern counterpart began to form.

The linking of North with South America united these landmasses for the first time in a couple of hundred million years. Evolution had been proceeding almost entirely independently on each for this entire time, so each continent had an entirely distinctive fauna and flora. With the creation of the Isthmus of Panama, animals could again move freely between the two remnants of Pangaea, resulting in what has been called the Great American Interchange. The flow was unbalanced. Only a handful of South American forms moved north, while at least twenty-nine North American placental genera moved south. They wreaked havoc on the South American natives, with the resident marsupial carnivores taking the heaviest hit. Today at least half the mammal genera living in South America are descended from North American immigrants. Ironically, many of them, including camel relatives and tapirs, belong to groups now extinct in the north.

THE CENOZOIC TIMESCALE

Geologists still disagree vociferously over how best to subdivide the Cenozoic. To cut a very long story short, seven epochs are generally recognized over this time (see Figure 1.2 on page 13), though how they should be grouped is disputed. Our current epoch, the Holocene, began some 11,000 years ago but is only arbitrarily separated from the preceding Pleistocene, the epoch of the Ice Age.

We’ve seen that ice ages have been an intermittent feature of Earth’s history for at least the last 2.7 billion years. But when

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