Lecture 16 Reading


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LATE PLEISTOCENE EXTINCTIONS : CAUSES AND CONSEQUENCES I. HUMAN EVOLUTION A. Summarize key events Bipedality 4-5 ma Enlarged brain 2-3 ma Tool manufacture 2-3 Widespread distribution 2-3 ma Fire 200,000 ybp Art 38,000 ybp Agriculture 10,000-12,000 ybp B. Much of this occurred during a time of climatic fluctuations – the Ice Ages. or Pleistocene Epoch. II. THE PLEISTOCENE EPOCH A. The Ice Ages began 1.8 million years ago, and we are probably still in them. Characterized by alternating periods of cold, dry (glacials) and warm, wet (interglacials). Geologic record indicates that there were four major glacial intervals during the past 1.8 ma. The last major advance of ice sheets began about 100,000 years ago and peaked around 18,000 years ago. It then ended rather suddenly around 12-10,000 years ago. The record of glaciation has been reconstructed using a variety of kinds of fossil data, including oxygen isotope ratios in microfossils of shelled marine organisms, shifting distributions of marine organisms with known environmental requirements, and shifting plant distributions as revealed by pollen analyses. B. Three large glacial centers developed in the Northern Hemisphere during glacial intervals: North America, Greenland, and Scandinavia. Around 18,000 years ago, glaciers extended as far south as New Jersey, as evidenced by the sedimentary record. Glaciers leave clues of their passing such as scratches/gouges on rocks and mixed, poorly sorted deposits of rocks, pebbles, sand and mud that they plow ahead of themselves as they grow. During glacials, the plants and animals would retreat southwards and then expand northwards again during interglacials, and this is documented in the fossil record. C. Although we have some understanding of what controls the timing of the glacials and interglacials, it is not at all clear what initiated the pattern. As was pointed out last quarter, the cycling of glacial and interglacial phases seems tied to changes in the nature of the earth's orbit around the sun. But these changes have been going on forever, and so why do we only see ice ages at certain times in earth history (late Ordovician, Carboniferous, and Pleistocene)? The initiation of ice ages seems tied to the presence of
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a permanent, year-round polar ice cap – and this may have occurred about 2-4 million years ago due to changes in oceanic circulation patterns that were caused by the rise of the Panamanian isthmus. D. The late Pleistocene fauna of North America included many more large mammals than are present today, including mammoths, mastodons, bison, camels, horses, giant ground sloths, sabertooth cats, dire wolves, and short-faced bears. There were also many more large predatory or scavenging birds such as condors. All of these go extinct between 12,000 and 10,000 years ago. Why and what effect has this had on the world we see today? III.
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