Condensed_Lecture_Notes_10-17

Condensed_Lecture_Notes_10-17 - LECTURE 10 EARLY ATMOSPHERE...

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LECTURE 10: EARLY ATMOSPHERE, EARLIEST ANIMAL LIFE, MODES OF FOSSILIZATION If the earliest atmosphere was of volcanic origin, it must not have contained oxygen gas (O 2 ), which is not emitted from volcanoes. Thus, the earliest organisms must have been anaerobic (able to live in the absence of free oxygen). Photosynthesizers emplaced oxygen, a by-product of photosynthesis, into the atmosphere. Only after a substantial buildup of O2 had occurred was it possible for animals, which are aerobic organisms, to exist. At first, the oxygen promptly combined with dissolved iron ions, causing hematite (Fe 2 O 3 ) or magnetite (Fe 3 O 4 ) to precipitate. Ferrous iron (Fe +2 ) dissolves in water, whereas compounds that contain ferric iron (Fe +3) , including minerals consisting of iron oxide (hematite and magnetite), are generally highly insoluble. This iron of sedimentary origin forms the world's greatest iron ore deposits. Only much later (perhaps at the Precambrian-Cambrian transition?) did oxygen begin to accumulate significantly in the atmosphere. Multiple-celled organisms (all of them eukaryotes) appeared in the fossil record nearly two billion years after the early prokaryotes. There is a fossil record of animals, known as the Ediacara (E-dee-ACK-ara) fauna that lived during latest Precambrian time. Many of these soft-bodied creatures had a very large surface area compared to body volume, and a number of them had body plans that are totally unknown today. For example, the body of one animal consisted of inflatable interconnected compartments like an air mattress. Lacking jaws and teeth, these animals were passive feeders. They could not be predators, but rather, they waited for food to be wafted to them. During Middle Cambrian time, fossils of numerous animals, both with and without hard parts, were elegantly preserved as the Burgess Shale fauna, in western Canada. Fossilization occurred by occasional underwater mudslide events that instantaneously smothered the organisms. They included a balanced ecological community with prey, predators, scavengers, filter feeders, mud grovelers, swimmers, etc. The Burgess Shale fauna also included organisms with no modern analogs -- for example, a critter with five eyes. We postulate that when animals first appeared there was an evolutionary explosion of "experimental" forms, some of which survived to this day (echinoderms, mollusks, arthropods, etc.), and some of which became extinct. We do not know what adaptive advantages the surviving forms must have had relative to organisms that became extinct. LECTURE 11: PRINCIPLES OF STRATIGRAPHY Before geology became an established science, people had incorrect notions about stratigraphy -- the study of strata. They conceived of concentric layers wrapping around the earth like onionskins.
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