bio25

Biology, 6th Edition

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Greg Steinberg AP Biology Chapter 25 Phylogeny and Systematics A. The Fossil Record and Geological Time a. Sedimentary Rocks i. Sedimentary rocks are the richest fossil source. ii. Sedimentary rocks are formed when rocks, sand, dirt, clay and silt pile up on top of each other. When extreme pressures force these together, they become a single sheet, or stratum. iii. Most organisms do not leave a fossil, but some do. iv. A fossil is an imprint of the organism, or a piece such as a bone or tissue. v. Most fossils are just replicas of organisms, like leaves, etc. vi. Some soft tissue may be preserved if bacteria and fungi can’t get to it. – These are very rare. b. Dating i. Scientists use two forms of dating, relative or absolute. ii. Relative dating lets scientists order events from beginning to end. iii. Absolute dating lets scientists precisely time when an event happened. iv. “The dog got up. The dog walked to its bowl. The dog drank water from the bowl” would be relative dating. v. “At 7:21PM the dog drank from its bowl” would be absolute dating. vi. Relative dating with fossils is determined by what layer of rock the fossil is found in. The older fossils are found towards the bottom, and the newer ones up top. vii. Scientists developed complex methods for absolute dating; Radiometric dating ( measuring radioactive isotopes such as Carbon-14 or Uranium-238). They can also measure the rate at which L-amino acids are turned into d-amino acids, which can also be used for dating.
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