Lab3W10Fossils

Lab3W10Fossils - GE 70B: Evolution of the Cosmos and Life...

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1 GE 70B: Evolution of the Cosmos and Life Name:___________________________ LABORATORY/DISCUSSION 2 WORKSHEET FOSSILS, FOSSILIZATION, AND EVIDENCE OF EARLY LIFE The purpose of today’s lab is to show you how fossils are preserved, as well as some of the evidence for early life. You should look closely at all of the fossils, trying to discern as much as you can about the organism whose remnants you are observing. Consult your lecture notes, pay attention to the clues in this write-up, and ask your TA for help in answering questions. There are three main types of fossils: body, trace, and chemical. Body fossils are the most common, and contain information about the physical form (i.e., morphology) of the ancient organisms. Trace fossils are preserved marks left by an organism on the sediment (e.g., tracks, trails) or on the remains of another organism (e.g., tooth marks on bone, drill holes in shells). Chemical fossils are the chemical traces of life. Life changes the chemistry of the environment around it, and these changes may be left in rocks. Sometimes body fossils are preserved as imprints where the flattened external shape of the organism compresses the sediment that it is buried within. In these cases, the organism decays, but the sediment surrounding it conforms to its shape and begins to harden prior to complete decay. Occasionally, some original organic matter can be seen on the fossil itself. This typically occurs in lower oxygen environments because this means less bacterial activity (aerobic bacteria are responsible for most decay). In these cases, the organic material has usually been degraded (via heat and pressurization) to a thin sheet of carbon, preserved as a black or brown colored film covering the imprint of the fossil. If the imprint fossil lacks any remnant organic matter, it is called an impression ; if it contains organic matter, it is called a compression . In other cases of preservation, a shell can be filled with sediment as it is buried, and then become dissolved away by acid in the groundwater (CO 2 becomes incorporated in rainwater as carbonic acid, which can dissolve away calcium carbonate shells–this reaction is also responsible for the formation of caverns in limestone). The resulting sediment “ cast ” or “ mold ” can then be preserved in the fossil record. Molds are produced when the shell dissolves away and all that remains is the petrified sediment that was deposited inside the shell, thereby preserving only the internal shape of a shell. A cast is produced when sediment later fills in the cavity produced by the dissolved shell, reproducing its external shape. In still other preservational cases, the original mineral of the shell can be replaced by a new mineral ( replacement ); often this occurs when pore waters precipitate minerals into a cavity produced by a dissolved shell. In other cases, the pore spaces within invertebrate shells, or in the cells of wood, can be infiltrated with a mineral ( permineralization
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This note was uploaded on 03/11/2010 for the course GE CLST 70B taught by Professor Morris,m.r./friscia,a.r./moldwin,m.b./vanvalkenburgh,b during the Winter '10 term at UCLA.

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Lab3W10Fossils - GE 70B: Evolution of the Cosmos and Life...

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