Fossils: Evidence of Past Life
Fossils, the remains or traces of prehistoric life are important in clusions in sediment and sedimentary rocks They are important basic tools for interpreting the geologic past. The scientific study of fossils is called paleontology. It is an interdisciplinary science that blends geology and biology in an attempt to understand all aspects of the succession of life over the vast expanse of geologic time. Knowing the nature of the life-forms that existed at a particular time helps researchers understand past environmental conditions. Furthermore, fossils are important time indicators and a key role in correlating rocks of similar ages that are from different places. Fossils are many The remains of relatively recent as teeth, may not have been altered at all(Figure 11.12) objects are bones, and shells are common examples. Far less common entire animals, flesh included, that have been preserved because of rather unusual circumstances. Remains of prehistoric elephants called mammoths that were frozen in the Arctic tundra of Siberia and Alaska are examples, as are the mummified remains of sloths preserved in a dry cave in Nevada. Given enough time, the remains of an organism are likely t be modified. Often, fossils become petrified(literally, turned into stone"), meaning that the small internal cavities and pores of the original structure are filled with precipitated mineral matter(Figure 1 In other instances replacement may occur. Here the cell walls and other solid material are removed and replaced with mineral matter sometimes the microscopic details of the replaced structure are faithfully retained of fossils. and casts constitute another common class When a shell or other structure is buried in sediment and then dissolved by underground water, a mold is created. The mold faithfully reflects only the shape and surface marking of the organ ism; it does not reveal any information concerning its internal structure. these hollow spaces are subsequently filled with mineral matter, casts are created(Figure l1.13B)
A type of fossilization called carbonization is particularly effective in preserving leaves an delicate animal forms. It occurs when fine sediment encases the remains of an organism. As time passes, pressure squeezes out the liquid and gaseous components and leaves behind a thin residue of carbon(Figure 11.13C) Black shales deposited as organic-rich mud in oxygen-poor environ is often contain abundant carbonized remains.