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Fossils

Fossils. Fossils are the remains or traces of past life forms They are most common in sedimentary rocks They are extremely useful for determining relative ages of strata Fossils provide some of the evidence for organic evolution. How do Fossils Form?.

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Fossils

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  1. Fossils • Fossils are the remains or traces of past life forms • They are most common in sedimentary rocks • They are extremely useful for determining relative ages of strata • Fossils provide some of the evidence for organic evolution

  2. How do Fossils Form? • Remains of organisms are called body fossils. • theyconsist mostly of durable skeletal elements • such as bones, teeth and shells • rarely we might find entire animals preserved by freezing or mummification

  3. Body Fossil • Skeleton of a mammoth, an extinct relative of today’s elephants, on display in the Museum of Geology and Paleontology in Florence, Italy.

  4. Body Fossils • Shells of Mesozoic invertebrate animals

  5. Trace Fossils • Indications of organic activity • including tracks, trails, burrows, and nests • are called trace fossils • A coprolite is a type of trace fossil • consisting of fossilized feces • that may provide information about the size • and diet of the animal that produced it

  6. Trace Fossils • This slab of rock • formed over the actual tracks of birds, • so it is a cast of the tracks

  7. Trace Fossils • Fossilized feces (coprolite) • of a carnivorous mammal • Specimen measures about 5 cm long • and contains small fragments of bones

  8. Fossil Record • The fossil record is very incomplete because • bacterial decay, • physical processes, • scavenging, • and metamorphism • destroy organic remains • In spite of this, fossils are quite common

  9. Body Fossil Formation • The most favorable conditions for preservation • of body fossils occurs when the organism • possesses a durable skeleton of some kind • and lives in an area where burial is likely • Body fossils may be preserved as • unaltered remains, • meaning they retain • their original composition and structure, • by freezing, mummification, in amber, in tar • or altered remains, • with some change in composition or structure • permineralization, replacement, carbonization

  10. Unaltered Remains • Insects in amber • Preservation in tar

  11. Unaltered Remains • Frozen baby mammoth • found in Russia in 1989

  12. Altered Remains • Petrified redwood tree • at the Petrified Forest in California • Numerous redwoods were blown down by a volcanic eruption • and covered with volcanic ash

  13. Altered Remains • Carbon film of a palm frond • Carbon film of an insect

  14. Molds and Casts • Molds form • when buried remains dissolve and leave a cavity • Casts form • if minerals or sediments fill in the cavity

  15. Mold and Cast Step a: burial of a shell Step b: dissolution leaving a cavity, a mold Step c: the mold is filled by sediment forming a cast

  16. Cast of a Turtle • Fossil turtle • showing some of the original shell material • body fossil • and a cast

  17. Fossil Record • The fossil record is the record of ancient life • preserved as fossils in rocks • Just as the geologic record • must be analyzed and interpreted, • so too must the fossil record • The fossil record • is a repository of prehistoric organisms • that provides our only knowledge • of such extinct animals as trilobites and dinosaurs

  18. Fossils and Telling Time • William Smith • 1769-1839, an English civil engineer • independently discovered • Steno’s principle of superposition • He also realized • that fossils in the rocks followed the same principle • He discovered that sequences of fossils, • especially groups of fossils • are consistent from area to area • Thereby he discovered a method • whereby relative ages of sedimentary rocks at different locations could be determined

  19. Fossils from Different Areas • Smith used fossils • To compare the ages of rocks from two different localities

  20. Principle of Fossil Succession • Using superposition, Smith was able to predict • the order in which fossils • would appear in rocks • not previously visited • Alexander Brongniart in France • also recognized this relationship • Their observations • led to the principle of fossil succession

  21. Principle of Fossil Succession • Principle of fossil succession • holds that fossil assemblages (groups of fossils) • succeed one another through time • in a regular and determinable order • Why not simply match up similar rocks types? • Because the same kind of rock • has formed repeatedly through time • Fossils also formed through time, • but because different organisms • existed at different times, • fossil assemblages are unique

  22. Distinct Aspect • An assemblage of fossils • has a distinctive aspect • compared with younger • or older fossil assemblages

  23. Matching Rocks Using Fossils • Geologists use the principle of fossil succession • to match ages of distant rock sequences • Dashed lines indicate rocks with similar fossils • thus having the same age

  24. Matching Rocks Using Fossils youngest • The youngest rocks are in column B • whereas the oldest ones are in column C oldest

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