Fossils

2. 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

3. 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

4. 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.

5. Body Fossils • Shells of Mesozoic invertebrate animals

6. 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

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

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

9. 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

10. 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

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

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

13. 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

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

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

16. 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

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

18. 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

19. 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

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

21. 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

22. 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

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

24. 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

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