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FossilsFossils are the remains or traces of past life formsThey are most common in sedimentary rocksThey are extremely useful for determining relative ages of strataFossils provide some of the evidence for organic evolution2How do Fossils Form?Remains of organisms are called body fossils.they consist mostly of durable skeletal elements such as bones, teeth and shellsrarely we might find entire animals preserved by freezing or mummification

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Body FossilSkeleton of a mammoth, an extinct relative of todays elephants, on display in the Museum of Geology and Paleontology in Florence, Italy.4Body FossilsShells of Mesozoic invertebrate animals

5Trace FossilsIndications of organic activity including tracks, trails, burrows, and nests are called trace fossilsA coprolite is a type of trace fossil consisting of fossilized fecesthat may provide information about the size and diet of the animal that produced it6Trace FossilsThis slab of rockformed over the actual tracks of birds,so it is a cast of the tracks

7Trace FossilsFossilized feces (coprolite) of a carnivorous mammalSpecimen measures about 5 cm long and contains small fragments of bones

8Fossil RecordThe fossil record is very incomplete because bacterial decay, physical processes, scavenging, and metamorphism destroy organic remainsIn spite of this, fossils are quite common9Body Fossil FormationThe most favorable conditions for preservation of body fossils occurs when the organismpossesses a durable skeleton of some kind and lives in an area where burial is likelyBody 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 structurepermineralization, replacement, carbonization10

Unaltered RemainsInsects in amberPreservation in tar

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Unaltered RemainsFrozen baby mammoth found in Russia in 198912Altered RemainsPetrified redwood treeat the Petrified Forest in CaliforniaNumerous redwoods were blown down by a volcanic eruptionand covered with volcanic ash

13Altered RemainsCarbon film of a palm frond

Carbon film of an insect14Molds and CastsMolds form when buried remains dissolve and leave a cavityCasts form if minerals or sediments fill in the cavity15Mold 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 cast16Cast of a TurtleFossil turtle showing some of the original shell material body fossiland a cast

17Fossil RecordThe fossil record is the record of ancient life preserved as fossils in rocksJust as the geologic record must be analyzed and interpreted, so too must the fossil recordThe fossil record is a repository of prehistoric organisms that provides our only knowledge of such extinct animals as trilobites and dinosaurs18Fossils and Telling TimeWilliam Smith 1769-1839, an English civil engineerindependently discovered Stenos principle of superpositionHe also realized that fossils in the rocks followed the same principleHe discovered that sequences of fossils, especially groups of fossils are consistent from area to areaThereby he discovered a method whereby relative ages of sedimentary rocks at different locations could be determined19Fossils from Different AreasTo compare the ages of rocks from two different localities

Smith used fossils

20Principle of Fossil SuccessionUsing 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 relationshipTheir observations led to the principle of fossil succession21Principle of Fossil SuccessionPrinciple of fossil successionholds that fossil assemblages (groups of fossils) succeed one another through time in a regular and determinable orderWhy not simply match up similar rocks types?Because the same kind of rock has formed repeatedly through timeFossils also formed through time, but because different organisms existed at different times, fossil assemblages are unique22Distinct AspectAn assemblage of fossils has a distinctive aspect compared with younger or older fossil assemblages23Matching Rocks Using FossilsGeologists use the principle of fossil succession to match ages of distant rock sequencesDashed lines indicate rocks with similar fossilsthus having the same age

24Matching Rocks Using FossilsThe youngest rocks are in column B whereas the oldest ones are in column C

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