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? Remains of organisms are called body fossils. they consist mostly of durable skeletal elements such as bones, teeth and shells rarely we might find entire animals preserved by freezing or mummification rarely we might find entire animals preserved by freezing or mummification

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.

Body Fossils Shells of Mesozoic invertebrate animals Shells of Mesozoic invertebrate animals

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

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

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

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

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

Unaltered Remains Insects in amber Preservation in tar Preservation in tar

Unaltered Remains Frozen baby mammoth Frozen baby mammoth found in Russia in 1989 found in Russia in 1989

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

Altered Remains Carbon film of a palm frond Carbon film of an insect Carbon film of an insect

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

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

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

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

Fossils and Telling Time William Smith , 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

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

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 Alexander Brongniart in France also recognized this relationship also recognized this relationship Their observations Their observations led to the principle of fossil succession led to the principle of fossil succession

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

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

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

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