Fossils
Imprints of Hard Parts in Sediment
Many fossils are simply imprints with no shell material present at all. Hard parts are commonly destroyed by decay or dissolution after burial, but may leave a record of their former presence in the surrounding sediment.
Preservation of Unaltered Hard Parts
(original material) The shells of invertebrates and single-celled organisms, or vertebrate bones and teeth may be preserved unaltered (body fossils).
Nicholaus Steno
1669 suggest fossils are remains of organisms.
Where fossils occur
Almost Exclusively in Sedimentary Rocks-Rocks made from sediments deposited in quiet water (lake beds or lagoons)preserve particularly fine specimens; -rocks from sediments deposited in high energy environments contain at best only small fragments of fossils mixed with other clastic grains. The heat of Melting or Metamorphism Would Destroy Almost Every Type of Fossil •Rare Exceptions: -Some Fossils in Low-grade metamorphic rocks; -Trees Buried by Lava Flow; -Some fossils are found in volcanic ash deposits.
Preservation of unaltered soft parts
In rare circumstances, the soft parts of an animal may be preserved.
PRESERVATION OF UNALTERED SOFT PARTS
In rare circumstances, the soft parts of an animal may be preserved. The two most common methods of soft part preservation are freezing and desiccation (drying or mummification). (Example: Pleistocene wooly mammoths frozen in Siberia and Alaska). Soft parts of organisms such as insects or small frogs may be preserved if the organism becomes trapped in pine resin (later altering to amber). Larger animals may become trapped in oily, tar-like asphalt (example: mammals preserved in the LaBreatar pits in Los Angeles, California).
IMPRINTS OF HARD PARTS IN SEDIMENTS
Molds are the imprints of an organism (or part of an organism) in the sediment. A shell buried in sandstone may be leached or dissolved by groundwater, leaving a mold of the shell in the surrounding sandstone. A cast may be produced if a mold is filled with sediment or mineral matter. A cast is a replica of the original. Casts are relatively uncommon.
THE GEOLOGIC COLUMN
No one locality on earth provides a complete record of Earth's history. By correlating rocks from locality to locality at millions of places around the world, geologists have pieced together a composite stratigraphic column, called the geologic column, that represents just about the entirety of Earth's history.
Preservation of Unaltered Hard Parts (2)
The direct remains of an organisms that exists pretty much in the same condition that the organism produced it. •Calcite is the most stable mineral that organisms produce, therefore those organisms that are calcite producers are the ones that commonly occur as body fossils.
Chemical Alteration of Hard Parts
The hard parts of many fossil organisms have been chemically altered by the addition, removal, or rearrangement of chemical constituents.
fossil succession states
The principle of fossil succession states that organisms evolve through time so that particular forms can be used as age markers wherever they are found.
FORMING A FOSSILS (FOSSILIZATION)
The remains of organisms may be fossilized in a variety of ways:
TRACE FOSSILS OR ICHNOFOSSILS
Trace fossils are markings in the sediment made by the activities of organisms. They result from the movement of organisms across the sediment surface, or the tunneling of organisms into the sediment, or the ingestion and excretion of sedimentary materials. The study of trace fossils is called ichnology. Trace fossils provide geologists with much useful information about ancient water depths, paleocurrents, availability of food, and sediment deposition rates. In many cases, tracks of animals are the only record of their existence. For example, in many places, dinosaur tracks are much more abundant than dinosaur bones. During its lifetime, a single dinosaur makes millions of tracks, but leaves only one skeleton, which may or may not be preserved.
William Smith (1769-1839)
developed the technique of using fossils to determine the age of certain rocks
Fossil
from the Latin word, fossil is ("dug up"); the remnant or trace of an ancient living organism that has been preserved in rock or sediment.
Carbonized fossil
impressions preserved plants or animals as a thin carbon film, usually in fine-grained sediments (shales). Fine details of the organisms may be preserved. Plant fossils, such as ferns, in shale generally are preserved by carbonization. Soft-bodies animals may also be preserved as carbonaceous films in black shales. (Example: Cambrian Burgess Shale fauna.)
replacement
is the molecule-by-molecule substitution of another mineral of different composition for the original material. The fine details of shell structures are generally preserved. Minerals which commonly replace hard parts are silica and pyrite.
Microfossils
microscopic fossils of single-celled prokaryotic organisms which evolved in the absence of oxygen
Permineralization
the filling of pores (tiny holes) in wood, shell, or bone by the deposition of minerals from solution. The added mineral matter makes the permineralized fossil much heavier than the original material. Petrified wood is a common example of permineralization. B. Replacementis the molecule-by-molecule substitution of another mineral of different composition for the original material. The fine details of shell structures are generally preserved. Minerals which commonly replace hard parts are silicaand pyrite.
Leonardo da Vincire
•1500 C.E. cognizes them as ancient life; he noted fragile shells that could not have been washed in land and which the organisms could not have moved great distances in short time spans. He noted layers that were fossil rich and layers that were barren and interpreted rocks as deposits of many events.
Aristotle
•300 B.C.E. Aristotle argues that these lives shapes had actually grown within rocks and had nothing to do with life.
Herodotus(Greek)
•450 B.C.E. Herodotus(Greek) suggest they were the remains of sea creatures that lived when the ocean covered what is now dry land.
