historical geology lecture 3. the fossil record
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HISTORICAL GEOLOGY LECTURE 3. THE FOSSIL RECORD. . Paleontology: "The study of the remains or traces of ancient life" - or fossils . Where are fossils found?. Mold of a tree trunk in a lava flow (igneous rock) marine fossils in marble (metamorphic rock). - PowerPoint PPT PresentationTRANSCRIPT
Harry Williams, Historical Geology 1
HISTORICAL GEOLOGYLECTURE 3. THE FOSSIL RECORD.
Paleontology: "The study of the remains or traces of ancient life" - or fossils.
Mold of a tree trunk in a lava flow (igneous rock)
marine fossils in marble (metamorphic rock)
Where are fossils found?
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It is possible to find fossils in igneous and metamorphic rocks, but it is very rare because fossils would normally be destroyed. Fossils are far more abundant in SEDIMENTARY rocks, which is one of the reasons sedimentary rocks are so important in historical geology. (Pass hand sample around class). How do fossils get to be fossils?...
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Fossilization
It is rare that a
fossil is made of the
original organic matter.
More often, the organic
matter is destroyed and
replaced by a mineral –
this is the process of
petrification (turning to
stone), by:
Mosquito in amber - the basis for Jurassic Park.
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i) Permineralization - minerals precipitate into pore spaces
Modern cow’s femur.
Permineralized dinosaur bone
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ii) Replacement/petrification - soft tissue replaced by harder minerals e.g. wood replaced by silica; calcium carbonate replaced by pyrite.
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Wood in Arizona’s petrified forest. Silica has replaced the wood.
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Live Fossil
Sometimes an organism has a hard mineral component that may be fossilized. Soft tissues decay quickly, but hard body parts (teeth, bones, shells) may be preserved (possibly with some chemical alteration - petrification).
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iii) Carbonization - soft tissue decomposed, leaving carbon film e.g. bee, fern leaf.
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iv) Molds - Applies especially to shells; shell dissolves away leaving a void that fills with a mineral precipitate or mud -> a CAST;
impression of the outside of the shell in sedimentary rock = EXTERNAL MOLD.
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Marine gastropod (snail) mold and cast.
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v) Trace Fossils - Evidence of animals in the form of trails, tracks, burrows, borings etc.
Dinosaur footprintGrazing trails
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Trace fossils tell you something about the environment e.g. clam burrows -> intertidal; dinosaur tracks = terrestrial.
Fossils are, of course, extremely useful, but it should be remembered that we are often dealing with an....
crawling
resting
grazing
feedingdwelling
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...Incomplete Record of Life
As discussed previously, most sequences of sedimentary rocks
contain GAPS in deposition or UNCONFORMITIES. These
may represent millions of years and fossils belonging to the
missing period will not be found. Some fossils have been
removed in places by erosion; some rocks are barren of fossils
due to unsuitable environmental conditions (i.e. it is rare to find
fossils in coarse river deposits, because organic remains get
worn away).
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Classifying Organisms
The modern system of classification is referred to as
TAXONOMY. The smallest taxonomic unit is the
SPECIES - “a group of organisms basically alike in their
structural and functional characteristics; can interbreed
and produce fertile offspring (proves genetically related)”.
The rest of the system is hierarchical levels of kinship:
http://en.wikipedia.org/wiki/Mule http://www.lovelongears.com/faq
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TAXONOMIC UNIT EXAMPLE
Species lupus
Genus (group of species) Canis
Family (" " genera) Canidae
Order (" " families) Carnivora
Class (" " orders) Mammalia
Phylum (" " classes) Chordata
Kingdom (" " phyla) Animalia
Domain (“ “ kingdoms) EukaryaAn organism is usually identified by genus and species e.g. Canis lupus (Wolf). Note that there can be subdivisions of the main categories above e.g. subclass, subphylum, superorder, etc.). (Note: you are not required to memorize taxonomic names for the lecture exams).
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Use Of Fossils In Stratigraphy:1. Relative dating (covered).
2. Correlation (covered).
3. Paleogeography - Species -> environment -> geography, e.g. = distribution of land and sea, based simply on presence of marine/terrestrial fossils. Mapping the location of fossils of intertidal species such as clams, can locate a former coastline - this is very important in historical geology, since many sea level changes have occurred in the past and coastlines have frequently shifted around.
4. Paleolatitude - fossils may also give an indication of LATITUDE, e.g. coral reefs usually form in low latitudes (under warmer climates).