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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)
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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
Mosquito in amber - the basis
for Jurassic Park.
stone), by:
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i) Permineralization - minerals
precipitate into pore spaces
Modern cow’s femur.
Permineralized dinosaur bone
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ii) Replacement - soft tissue
replaced by harder minerals
eg. 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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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 -> 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 footprint
Grazing trails
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Trace fossils tell you something about the environment e.g.
clam burrows -> intertidal; dinosaur tracks = terrestrial.
grazing
crawling
resting
dwelling
feeding
Fossils are, of course, extremely useful, but it should be
remembered that we are often dealing with an....
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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)
Eukarya
An 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).
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