Harry Williams, Historical Geology1 HISTORICAL GEOLOGY LECTURE 3. THE FOSSIL RECORD. Paleontology:...

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?


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?...


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.


i) Permineralization - minerals precipitate into pore spaces

Modern cow’s femur.

Permineralized dinosaur bone


ii) Replacement - soft tissue replaced by harder minerals eg. wood replaced by silica;

calcium carbonate replaced by pyrite.


Wood in Arizona’s petrified forest. Silica has replaced the wood.


iii) Carbonization - soft tissue decomposed, leaving carbon film e.g. bee, fern leaf.


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.


Marine gastropod (snail) mold and cast.


v) Trace Fossils - Evidence of animals in the form of trails, tracks, burrows, borings etc.

Dinosaur footprint

Grazing trails


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


...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).


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

http://en.wikipedia.org/wiki/Mule

http://www.lovelongears.com/faq


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).


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).

Harry Williams, Historical Geology1 HISTORICAL GEOLOGY LECTURE 3. THE FOSSIL RECORD. Paleontology:...

Documents

Transcript of Harry Williams, Historical Geology1 HISTORICAL GEOLOGY LECTURE 3. THE FOSSIL RECORD. Paleontology:...