fossil record - why it is incomplete - index fossils - principle of superposition - principle of fossil succession - lithostratigraphic units - biostratigraphic units - time stratigraphic units - facies fossils - correlation - biozones
2. What is the Fossil record ?
Provides evidence about the history of life on earth and its
The total number of fossils that have been discovered ,
as well as to the information derived from them.
3. M8 Why is it incomplete ?
The fossil record is very incomplete because of :
1 ) fossils are rare , because fossilization require specific
E.g., only animals have hard body , can fossilize.
2 ) not all the fossils have been found.
4. Seek the
peak Why is it incomplete ?
3 ) dead organisms are destroyed by :
Predators ( wild animals )
Climate , before they fossilize .
4 ) fossils destroyed and degenerate over time
Thus the oldest fossils might turn to dust.
5. Seek the
peak Romer's Gap
is an example of an apparent gap in the tetra pod fossil record
Such gaps represent periods from which excavators have not yet found
Romer's gap spanned from approximately 360 to 345 million years ago,
corresponding to the first 15 million years of the Carboniferous Period.
Some scientists have suggested that the geochemistry caused bad
conditions for fossil formation, so few organisms were fossilized.
Another theory suggests that scientists have not yet discovered an
excavation site for these fossils, due to inaccessibility or random
7. Seek the
peak Principle of superposition
•Principle of Original Horizontality:
because strata are deposited under gravity, they form horizontal layers. If
the strata are no longer horizontal, something has disturbed the sediments
•Principle of Superposition:
unless they have been disturbed, the strata at the bottom of a stack
were deposited first, the ones on top of that are next oldest, and so on, with
the youngest strata being the ones on top.
•Principle of Cross-cutting Relationships:
any structure (fold, fault, weathering surface, igneous rock intrusion, etc.)
which cuts across or otherwise deforms strata is necessarily younger than the
rocks and structures it cuts across or deforms.
10. Seek the
peak FOSSIL SUCCESSION
Fossils show up in the geologic record in a definite , determinable
and regular order .
don’t present randomly
Any period of geologic time can be recognized by its fossil content
Example : age of mammals ( Cenozoic era )
So fossils can be used to identify the relative age of the layers of a
rock formation .
so the organisms found in the top layers appeared after the
organisms found in the layers below them .
21. Index Fossils
Are useful for correlation and the biostatigraphy.
To be a goodindex fossil, the species should:
1 ) Have been VERY common, so chances of individuals being
buried is good.
2 ) Have hard parts, so chances of fossilization are good.
3 ) Have a wide geographic range, so that correlation over wide
region is possible Lived in (or could be deposited in) different
environments, so can be found in different formations.
useful for correlation. To be a good index fossil, the species should:
22. Index Fossils
4 ) Have some distinctive features, so it can be
recognized from closely related forms.
5 ) Have a short geological duration (a few million years
23. Index Fossils
1 ) Nannofossils
Are microscopic fossils
(the remains of calcareous nannoplankton ) from various
Nannofossils are very abundant, widely distributed
And time-specific, because of their high evolutionary
24. Index Fossils
**** There are enormous numbers of useful nanofossils,
including radiolarians and foraminifera.
**** Nanofossils are the primary method of dating
25. Seek the
peak Index Fossils
Are extremely small objects (less than 25 microns)
produced by planktonic unicellular algae .
28. Seek the
peak Index Fossils
were common during the Paleozoic Era ( Cambrian age )
about half of the Paleozoic fossils are trilobites. ( common)
They extinct during the late Permian period.
32. Seek the
Some fossils can be found only in certain environments ,
for long periods of geologic time.
They allow us to learn about the environmental conditions
in the period when the covering layer was deposited.
36. Biozones- rock unit
Biozone (or "zone"):
Rock unit characterized by one or more taxa that permit it to be
distinguished from adjacent rocks.
Last Appearance Datum (LAD): either local or global
First Appearance Datum (FAD): either local or global