Index Fossils: Evolution
and Biostratigraphy
Evolution
 Variations exist within a population
 Result from mutations and other
genetic accidents
 Some variations are advantageous
 Others are not
 Some are neutral
 Natural Selection works on these
variations
 Characteristics of population shift
through time = evolution
What is Natural Selection?
 What phrase is commonly used to
describe Natural Selection?
 “Survival of the fittest”
 What does it mean?
 Organisms best suited to the environment
survive long enough to reproduce.
 Advantageous traits are passed on
How do new species arise?
 When accumulated variations are great
enough that some individuals within the
population cannot interbreed, a new
species has arisen.
 New species commonly arise in
geographically or environmentally
isolated areas.
 New species and original species may
exist at same time
Bio-Events
 First appearances of new species
 First appearances of new higher taxa
 Extinctions of species
 Mass extinctions of multiple taxa
 Bio-events are unique points in geologic
time
Index Fossils
 Some fossils are more useful than others for
relative age determinations
 Fossils that are most useful are called INDEX
FOSSILS
 What factors would maximize a fossil’s
usefulness? (i.e., What makes a good index
fossil?)
What makes a good index
fossil?
 Distinctive appearance/easy to
recognize
 Short duration between first
appearance and extinction (a.k.a.
RANGE)
 Widespread geographic distribution
(makes correlation possible across a
wide area/multiple continents)
Characteristics of Index Fossils
 Limited Stratigraphic Range
 Widespread Geographic Distribution
 Commonly Pelagic
 Or, tolerant of a wide variety of
environments (found in many facies)
Mile Markers in Deep Time
 Life is unique for each interval of time
because of evolution and extinction
 Volcanos, earthquakes, mountains,
types of rocks repeat throughout history
 Life does not repeat – it is directional
 Therefore, we can place other Earth
events in a framework based on fossils
Mile Markers in Deep Time
 The fossil-based framework for Earth history is
known as the GEOLOGIC TIME SCALE
 Units: Eras, Periods, Epochs, Ages
 Boundaries between units are based on bio-events
Index Fossils and
International
Stratotypes
The Case of the Silurian –
Devonian Boundary
Dr. Ivo Chlupáč and Karlstejn
Index Fossils and the
Silurian - Devonian Boundary
 Graptolites *
 Conodonts *
 Chitinozoans *
 Trilobites
 Brachiopods
 Cephalopods
Definition of
the S-D Boundary
 Stratotype: Klonk,
near Suchomasty,
Czech Republic
S-D GSSP
 GSSP = Global
Stratotype, Sequence
and Point
 First appearance of
Monograptus uniformis
uniformis
 Bed 20
 Klonk, Czechia
Preserving the Stratotype