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