Index fossil indicates age of strata

thank you linda..I need more elaborate explaination. please write ..

we have two main kinds of fossils: index or stratigraphic fossils, which could give us the age of strata, for example trilobites for the Paleozoic era, ammonites for the Mesozoic...

And we have facies fossils, that allow us to define ancient paleoenvironments (salinity, temperature,....), like most of bivalves, gastropods...

Zonal fossils are in use globally, while index fossils regionally.

• Zonal fossil is a fossil (specie) that is restricted within a locality or rock formation while an index or guide fossil is a fossil (specie) with a wide geographical spread and short stratigraphic interval (short life-span).

For more clarity look at: http://en.wikipedia.org/wiki/Zone_fossils

index fossils are found in certain time period in geological past for example Trilobite and ammonite, however zonal fossils are find particularly in a strata (horizon at particular locality) on which basis a biozone can be decide.

In gross terms, index fossils can be used to define biozones (total range zone TRZ, partial range zone PRZ -former co-occuring zone, concurrent zone, exclusion zone-, etc) while zonal fossils are not necessarily confined into the biozone. It only depends on the type of the zone you are dealing with.

You can find all explanations and subtle différences between zonal and index fossils in:

Stratigraphy

Terminology and Practice

REY Stratigraphy Authors : REY Jacques Eds. GALEOTTI Simone

Work initiated by the French Committee of Stratigraphy

ISBN : 9782710809104

Price : 45 Euros, 54 $US, 295,19 FF

Publication Date : Mar 2008

trade paperback, 210 x 297 mm, 176 p.

Press reviews

This book, written by 33 stratigraphic experts, presents various processes available which will enable the location in time of all rock types: sedimentary, metamorphic, plutonic, and eruptive, whether they are in outcrop or at subsurface. The terminology and the appropriate practices for each method are presented in separate chapters and illustrated with concrete examples. The order of the chapters is modeled on the progression of the stratigraphic process, from the descriptive to the interpretative, from the methods of the geometric stratigraphy (lithostratigraphy and genetic stratigraphy, chemostratigraphy, magnetostratigraphy) to the chronological stratigraphy (biostratigraphy), followed by the chronometric stratigraphy (isotopic geochronology). The final two chapters are dedicated to chronostratigraphic units and correlations which combine the contributions of various methods and to the presentation of the 2007 version of the Geological Time Scale. The definitions of stratigraphic terms can be found in a glossary at the end of the work.

The book is addressed to all professional geologists, from the industrial sector as well as those in universities, including teachers and researchers who would like to deepen their knowledge of the vocabulary, the concepts, the methods and the practical applications of different approaches of stratigraphy, a reference discipline for the entirety of the geological sciences.

Contents: 1. Stratigraphy: Foundations and Perspectives. 2. Lithostratigraphy. 3. Chemostratigraphy. 4. Magnetostratigraphy. 5. Biostratigraphy. 6. Isotope Geochronology. 7. Specific Stratigraphies. 8. Chronostratigraphic Units and Correlations. 9. The Geological Time Scale. Glossary. References.

[This book is available in French under the title "Stratigraphie. Terminologie française".]

With best wishes from the French Committee of Stratigraphy (affiliate of IUGS)

Zonal fossil is the one that characterizes or confined to certain biostratigraphic zone while the index fossil is a more general term for all fossils that help in biostratigraphy or chronostratigraphy