Application of Paleontology with special reference to- 'CORRELATION'

For correlation, useful fossils are those that are abundant, widespread, rapidly evolving (and therefore of short range), and distributed in a variety of facies. For example, ‘Trilobites’ are main tool of correlation in Cambrian and Lower Ordovician; ‘Ammonites’ for Mesozoic; ‘Graptolites’ for Ordovician to lower Devonian; ‘Ammonoids’ are important in Devonian through Permian; ‘Conodonts’ used from Ordovician through Triassic; ‘Foraminiferans’ are used for correlating the rocks of Upper Paleozoic and post Jurassic time.

  Interpreting historical sequence or stratigraphy based on the fossil contents of the rock is ‘Biostratigraphy’. It is the only practical method of dating and correlating most sedimentary rocks, and is the fundamental tool of determining geologic history in most regions. In biostratigraphy, pelagic organisms such as ammonoids, conodonts and graptolites are more useful in globally, while bottom dwellers like brachiopods, bivalves, gastropods and corals are used locally.

Zone fossils: Certain fossil species which have good stratigraphical markers. They characterize short sections of the geological succession, e.g. Pecten sp. (Bhuban formation of Mizoram), Ophiceras sakuntala (Ophiceras zone to Triassic of Spiti), etc.

   Zonations are used to correlate and date sedimentary strata. A ‘zone’ is a fundamental unit in biostratigraphy. The most common types of zones are:

• Range Zone: defined by stratigraphic range of a selected taxon in a fossil assemblage
• Concurrent Range Zone: the overlapping part of the ranges of two or more selected taxa
• Interval Zone: the stratigraphical interval between two successive biostratigraphic events
• Assemblage Zone: defined by a distinctive assemblage of fossil taxa
• Phylo Zone: defined on the basis of the range of a taxon representing a segment of an evolutionary lineage (between the predecessor and the successor)

Index fossil and correlation of rocks: The fossil (or species) which have limited geological range or vertical distribution and a very wide geographical range or lateral distribution are called Index fossils or Guide fossils or Marker fossils. Such fossils are restricted to specific rock formations. The rocks having index fossils in the different parts of the world can be correlated with each other. The correlation may be through the study of index fossils, study of fossils assemblage, and study of stages of evolutionary development.

Important index fossils of Mizoram, India:- Chlamys (Argopecten) senatoria (Gmelin), Diplodonta (Diplodonta) incerta d’Archiac an index fossils of Aquitanian-Burdigalian (Lower Miocene). Pseudotaberina malabarica, an important index species of Lower Miocene. Turritella elongata, index fossil of Lower Eocene. Nummulites ghizehensis, an index fossil of Middle Eocene. Ostrea latimarginata, an index fossil of Lower Miocene. 

References:

Jain P.C. and Anantharaman M.S. (2010). Palaeontology (Palaeobiology): Evolution and Animal Distribution. Vishal: Delhi.

Laldinpuia (2006). Mega-Invertebrates Palaeontology of Upper Bhuban Formation in and around western Aizawl. M.Sc. Dissertation (Unpublished).

Prothero D.R. (1998). Bringinging Fossils to Life (An Introduction to Paleobiology). WCB McGraw-Hill: USA.

Saraswati P.K. (2000).  Quantitative Biostratigraphy. In: Lecture Notes on Physical Stratigraphy and Paleobiology. DST: New Delhi.

Stearn C. and Carroll R. (1989). Paleontology: The Record of Life. Wiley: Canada.

Tiwari R.P. and Kachhara R.P. (2003). Molluscan Biostratigraphy of the Tertiary Sediments of Mizoram, India. Journ Pal. Soc Ind, 48: 65-88.