FOSSILS and process of fossilization BSc 4 th

FOSSILS and process of fossilization BSc 4 th Semester (Open Elective) Fundamentals of Geology (BOE) OPEN ELECTIVE COURSE (OTHER THAN GEOLOGY MAIN STUDENTS) Dr Abdullah Khan Associate Professor Department of Geology AMU

• Fossils are the ancient remains or traces of prehistoric life. • They are important tools to have some idea about geologic past. • The study of fossils is called Palaeontology. • By knowing the nature of the life that existed at a particular time helps researchers to understand ancient environmental conditions. • Furthermore, fossils are important time indicators and plays a key role in correlating rocks of similar ages that are from different places. For such studies index fossils are used.

• Fossils are of many types. • Teeth, bones and shells are some common parts that preserve as fossils. • 1. Pertified: Given enough time, the remains of an organism are likely to be modified. Often, fossils become petrified (literally, “turned into stone”) that means internal cavities/opening and pores of the original structure got filled with mineral matter. • 2. Replacement: Replacement may occur in which the cell walls and other solid material are replaced with mineral matter.

Petrified Wood

• 3. Molds and casts: constitute another common class of fossils. • When a shell or other structure is buried in sediment and get dissolved by underground water, a mold is created. Mold is the external expression. • Molds reflects only the external shape and surface of the organism. • It does not reveal any information about internal structure. • Further, if these hollow spaces are subsequently filled secondary material, casts are created.

• 4. Carbonization: In Carbonization process leaves and delicate animal forms preserve. • As time passes, pressure squeezes out the liquid and gaseous components • Leaves behind a thin residue of carbon.

• In addition to the fossils already mentioned, there are some other numerous types. 1. Tracks—animal footprints made in soft sediment. 2. Burrows—tubes within sediments, wood or rock made by an burrowing animal. These holes may later become filled with mineral matter and preserved. 3. Coprolites—fossil dung and stomach contents that can provide useful information of food habits of organisms. 4. Gastroliths—highly polished stomach stones that were used in the grinding of food by some extinct reptiles.

Burrows Tracks Coprolites Gastroliths

(A) Body Fossils 1. Original or Unaltered soft parts 2. Original or Unaltered hard parts 3. Altered hard parts 3. 1. Carbonization 3. 2. Petrifaction or Permineralization 3. 3. Replacement 3. 4. Recrystalization (B) Trace Fossils 1. Mould and Cast 2. Tracks and Trails 3. Burrows and Borings 4. Coprolite 5. Gastrolith

Body Fossils 1. Original or Unaltered soft parts: • Such fossils are rare but often excellently preserved • Require special medium or environment like glaciers • Wooly mammoth of Siberia 2. Original or Unaltered hard parts: • Mostly hard parts are fossilized • Hard parts are skeleton/teeth composed of calcite, aragonite, phosphate, silica etc.

3. Altered Hard parts: • Most hard parts are altered • A variety of processes are responsible for altering hard parts like Carbonization, Petrifaction or Permineralization, Replacement and Recrystalization • Majority of fossils belong to this category 3. 1. Carbonization: In Carbonization process leaves and delicate animal forms preserve. As time passes, pressure squeezes out the liquid and gaseous components. Leaves behind a thin residue of carbon. 3. 2. Petrifaction or Permineralization: Pores and cavities of original hard part are filled with minerals deposited by ground water and original hard part becomes heavier, stone-like and more resistant to destruction. Some common permineralizers are silica, calcium carbonate and iron compounds. Generally, Tree trunks are commonly preserved by this process

3. 3. Replacement: Molecule by molecule solution of original hard part and simultaneous deposition of minerals by ground water take place in such a way that external and internal structures of the original hard part remain intact/same. 3. 4. Recrystalization: Original mineral of hard parts changes into a more stable mineral. e. g Aragonitic hard parts change in to calcitic hard parts.

Trace Fossils • Fossils in which no organic remain or its replacement is preserved but which provide an indirect evidence of the presence of organism. 1. Mould and Cast: Mould is the impression of an organism or organic structure (bone, shell, teeth etc. ) in the surrounding material. It forms when the organic material is removed by solution or erosion. Cast is formed when a mould is filled with some secondary minerals. 2. Tracks and Trails: Tracks are foot prints of animals. A series of tracks may indicate length of limbs, posture etc. Tracks are mostly preserved in arid and semiarid areas. Trails are impression made by the bodies, tails, or other associated parts. 3. Burrows and Borings: Burrows are excavations made by animals in soft sediments. • Borings are holes made by animals within rocks and wood for food, shelter. Generally they are circular in cross-section. 4. Coprolites: Fossil fecal pellets of animals that are nodular, tubular or pellet like in shape and phosphatic in composition. They provide valuable information related to food habits. 5. Gastrolith: Highly polished, rounded stones that are used for grinding in the stomach.

Preservation of fossils • Two conditions are very important for preservation of fossils: Immediate Burial & Possession of hard parts

Fossils and Correlation Index fossil: Widespread geographically and are limited to a short span of geologic time, so they help in determining the age rocks and correlation of rocks at two different places.

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