Modes of Preservation of Fossils | Geology Optional Notes for UPSC PDF Download

Modes of Fossil Preservation

• Unaltered remains
  • Unaltered remains refer to fossils that are preserved in their original mineral composition, despite potential compression or separation due to burial sediments.
  • It is crucial to understand the expected mineralogy of a fossil organism to determine the preservation mode applicable to the specimen.
  • Examples of common fossil mineralogy:
    • Calcite (CaCO3): Foraminifera, coccolithophores, sponges, corals, brachiopods, trilobites, molluscs, echinoderms
    • Aragonite (CaCO3): Modern corals, molluscs
    • Quartz (SiO2): Radiolarians, diatoms, sponges
    • Hydroxyapatite Ca5(PO4)3: Vertebrate bone and teeth
  • In rare cases, soft tissues like skin, feathers, or fur can be preserved, often found in materials like amber or glacial ice rather than sediments.

Permineralization

• Permineralization is a fossilization process where mineral crystals fill open spaces within porous tissues after burial.
• During permineralization, groundwater carrying dissolved minerals flows through the fossil, and upon evaporation, minerals fill the available space.
• Common infilling minerals in permineralization include quartz, calcite, and iron oxides, which are prevalent in sedimentary rocks.
• Vertebrate bones, wood, and some shells can undergo permineralization for preservation.
• Petrifaction, such as petrified wood, involves minerals filling pores and replacing organic material in plant tissues.
• Permineralized fossils retain high detail, preserving internal structures and the original organism's three-dimensional shape.
• Due to crystal growth in prior open spaces, permineralized fossils are denser and heavier than their original forms.

Carbonization

• Soft tissues, when rapidly buried in calm water, undergo pressure and temperature changes during lithification.
• This process affects the remaining organic material, leading to the release of volatile compounds.
• Following the release of volatiles, a dark carbon film, which is chemically stable, remains.
• Carbonization can preserve intricate details of delicate soft tissues, such as individual cell structures in plants, fish scales, insect wings, and even internal organs.

Replacement

• Under high pressure and temperature deep within the Earth's crust, fluids containing mobile chemical ions are released from sediments.
• These fluids engage with buried fossils, causing the original mineral or organic material to dissolve entirely.
• The dissolved material is then substituted with different ions that are deposited from the moving fluids.
• Common minerals involved in this replacement process are silica and pyrite.
• Decomposition of organic matter necessitates the presence of bacteria, aiding in sulfur reduction and the formation of iron sulfide mineral, pyrite.
• Through replacement, a significant amount of the original organism's structure can be preserved.

Recrystallization

• Recrystallization is a vital process for the preservation of fossils composed of aragonite, a commonly found yet chemically unstable mineral over long geological periods.
• Various marine invertebrates such as molluscs and modern corals have the ability to produce aragonite as their skeletal material due to their metabolic processes.
• Aragonite, which is a polymorph of CaCO3, can exhibit a striking appearance known as "mother of pearl" in the nacreous layer inside shells.
• Once removed from the living tissue of the organism, aragonite lacks chemical stability compared to calcite, prompting it to undergo spontaneous recrystallization into calcite after burial.
• The transformation into calcite enhances the longevity of the shell, increasing the likelihood of its preservation in the geological record and subsequent discovery by geologists.
• Despite the crystal structure changing during recrystallization, the fundamental chemical composition of the mineral remains unaltered, with the same atoms rearranging into a more stable form.
• While recrystallization maintains the general shape of the fossil, it may lead to the loss of finer details as new crystals develop.

Internal molds

• Definition of Internal molds:
  • Internal molds are formed when the inside space of an organism is filled with sediment that later hardens, preserving the internal shape.
• Example of Internal molds:
  • Imagine creating a sandcastle by filling a bucket with wet sand, flipping it over, and lifting the bucket. The shape left in the sand is akin to an internal mold.
• Occurrence in Geology:
  • Internal molds are commonly found in bivalves, brachiopods, or snails with a single open chamber that allows sediment to fill the space.
  • This preservation method can reveal details like muscle scars, but identifying mollusks may become challenging without the original shell.
• Geopetal Structures:
  • Small cavities within shells can retain sediment, forming pockets that may close with minerals over time, creating geopetal structures.
  • Geopetal structures act as indicators of the 'up' direction during the deposition of sedimentary layers, aiding geologists in understanding tectonic processes' effects.
• Visual Clue Interpretation:
  • Examining differences in infilled calcite crystals can indicate the 'up' direction in a geological formation, as shown by variations in crystal sizes.

External Molds

• An external mold is a preserved impression of an organism's outer shape, often formed when a fossil is pressed into soft sediment. The original hard parts of the organism may decay or be removed, leaving behind an imprint of its exterior.
• When external molds are found, they show a representation of the organism's surface, but not an exact replica. This is due to negative relief, where raised areas on the mold correspond to indentations on the original fossil, and vice versa.
• For example, imagine the footprints you leave in the sand on a beach. If those footprints were instantly turned into solid rock, they would resemble an external mold of your foot.
• Fossil trackways, like dinosaur tracks, are examples of external molds, showcasing the imprints left by organisms. These molds are commonly formed by hard shells, bones, and occasionally even soft tissues under specific conditions.
• Bioimmuration is another process through which external molds can be created. It involves the formation of molds by organisms encrusting external surfaces.
• For instance, barnacles that attach to boats can leave behind impressions of the boat surface when removed. This process can involve various organisms such as barnacles, corals, brachiopods, and sponges, showcasing a history of successive encrustations.
• Bioimmuration occurs when the original hard substrate dissolves or decays, leaving behind the encrusting organisms. This phenomenon provides insights into past ecological scenarios through the layers of encrustations.

Casts

• Casts are replicas of organisms, essentially recreating their original shape.
• Organisms are initially preserved as external molds or open burrows that are later filled with sediment to create a three-dimensional form resembling the original fossil.
• Imagine pressing clay into the external mold of a trilobite to create a cast that mimics the original fossil's shape.
• While creating casts, some details may be lost, but they still offer valuable insights into the organism.
• Many fossil skeletons displayed in museums are primarily casts, not fakes, as they are based on the original fossils.
• Reasons for using casts include challenges in mounting heavy permineralized bone specimens, fragile originals requiring controlled environments, sharing specimens among museums, and ongoing research on the fossils.