GS

Fossils

1. Fossil Preservation

Conditions Favoring Preservation

  1. Rapid Burial:

    • Protects remains from scavengers, decay, and environmental conditions.

    • Common in river deltas, volcanic ash, and landslides.

  2. Hard Parts:

    • Bones, teeth, shells (made of minerals like calcite, aragonite) fossilize better than soft tissues.

    • Example: Mollusks and vertebrate skeletons.

  3. Low Oxygen Environments:

    • Inhibits decay by slowing microbial activity.

    • Found in deep-sea floors, stagnant lakes, and anoxic swamps.

  4. Escaping Destruction:

    • Fossils must avoid physical and chemical destruction (e.g., metamorphism, erosion).

Common Modes of Preservation

  1. Petrification:

    • Organism material replaced or filled by minerals.

    • Permineralization: Minerals fill pores or cavities (common in bone and wood).

    • Mineral Replacement: Original material replaced by minerals.

      • Silicification: Silica replaces organic material.

      • Pyritization: Pyrite replaces organic tissues in low-oxygen environments.

      • Phosphatization: Phosphate minerals replace soft tissues or bones.

  2. Casts and Molds:

    • External Mold: Impression of the organism's exterior.

    • Internal Mold (Steinkern): Formed inside hollow structures (e.g., shells).

  3. Imprints:

    • Surface impressions, such as plant leaves or thin organism traces.

  4. Carbonization:

    • Organic materials compressed under heat and pressure, leaving a carbon residue.

    • Common for plant fossils.

  5. Unaltered Remains:

    • Fossils retain original material, e.g., shells, teeth, and bones.

    • Examples include ancient mammoth tusks and coral skeletons.

Uncommon Modes of Preservation

  1. Encasement in Amber:

    • Organisms trapped in tree resin, preserving soft tissues and fine details.

    • Common for insects and small plants.

  2. Mummification:

    • Preservation in arid environments, drying tissues without decay.

  3. Freezing:

    • Permafrost preserves entire organisms.

    • Example: Woolly mammoths in Siberia.

  4. Tar:

    • Sticky tar traps and preserves animals, as in the La Brea Tar Pits.

2. Bias in the Fossil Record

  • Animals with Hard Parts:

    • Fossils with skeletons or shells are more likely preserved due to durability.

    • Example: Corals and brachiopods vs. soft-bodied jellyfish.

  • Aquatic vs. Terrestrial:

    • Aquatic organisms have a higher chance of preservation due to sediment deposition in water.

    • Terrestrial organisms need rapid burial by landslides, floods, or volcanic ash.

3. Dating Fossils

Relative Dating Techniques

  1. Law of Superposition:

    • In undisturbed layers, the oldest rocks are at the bottom, and the youngest are at the top.

  2. Original Horizontality:

    • Sediments are initially deposited horizontally; tilting/folding occurs later.

  3. Cross-Cutting Relationships:

    • Intrusions or faults cutting through layers are younger than the layers they cross.

  4. Unconformities:

    • Gaps in the rock record from erosion or non-deposition.

  5. Faunal Succession:

    • Fossils follow a predictable sequence through layers.

  6. Correlation:

    • Matching layers and fossils between regions.

Absolute Dating Techniques

  1. Radiometric Dating:

    • Measures radioactive decay of isotopes.

    • Common isotopes:

      • Carbon-14: Half-life ~5730 years; for dating recent fossils (<50,000 years).

      • Potassium-Argon (K-Ar): Half-life ~1.3 billion years; for volcanic rocks.

      • Uranium-Lead (U-238/Pb-206): Half-life ~4.5 billion years; for very old rocks.

  2. Limitations:

    • Relative dating lacks precision.

    • Radiometric dating requires igneous or volcanic material near the fossil.

Combined Dating Methods

  • Use radiometric dating of volcanic ash layers with relative dating of sedimentary layers.

4. Geologic Time Scale

  • Organization:

    • Eons → Eras → Periods → Epochs.

    • Example: Phanerozoic Eon → Mesozoic Era → Cretaceous Period.

  • Key Events:

    • Mass Extinctions:

      1. End-Ordovician (440 MYA)

      2. Late Devonian (375 MYA)

      3. End-Permian (252 MYA) – largest.

      4. End-Triassic (201 MYA)

      5. End-Cretaceous (66 MYA) – asteroid impact.

    • Pleistocene-Holocene Extinction: Loss of megafauna due to climate and human activity.

5. Fossil-Bearing Sedimentary Rocks

  • Amber: Preserves small organisms in tree resin.

  • Chalk: Made of microscopic marine organisms (e.g., coccolithophores).

  • Chert: Silica-rich; preserves microfossils like radiolarians.

  • Coquina: Shell fragments cemented together.

  • Fossil Limestone: Limestone with abundant marine fossils.

  • Sandstone/Shale: Common for fossils due to sediment layering.

6. Modes of Life & Ecology

  • Life Modes:

    • Benthic:

      • Infaunal: Burrowed in sediment.

      • Epifaunal: On sediment surface.

      • Sessile: Stationary.

      • Vagrant: Mobile.

    • Planktonic: Floating organisms.

    • Nektonic: Active swimmers.

    • Terrestrial: Land-dwelling.

  • Trophic Roles:

    • Producers (plants, algae), predators, scavengers, detritivores, filter feeders.

7. Environments

  1. Marine:

    • Shallow marine (reefs), lagoons, deep ocean.

  2. Terrestrial:

    • Forests (tropical, temperate), grasslands, deserts, tundra.

  3. Freshwater:

    • Lakes, rivers, swamps.

8. Paleontological Significance

  1. Important Discoveries:

    • Tiktaalik: Transition from fish to tetrapods.

    • Archaeopteryx: Dinosaur-bird transition.

    • Feathered Dinosaurs: Link to birds.

  2. Lagerstätten Sites:

    • Burgess Shale, La Brea Tar Pits, Solnhofen Limestone.

9. Trace Fossils (Ichnofossils)

  • Evidence of behavior:

    • Tracks/Trackways: Walking or running patterns.

    • Burrows/Tubes: Dwelling traces.

    • Coprolites: Fossilized dung, revealing diet.

10. Dinosaur Trackway Calculations

  1. Hip Height:
    Hip Height=Footprint Length×4\text{Hip Height} = \text{Footprint Length} \times 4Hip Height=Footprint Length×4

  2. Head-to-Tail Length:
    Length=Footprint Length×10\text{Length} = \text{Footprint Length} \times 10Length=Footprint Length×10

  3. Relative Speed Ratio:
    Speed Ratio=Stride LengthHip Height\text{Speed Ratio} = \frac{\text{Stride Length}}{\text{Hip Height}}Speed Ratio=Hip HeightStride Length​

    • <2.0<2.0<2.0: Walking.

    • 2.0−2.92.0 - 2.92.0−2.9: Trotting.

    • >2.9>2.9>2.9: Running.