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.