Sedimentary Rocks and Fossilization

Environmental Scene and Preservation of Fossils

  • The environmental scenario presented includes:

    • Mountains

    • River and its floodplain

    • Forest

    • Lake

    • Desert

  • Focus on determining which dead dinosaur carcasses are likely to be preserved as fossils.

Key Concepts in Fossilization

  1. Necessity of Burial for Fossilization

    • Bones require burial to have a chance of being fossilized.

  2. Natural Phenomenon: Rain

    • Rainfall occurs in mountains, causing:

      • River flooding

      • Lake filling

    • Resulting of the flooding leads to

      • Carcasses getting buried near the river and lakeshore.

Areas of Carcasses

  • Buried Carcasses

    • Locations:

    • Near the river

    • Lakeshore

  • Non-Buried Carcasses

    • Locations:

    • Forest: risk of decomposition from:

      • Plant roots

      • Soil microbes

    • Desert: dry conditions lead to deposition by wind, slower than flood burial.

    • Mountainside: unlikely to be covered and will likely decompose, rot, and weather away.

Marine Environments and Dinosaur Fossils

  • Sedimentary environments also exist in oceans.

  • Most preserved fossils in marine sediments are of marine animals.

  • Rare instances of dinosaur fossils found in marine sediments due to:

    • Coastal living of many dinosaurs increasing chances of being washed out to sea.

  • Evidence includes skeletons preserved in ancient sea floor rocks.

Types of Sedimentary Rocks

  1. Sandstone

    • Formed from sand deposits in:

      • Rivers

      • Beaches

      • Dunes

  2. Mudstone/Shale

    • Formed from mud or silt deposits, typical in lakes.

  3. Coal

    • Formed in swampy areas due to the fossilization of compressed plant remains.

  4. Limestone

    • Formed from chalky sediments composed of plankton exoskeletons, typical of marine environments.

Complete vs. Disarticulated Dinosaur Skeletons

  • Factors influencing fossil discovery:

    • Complete Skeletons Found In:

    • Flood plains

    • Lakes

    • Collapsed sand dunes

    • Reasons for Not Finding Complete Skeletons:

    • Skeleton in a flowing river.

    • Scavenging by predators.

    • Burial in forests results in plant decomposing bones.

    • Scattered bones due to animal trampling.

Environmental Effects on Bone Discovery

  • Predators can scatter bones by pulling off parts while feeding.

  • Other dinosaurs may trample remains, breaking bones further.

  • Consequences of flowing water on bones:

    • Fast flowing rivers cause bones to be broken and abraded.

    • Long bone alignment indicates directionality depending on water flow.

Fossilization Processes

  1. Preservational Styles:

    • Permineralization:

      • Internal spaces of bones filled with dissolved minerals from water.

      • Example: Fossilized plants showing mineralization.

    • Replacement:

      • Original material replaced by minerals creating a cast or impression of the original object.

      • Example: Fossilized tree root (Stigmaria) mentioned.

  2. Comparison with Dental Impressions:

    • Similar to dental molds creating casts for braces.

  3. Taphonomic Effects:

    • Plastic Deformation:

      • Asymmetrical preservation of bones (like a tyrannosaur skull) due to excessive weight and pressure from sediments.

Summary of Fossilization Process

  • Correct Order of Major Events from Dinosaur Death to Excavation:

    • A. Death

    • B. Burial

    • C. Fossilization

    • D. Erosion

    • E. Excavation

  • The correct sequence is:

    • B: Death, Burial, Fossilization, Erosion, Excavation.