Fossils and Geologic Time

Fossils and Geologic Time

Introduction

  • Understanding Earth’s past helps to:

    • Comprehend current changes.

    • Analyze how human choices influence future population evolution.

Learning Goals (Bio.7.d; 6.b.2, 7.a.1-2)

  • Identify the processes involved in fossil formation.

  • Compare and contrast Relative and Radioactive Dating.

Fundamental Concepts

  • What:

    • Fossil Formation

    • Geologic Time

Fossils

  • Definition:

    • Fossils are the preserved remains or evidence of an organism.

The Fossil Record

  • Purpose of Fossil Records:

    • Illustrate changes in different groups of organisms (species) over time.

  • Benefits of Fossil Records Include:

    • Inferring:

    • The time period the organism inhabited the Earth.

    • The anatomical structures of organisms.

    • Dietary habits (what food they consumed and what predated on them).

    • The environmental conditions of their habitats.

  • Disadvantages:

    • Existence of large gaps in the fossil record which can lead to inaccuracies in our understanding of evolutionary history.

Paleontology

  • Definition:

    • Paleontology is the scientific study of prehistoric life through the analysis of fossils, rocks, and historical climate data.

Types of Fossils

  1. Trace Fossils:

    • Evidence of movement or activity of organisms (e.g., footprints).

  2. Imprint Fossils:

    • Formed before sediment hardens (e.g., leaves impressed in sediment).

  3. Mold Fossils:

    • Negative imprint left after an organism decays, resulting in empty space.

  4. Cast Fossils:

    • Created when a mold is filled with minerals, resulting in an exact replica of the organism.

  5. Petrified Fossils:

    • Organic materials replaced by minerals, transforming them into stone.

  6. Amber Fossils:

    • Organisms trapped in hardened tree sap.

How Fossils Form

  • Most fossils are formed in sedimentary rock, created over time through:

    1. The deposition of broken-down rock layers.

    2. Water transporting small particles to lakes and seas.

    3. The burial of deceased organisms by layers of sediment, which eventually hardens into rock, preserving the remains for future study.

Dating Fossils

Relative Dating

  • Method of determining a fossil's age based on its depth in layers of sediment and the presence of index fossils.

    • Index Fossil:

    • A species that is easily recognizable and used to compare the relative age of fossils.

    • Relatively older fossils are found in lower sediment layers, while younger fossils are closer to the surface.

Radioactive Dating (aka Radiometric or Absolute Dating)

  1. Definition:

    • A precise method of dating fossils based on the decay rate of radioactive isotopes over time.

  2. Concept of Half-Life:

    • The half-life is defined as the time required for half of the radioactive atoms in a sample to decay.

    • Example: The half-life of Carbon-14 is approximately 5730 years.

Key Radioactive Isotopes and Their Half-Lives

Isotope

Parent

Daughter

Half-life of Parent (years)

Useful Range (years)

Carbon-14

Nitrogen-14

5730

100 - 30,000


Potassium-40

Argon-40

1.3 billion

100,000 - 4.5 billion


Rubidium-87

Strontium-87

47 billion

10 million - 4.5 billion


Uranium-238

Lead-206

4.5 billion

10 million - 4.6 billion


Uranium-235

Lead-207

710 million

10 million - 4.6 billion


Comparing Relative and Radioactive Dating of Fossils

Relative Dating

  • Objective:

    • Determines the age of fossils with respect to other fossils (older or younger).

  • Methodology:

    • Involves comparing fossil depths and sedimentary layers.

  • Drawbacks:

    • Imprecision in dating and inherent limitations in age data.

Radioactive (Absolute) Dating

  • Objective:

    • Provides a more precise age of fossils measured in years.

  • Methodology:

    • Involves determining the relative amounts of radioactive isotopes versus non-radioactive isotopes in a specimen.

  • Drawbacks:

    • Requires complex laboratory methods.

Key Learning Objectives

  • Understanding Evaluation:

    • Describe each step involved in fossilization.

    • Use both relative and radioactive dating to ascertain the ages of fossils.

    • Identify processes of fossil formation.

    • Compare and contrast relative and radioactive dating methods.

Geologic Time Scale

  • Development:

    • Established through the study of rock layers and the fossil record, though many gaps exist, making it imprecise.

  • Divisions:

    • Represent evolutionary time.

  • Four Major Eras:

    1. Precambrian Era (oldest)

    2. Paleozoic Era

    3. Mesozoic Era

    4. Cenozoic Era (current)

Analyzing Geologic Time

Constructing the Timeline:

  1. Gather Era cards and arrange them in chronological order from oldest to youngest.

  2. Utilize information from subsequent slides to appropriately place organism cards within the correct eras.

Analytical Questions:

  1. What evolutionary events occurred since the Precambrian Era?

  2. How did photosynthetic bacteria alter the atmospheric conditions during the Paleozoic era to enable increased diversity?

  3. In what era does the Endosymbiotic Theory belong?

  4. Why did the emergence of eukaryotes lead to increased diversity of life?

  5. Discuss the significance when finding a bird fossil dated at 300 million years ago.

  6. Identify one adaptation that allowed mammals to survive changing environments.

  • Explore how early life events shaped the present day Earth.

Precambrian Era

  • Timeframe:

    • Approximately 4.6 billion to about 550 million years ago.

  • Dominant Organisms:

    • Bacteria and single-celled organisms:

    • Heterotrophic prokaryotes.

    • Chemoautotrophic prokaryotes.

    • Photoautotrophic prokaryotes (e.g., cyanobacteria).

    • Appearance of simple eukaryotes (e.g., algae).

  • Major Events:

    • Oxygen Revolution.

Paleozoic Era

  • Timeframe:

    • Approximately 550 million to about 250 million years ago.

  • Dominant Life Forms:

    • Invertebrate organisms (those lacking backbones) dominated including:

    • Sponges, jellyfish, squids & octopuses, worms, trilobites, and nautilus (living descendant).

    • Emerging vertebrates towards the era's end.

Significant Developments in Paleozoic Era:

  • First appearance of:

    • Sharks and fish

    • Amphibians

    • Insects

    • Land plants (ferns, mosses)

    • Gymnosperms (cone-bearing plants, including conifers).

Mesozoic Era

  • Timeframe:

    • Approximately 250 to 65 million years ago.

  • Characterization:

    • Age of Reptiles and Dinosaurs.

  • Dominant Organisms:

    • Reptiles, dinosaurs, and the first birds (towards the era's end).

  • Key Events:

    • Climate shift towards dryness.

    • Complete breakup of the supercontinent Pangea.

Cenozoic Era

  • Timeframe:

    • Approximately 65 million years ago to the present.

  • Characterization:

    • Age of Mammals.

  • Dominant Organisms:

    • Mammals found in diverse habitats (land, water, air).

    • Emergence of the first primates and humans (appeared around 4.5 million years ago).

    • Dominance of flowering plants.