Changes in Species Over Geological Time: Fossil Record & Dating

Fossil Record: Evidence of Change

• The fossil record provides a historical sequence of life, showcasing changes in organisms over time.
• Fossils are the preserved remains or traces of past organisms.
• Fossil formation requires specific environmental conditions that prevent decomposition (e.g., rapid burial, presence of hard parts).
• Fossils are predominantly found in sedimentary rock, which forms layers over time.

KEY TAKEAWAY: The fossil record is incomplete but provides crucial evidence for evolution by showing changes in organisms across geological time.

Faunal (Fossil) Succession

• Faunal succession is the principle that fossil organisms are found in sedimentary rocks in a specific, predictable order.
• Older rock layers contain fossils of simpler organisms, while younger layers contain fossils of more complex organisms.
• This ordered appearance reflects the evolutionary history of life on Earth.
• The sequence of fossils provides evidence that life has changed over time.

EXAM TIP: Understand that faunal succession is a relative dating method based on the predictable sequence of fossils.

Index Fossils

• Index fossils are fossils of species that:
  • Lived for a relatively short period.
  • Were geographically widespread.
  • Are abundant in the fossil record.
• Index fossils are used to determine the relative age of rock layers and other fossils.
• When an index fossil is found in different rock layers in different locations, it indicates that those layers are of similar age.

COMMON MISTAKE: Confusing index fossils with transitional fossils. Index fossils are for dating, while transitional fossils show evolutionary links.

Transitional Fossils

• Transitional fossils (also known as intermediate fossils) exhibit characteristics of two different groups of organisms.
• They provide evidence for evolutionary transitions, showing how one group evolved into another.
• Examples:
  • Archaeopteryx (reptile-like bird): feathers and wings (bird-like) and teeth and a bony tail (reptile-like).
  • Tiktaalik (fish-like amphibian): fins with wrist-like bones (amphibian-like) and scales and gills (fish-like).

VCAA FOCUS: Be prepared to discuss specific examples of transitional fossils and the evolutionary relationships they illustrate.

Dating Fossils

Relative Dating

• Relative dating determines the age of a fossil relative to other fossils or rock layers. It does not provide a numerical age.
• Methods:
  • Stratigraphy (Law of Superposition): In undisturbed sedimentary rock layers, the oldest layers are at the bottom, and the youngest layers are at the top. Fossils in lower layers are older than those in upper layers.
  • Faunal Succession: Using the known sequence of fossils to determine relative ages.
  • Index Fossils: Comparing the presence of index fossils in different rock layers.

STUDY HINT: Practice drawing diagrams of rock layers and placing fossils in the correct relative order.

Absolute Dating

• Absolute dating (also known as radiometric dating) provides an approximate numerical age for a fossil or rock.
• It is based on the decay of radioactive isotopes.
• Radioactive isotopes decay at a constant rate, measured by their half-life (the time it takes for half of the isotope to decay).
• Common methods:
  • Radiocarbon dating (Carbon-14 dating): Used for dating organic material up to ~50,000 years old. Based on the decay of $^{14}C$ to $^{14}N$.
  • Potassium-Argon dating (K-Ar dating): Used for dating volcanic rocks millions of years old. Based on the decay of $^{40}K$ to $^{40}Ar$.
  • Uranium-Lead dating (U-Pb dating): Used for dating very old rocks (billions of years old). Based on the decay of $^{238}U$ to $^{206}Pb$ and $^{235}U$ to $^{207}Pb$.

Half-life is constant for each isotope:
$$ N(t) = N_0 e^{-\lambda t} $$
Where:
* $N(t)$ is the quantity of the isotope remaining after time $t$.
* $N_0$ is the initial quantity of the isotope.
* $\lambda$ is the decay constant, related to the half-life ($t_{1/2}$) by $\lambda = \frac{ln(2)}{t_{1/2}}$.
* $t$ is the time elapsed.

REMEMBER: Different dating methods are suitable for different time scales and materials. Radiocarbon dating is useless for dinosaur fossils.

APPLICATION: Radiometric dating is used to construct the geological timescale and to understand the timing of major evolutionary events.