Fossils and Fossilization

What are Fossils?

We're moving into the paleoanthropology section of the course. Remember that paleoanthropology is the study of human evolution through hominin fossils. We will be studying macroevolution, or species-level changes.

Fossils can include:

  • Trace fossils: Tracks/impressions of past activities.

  • Coprolites: Fossilized poop.

  • Whole or partial fossils: Remains of organisms.

The Fossilization Process

Fossil: The remains of an organism that have been partially or wholly transformed into rock.

Understanding taphonomic processes is key to understanding how fossils are made.

Taphonomy: The study of the deposition of plant/animal remains and the environmental conditions affecting their preservation.

During fossilization, replacement occurs, where calcium and phosphorous in bones and teeth are replaced with silica and iron.

Limitations of the Fossil Record

There are several biases and limitations in the fossil record:

  • Taphonomic processes: Specific conditions are needed, such as the right soil and burial conditions.

  • Search limitations: Paleontologists and paleoanthropologists can only search in certain places, and war can limit these searches.

  • Geological alterations: Geologic processes can alter the strata (rock layers) in some places.

  • Environmental conditions: Not all areas of the world that could have fossils have the right conditions at all times.

An example of a location where important fossil discoveries have been made is the Fayum Depression in Egypt.

⏱ Molecular Clock

Molecular clock: An evolutionary chronology based on the assumption that mutations occur at a constant rate over time, verified with the fossil record.

The molecular clock is used to understand divergences in primate lineage and is a reliable dating technique.

🗓 Relative vs. Absolute Dating Techniques

Relative Dating

Relative dating: Provides an age estimate for a fossil compared to something else.

It is based on Steno’s Law of Superposition: the lower the stratum, the older the fossil; the higher the stratum, the younger the fossil.

Relative dating techniques include:

  • Stratigraphic comparisons

  • Biostratigraphic comparisons

  • Fluorine dating

  • Cultural dating

  • Geomagnetic polarity

Relative Dating Methods

  • Stratigraphic comparisons: Matching up strata from several sites to provide an age. This can be done through chemical or comparative means using diagnostic artifacts, or by comparing strata within a site.

  • Biostratigraphic dating: Using the association of fossils within strata to estimate the approximate age for each layer.

    • Index fossil: A fossil from a specific and known time and geographic range that can be used to estimate the age of a stratum.

    • Example: Mammoths in Europe, Asia, and North America. Molars became increasingly complex after 2.5 million years ago, which can be used to estimate a site’s age.

Absolute Dating

Absolute dating: Providing an actual age range for a fossil, mostly based on the decay of radioactive isotopes.

Absolute dating techniques include:

  • Dendrochronology

  • Radiocarbon dating

  • Radiopotassium dating (K/Ar) aka potassium-argon method

  • Argon-argon method

  • Fission-track

Relative Dating Methods Continued

  • Fluorine dating: Compares the accumulation of fluorine in animal and human bones from the same site.

    • Bone within the ground loses nitrogen and gains fluorine.

    • The amount of fluorine depends on groundwater, so it's site-specific.

    • Good for comparing fossils found near each other.

    • Example: Piltdown Man from Sussex, England in 1912. Fluorine dating revealed it to be only 50,000 years old. Carbon dating then moved that to less than 600 years old for the cranium. It was a human cranium and orangutan jaw.

  • Cultural dating: Based on the approximate time range of a material culture.

    • Example: Oldowan tool culture began around 2.5 million years ago.

🧪 Absolute Dating (aka Chronometric)

  • Radiopotassium dating: Measures the ratio of Potassium (K)-40 to Argon (Ar)-40 in material older than 200,000 years.

    • 40K40K decays into 40Ar40Ar. Argon is not naturally found in volcanic/igneous rock.

    • Half-life is 1.25 billion years.

    • Dates the rock around the artifact/fossil, not the artifact/fossil itself.

    • First used by the Leakeys to show that a skull from Tanzania was 1.8 million years old.

    • 40Ar/39Ar40Ar/39Ar is the more up-to-date and more accurate version of this test but functions on the same principle and can use smaller samples.

  • Fission-track dating: Based on 238U238U decay to 235U235U, used to check K/Ar dates, and is good for non-crystalline samples older than 100,000 years, providing a minimum age.

  • Dendrochronology: Tree-ring count to determine the age of a site.

    • Count the rings in structural remains.

    • Examine the width of the rings to assess seasonality, drought, and rainy years.

    • A master sequence of dating for various world regions is created by overlapping the ring sequences of living trees with those of dead ones.

    • Match the thin rings with dry years, thick ones with wet years from historical records to get the exact year the tree was cut down.

🌡 What Else Can Isotopes Do?

  • 18O18O: A stable form of oxygen that can tell us about past climate change.

    • Samples of foraminifera (sea-dwelling microorganisms) are taken from ice cores and ocean floor sediments.

      • In periods where temperatures were low, there are high levels of 18O18O.

      • Periods of high temperatures have low levels of 18O18O.

    • Ancient soils and plants also inform environmental conditions.

  • Radiocarbon dating: Ratio of Carbon (C)-14 measured to provide a date for material less than 50,000 years old (preferably no older than 40,000 years).

    • Became possible to use this method after WWII, during the nuclear age.

    • 14C is the radioactive isotope of carbon; 12C and 13C are stable isotopes.

    • Isotope: Two or more forms of an element that have the same number of protons but vary in the number of neutrons.

    • 14C decays into 14N: Compare this to the amount of 14C and multiply percentage by half-life.

    • Half-life of 5,730 years.

    • Dates any organic material that contains carbon; burned material preserves better.

  • 12C and 13C: Can inform us about past dietary habits/environment.

    • Levels of these ratios can tell us if animals or people were eating C3 or C4 plants.

      • C3 plants: Wheat, rice, barley, rye, spinach, peanuts – less 13C.

      • C4 plants: Corn, millet, sugarcane, CAM plants (agave), tropical grasses – more 13C.

Ask me any question about your notes or content!

What are the key limitations of the fossil record?

Explain the main difference between relative and absolute dating.

How does the molecular clock help understand evolution?

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