Fossils and the Geologic Time Scale

What is a Fossil?

  • A fossil is an impression, cast, original material or track of any animal or plant that is preserved in rock after the original organic material is transformed or removed.
  • Preserved remains or traces of an organism that lived in the past.
  • Fossil are formed when organisms die and are buried in sediment. Eventually sediment builds up and hardens to become sedimentary rock.
  • Sediments are pieces of solid material that have been deposited on Earth’s surface by wind, ice, gravity, or chemical precipitation.
  • Paleontologist – scientist who study the remains of organisms in the rock record.

A fossil may be:

  • An original skeleton or shell
  • A mold or cast
  • Material that has replaced the once living thing;
  • Traces such as footprints or warm tubes.

What conditions promote fossilization?

  • Hard body parts such as skeletal bones or exoskeletons.
  • Rapid burial and/or lack of oxygen.

What are the fossil types?

  • Body fossils – actual parts of an organisms, unaltered or altered bones, shells, leaf imprints.


  • Trace fossils – evidence of life that is not a body fossil: tracks, burrows, casts.


What are the modes of fossil preservation for body fossils?


  1. Original Material – original, unaltered material from the living organism unaltered bone or shell.
  2. Encrustrations or entombments – material is trapped inside coating such as amber.
  3. Mummification – quickly dried material.
  4. Refrigeration – material is trapped inside ice and tissue is preserved.


  1. Permineralization – pores in tissue are filled by minerals.
  2. Replacement – replacement of tissue with minerals.
  3. Carbonization – tissue material is decomposed or reduced to a film of carbon.

Kinds of Fossils:

  • Petrified – when minerals replace the remains and they become rock things like wood.
  • Mold – when the shell remains and the contents dissolve (hollow) like in art class you use a mold to get the correct shape of a bowl.
  • Cast – when the mold becomes filled with minerals that are not a part of the original organism.
  • Index – a fossil found in a narrow time range but widely distributed around the earth: used to date rock layers.
  • Trace fossil – a fossilized mark that is formed in soft sediment by the movement or actions of an animal.
    • burrows or borings – spaces dug out by living things and preserved as is or filled in.
    • gastroliths – smooth stones from abdominal cavity of dinosaurs.
    • coprolites – fossilized excrement; usually preserved by replacement.
    • tracks – impressions of passage of living things.

Determining the Age of Rocks / Fossils

Dating techniques are procedures used by scientists to determine the age of a specimen.

  1. Absolute dating is the process of determining an age on a specified chronology in archaeology and geology. Some scientists prefer the terms chronometric or calendar dating, as use of the word “absolute” implies an unwarranted certainty of accuracy. Absolute dating provides a numerical age or range in contrast with relative dating which places events in order without any measure of the age between events. In archaeology, absolute dating is usually based on the physical, chemical, and life properties of the materials of artifacts, buildings, or other items that have been modified by humans and by historical associations with materials with known dates (coins and written history). Techniques include tree rings in timbers, radiocarbon dating of wood or bones, and trapped charge dating methods such as thermoluminescence dating of glazed ceramics.
  2. Relative dating methods determine whether one sample is older or younger than another. They do not provide an age in years. Before the advent of absolute dating methods, nearly all dating was relative. The main relative dating method is stratigraphy .


Geologic Time Scale

  • Geologic time: it is very, very long. Earth is estimated to be around 4.6 billion years old based on the rock and fossil record. Geologic time is broken up into sections based on major changes in Earth.
  • Largest division: Eons.
  • Eons divided into Eras:
    • PreCambrian, Paleozoic, Mesozoic, Cenozoic
    • Ceno – recent
    • Meso – middle
    • Paleo – ancient
    • Zoic -life
  • Eras are divided into Periods
  • Periods are divided into Epoch.

PRECAMBRIAN (543 million years ago to ~4 billion years ago)

Age of Bacteria

Pre-Cambrian Eons / Era made up the following periods;

  • Hadean
  • Archaen
  • Proterozoic

This was the time to which no living organisms existed yet with the exception of a bacteria known as stromatolites. Pre-Cambrian began with the formation of the Earth 4.6 billion years ago. Bacteria appeared 3.5 billion years ago, followed by algae and fungi.


The Phanerozoic Eon brought about three Eras.

a. Paleozoic Era – 543 million years ago to 248 million years ago

“Age of the Fish”

  • Explosion of life in the sea: trilobite, shellfish, fish
  • Life appears on land: ferns, amphibians
  • Pangaea formed
  • Largest mass extinction ever at the end of the Paleozoic. 90% of species became extinct.
  • The Paleozoic Era is known to be start up for life. Explosion of life started with Cambrian Period to which most organisms present had hard shell covering.
  1. Cambrian Period – Sponges, snails, clams and worms evolve.
  2. Ordovician Period – first fishes evolved and other species become extinct. Ordovician Period’s typical marine community consisted of organisms such as green algae, primitive fishes, cephalopods, corals and gastropods. One important creature that existed in this time are the nautiloids which is a tentacled mollusk.
  3. Silurian Period was the age where terrestrial plants started to flourish thus pushing some marine organisms out of the water and into the land. Land plants, insects and spiders appear.
  4. Devonian Period is also known as the age of fishes. The first amphibians breathed through simple lungs and their skin. They may have spent most of their lives in the water, leaving it only to escape the attentions of predatory fish. Cone-bearing plants start to appear.
  5. Carboniferous Period is famous for its vast swamp forests. Vegetation included giant club mosses, tree ferns, great horsetails and towering trees with strap-shaped leaves. Insects also flourished during this period. Tropical forests appear and reptiles evolve. In Europe the Missippian and Pennsylvanian periods are together called the Carboniferous period. Vast swamps and forests from that time created the huge coal beds found throughout eastern United States.
  6. The Permian period, which ended in the largest mass extinction the Earth has ever known, began about 299 million years ago. Early reptiles were well placed to capitalize on the new environment. Shielded by their thicker, moisture-retaining skins, they moved in where amphibians had previously held sway. Seed plants become common and insects and reptiles become widespread. Sea animals and some amphibians begin to disappear.

b. Mesozoic Era – 246 million years ago to 65 million years ago

“Age of the Reptiles”

  • Life: Reptiles are the dominant life on land. Dinosaurs exist. Birds appear. Forests of trees appear.
  • Pangea broke apart during this period.
  • Dinosaurs become extinct in a mass extinction at the end of the Mesozoic.
  • Mesozoic Era was divided into 3 periods; Triassic, Jurassic, and Cretaceous.
  1. Triassic Period was a time of tremendous change and rejuvenation. Life that survived the so-called Great Dying repopulated the planet, diversified into freshly exposed ecological niches, and gave rise to new creatures, including rodent-size mammals and the first dinosaurs. The Triassic closed in the same way it began. Something—perhaps a volcanic belch or an asteroid collision—caused another mass extinction. Dinosaurs, however, survived and went on to dominate the Jurassic.
  2. The Jurassic period (199.6 million to 145.5 million years ago) was characterized by a warm, wet climate that gave rise to lush vegetation and abundant life. Many new dinosaurs emerged—in great numbers. Among them were stegosaurs, brachiosaurs, allosaurs, and many others.
  3. Long before the carnage began, the Cretaceous picked up where the Jurassic left off: Gigantic sauropods led parades of dinosaurs through the forests, over the plains, and along the coasts; long-necked and toothy marine reptiles terrorized fish, ammonites, and mollusks in the seas; pterosaurs and hairy-feathered birds filled the skies. Flowering plants appear, mammals become more common, dinosaurs become extinct in the Cretaceous period.
  • Some of the dinosaurs are thought to have evolved into birds. Archaeopteryx is the oldest fossil bird found to date.

Dinosaur Extinction

  1. Volcanoes erupt and fill the atmosphere with CO2 and gases that block the sun light preventing plant life.
  2. Meteorite impact causes multiple natural disasters.

c. Cenozoic Era – 65 million years ago to present

“Age of the Mammals”

  • Life: Large mammals appear, humans appear.
  • Ice ages occur and ice sheets advance.
  1. Tertiary period – first primates appear and flowering plants become the most common.
  2. Quaternary period – humans evolve and large mammals like woolly mammoths become extinct.

Paleogene period is most notable for being the time during which mammals diversified from relatively small, simple forms into a large group of diverse animals in the wake of the Cretaceous- Paleogene extinction event that ended the preceding Cretaceous period. Cooling and drying period.

  • Paleocene epoch
  • Eocene epoch
  • Oligocene epoch


Neogene period –  Sharks grew and dominated the seas once again. Megalodon, the biggest shark of all, was nearly 50 feet (15 meters) long. During this period, mammals and birds continued to evolve into roughly modern forms, while other groups of life remained relatively unchanged. Early hominids, the ancestors of humans, appeared in Africa near the end of the period.

  • Miocene epoch
  • Pliocene epoch


Quaternary period is typically defined by the cyclic growth and decay of continental ice sheets associated ice sheets associated with Milankovitch cycles and the associated climate and environmental changes that occurred.

  • Pleistocene epoch
  • Holocene epoch




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