Section 3: The Fossil Record

Fossils are imprints of organisms that lived in the past, highlighting the differences between the past and the present.  Fossils vary in size, type, shape, and form only under certain conditions. Because not all organisms leave a trace behind when they die, the fossil record is not complete.

While the picture of the past is incomplete, the fossil record shows how fossils were organized to provide evidence about the history of life on Earth, including how organisms have changed over time. It revealed when an organism lived, how it lived, and how it may have died.  A paleontologist is a scientist who studies fossils. Paleontologists observe skeletal features to make inferences about an organism’s behavior or use them to compare similar skeletal structures to hypothesize evolutionary relationships of species.

The formation of fossils is rare but can occur in three different ways.  First, petrified fossils are found where remains are buried in sediment and then change to rock over time.  Molds and casts are created from the hollow spaces in sediment in the shape of an organism.  The form leaves a mold.  When a mold becomes filled with hardened minerals, it becomes a cast.  Finally, preserved remains can be found when quickly buried organisms are preserved by ice, volcanic ash, or clay before they decay.

Earth’s surface is made up of many layers of rock.  A fossil’s age can be determined depending on which layer of rock it was found in.  Geologists use relative dating and radioactive dating to determine the age of rocks and fossils.  Relative dating is when the age of fossils is determined by comparing their placement with that of fossils in other rock layers.  Radioactive dating involves measuring the amounts of radioactive isotopes in a sample to determine its actual age.

Paleontologists use the geological time scale, or a “calendar” of Earth’s history, to represent evolutionary time.  After Precambrian Time, about 570 million years ago, the geological time scale is divided into three eras: Paleozoic, Mesozoic, and Cenozoic.  Those eras are then subdivided into periods ranging from tens of millions of years to less than two million years.

During the Precambrian time, the first photosynthetic bacteria formed structures called stromatolites.  By the end of this era, organisms such as algae, sponges, and jellyfish filled the ocean.  During Paleozoic times, life exploded with many types of invertebrates, including amphibians, reptiles, and plant phyla.  During the Mesozoic era, flowering plants and mammals appeared. Dinosaurs roamed the Earth. This era is divided into three periods: Triassic, Jurassic, and Cretaceous.  During the Cenozoic time period, primates evolved and diversified.


  1. What is the fossil record?
  2. How do geologists determine the ages of rocks?
  3. What is the purpose of the geological time scale?

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