Fossils 101

Fossils are the preserved remains, or traces of remains, of ancient organisms. So, contrary to what my granddaughters believe, Grandpa is not a fossil – yet.


Paleontology is the study of fossils: their age, method of formation. Examples of fossils include:

  • Petrified Wood
    Petrified Wood


  • shells,
  • exoskeletons,
  • stone imprints of animals or microbes,
  • objects preserved in amber,
  • hair,
  • petrified wood,
  • oil,
  • coal, and
  • DNA remnants.

Fossils are commonly not the actual physical remains of the organism itself, but rather the remains are now rocks. A fossil can preserve an entire organism or just part of one. For example, bones, shells, feathers, wood, and leaves can all become fossils.

One of the most common fossils is petrified wood. The petrification replacement process can preserve details of the nature of the wood, so one can determine what type of tree it was. This process gives the former wood material different colors and textures. Good quality agate or opalized petrified wood is considered a gemstone.

Macrofossils and Microfossils

Fossils can be very large or very small. Microfossils are only visible with a microscope. Bacteria and pollen are microfossils. Macrofossils can be several meters long and weigh several tons. Macrofossils can be petrified trees or dinosaur bones.

How old are Fossils?

Fossils can come from the Archaean Eon (which began almost 4 billion years ago) all the way up to the Holocene Epoch (which continues today). The observation in the 19th century that certain fossils associated with specific layers or types of rock led to the discovery and development of a geological timescale and the relative ages of different fossils. The development of age-dating techniques in the early 20th century allowed scientists to quantitatively measure the absolute ages of rocks and the fossils they host.

Dinosaur + Ostridge Egg
Dinosaur and Ostridge Egg

Some of the most famous fossils are those of the Dinosaurs. Dinosaurs lived on the planet from about 240 million years to 65 million years ago. An interesting comparison is to look at a dinosaur egg and an Ostridge egg side by side. They are very similar in shape, size, and texture of the eggshell. One can see why it is believed that birds relate directly to dinosaurs.

Another common fossil seen in stores is that of a Trilobite. These ocean-dwelling creatures existed from about 600 to 400 million years ago. Their preservation is the detail of the body replaced by rock. I have seen some of the smaller specimens made into neckless pendants.


Collecting Fossils

While collecting fossils can be fun, you must be very careful to have permission to collect them. For instance, the collection should not take place on public lands, such as parks and preserves. Unfortunately, amateur collectors more often than not cause damage to the site and destroy the scientific value of the fossils for future generations.

So the next time you read the book or see the movie “Jurassic Park”, you can say in confidence, “See, they are using a fossil that is trapped in amber.”

National Fossil Day

Wednesday, October 13th, 2021 is National Fossil Day and the 12th Anniversary of the annual celebration. It happens each year to highlight the scientific and educational value of paleontology. For more information, please check out

Alex Schriener
Alex Schriener, Jr.

Alexander Schriener, Jr. – Associate Geologist, PG has been employed as a geologist for over 40 years and a California Licensed Professional Geologist for nearly 20 years, most of it in geothermal energy development and exploration. For twenty years he has worked as a geologist and Resource Manager at the Salton Sea geothermal field.