Discovery Science: Earth – Petrifactions – Other Fossils and Living Fossils

Earth Science: Petrifactions – Other Fossils and Living Fossils

Fossils are the legacies of former life. They vary in size from giant skeletons of dinosaurs to tender prints of dragonfly wings or minute traces of microscopically small bacteria.

Fossils document the Earth’s history, each being associated with a geological epoch prior to the Holocene. They can be remains such as bones and teeth or traces
such as footprints and feces.

Even fragile plant and animal parts may leave imprints. Black carbon film can preserve the shape of leaves or mollusks, and detailed organic structure can be fossilized in petrified forests through silification. Among the more spectacular fossils are dinosaur remains: footprints, bones, and even the occasional egg have been found on most continents.

Fully fossilized organisms allow scientists to reconstruct extinct plants and animals as accurately as possible.

Index fossils

Fossils are also excellent tools for determining the age of rocks in a method known as biostratigraphy. The plant and animal groups best used for this—called index fossils-are those that evolved quickly to reach a high degree of diversity.

Ideally, they would have existed for geologically short time periods (several hundreds or millions of years) with a wide geographical distribution.

They should be abundant and easily identifiable. Most index fossils were marine organisms such as trilobites (the arthropods of the Cambrian and Ordovician periods) or ammonites and cephalopods (of the Triassic and Jurassic periods).

Living fossils

Evolutionary scientists are also very interested in living fossils. These species are not extinct but belong to genera whose basic structures have remained similar for millions of years. They live predominantly in isolated habitats that have had little to no environmental changes over over time, slowing evolutionary change.

Living fossils are often the only species left in an order or belong to their own class. Examples include nautilus, coelacanth, horse- shoe crabs, duck-billed platypus, and marsupials. Primeval plants that are living fossils include dawn redwood and ginkgo.


THE TERM FOSSIL first occurred in the classic reference book De natura fossilium by Georgius Agricola, founder of mineralogy.

The study of fossils is called paleontology. The study of decay and fossilization processes of organisms is called taphonomy.