Types of Fossilization: How Fossils Form

Fossils are the remains and traces of ancient, once-living organisms. To qualify as a fossil, an organism usually must be more than 10,000 years old. Out of all the millions of species that have evolved and lived on this planet since life first began 3.9 billion years ago, very few have been preserved as fossils.

There are many types of fossilization. Organisms with hard parts, such as bones, scales, teeth, or shells, are better documented in the fossil record because they are more likely to fossilize. Soft body parts, such as skin, muscle tissue, and organs, decompose rapidly and are often consumed by predators before fossilization can take place.

Replacement

Replacement, or petrification, occurs when the original material (bone, shell, organic plant matter) is replaced by minerals. The original plant material of this Cunninghamia cone was replaced during fossilization by silica, a common quartz mineral. This specimen was discovered in an ironstone bed in Midland Provincial Park near the Museum.

Cunninghamia cone. 72 – 66 million years old. Late Cretaceous, Horseshoe Canyon Formation. Midland Provincial Park, Alberta.

Cunninghamia cone. 72 – 66 million years old. Late Cretaceous, Horseshoe Canyon Formation. Midland Provincial Park, Alberta.

Permineralization

Once buried, minerals carried in ground water may be deposited in the empty spaces within plant or animal matter, resulting in permineralization. This type of fossilization often shows incredible detail, like the microstructure of bones. One of the best ways to determine how old a dinosaur was when it died is to take a cross section of a limb. Thin slices were removed from this hadrosaur tibia, and then examined under a microscope. Researchers have determined that this hadrosaur was three years old when it died.

We removed a section of this hadrosaur tibia to make a mold of it, and cast a copy. We then glued the cast back into the limb so the specimen retained its original shape.

We removed a section of this hadrosaur tibia to make a mold of it, and cast a copy. We then glued the cast back into the limb so the specimen retained its original shape.

Changes in the bone texture show a cyclical variation, indicating the yearly growth of the animal, like rings in a tree stump.

Changes in the bone texture show a cyclical variation, indicating the yearly growth of the animal, like rings in a tree stump.

Carbonization

When a plant or animal dies and is buried by sediment, hydrogen and oxygen are eliminated, resulting in a carbon etching. Plant leaves are often fossilized in this manner. This fern fossil was found by Museum staff at the Grassy Mountain Coal Mine, 7 km northeast of Blairmore, Alberta. It’s an excellent example of the exquisite plant fossils found in the area since the mine has been operating (on and off for nearly 100 years).

Unidentified fern. 150 – 120 million years old. Late Jurassic/Early Cretaceous, Mist Mountain Formation. Blairmore, Alberta.

Unidentified fern. 150 – 120 million years old. Late Jurassic/Early Cretaceous, Mist Mountain Formation. Blairmore, Alberta.

Molds and Casts

Natural molds occur when the original skeletal part dissolves, leaving an impression in the surrounding rock.  A natural cast is created when mud or other material fills the mold. These tracks are from a large ornithopod, a bipedal, herbivorous dinosaur, and show that they had large fleshy pads beneath the bones of their feet.

Natural mold (left) and natural cast (right). 130 - 112 million years old. Early Cretaceous, Gething Formation. Peace River Country, British Columbia.

Natural mold (left) and natural cast (right). 130 – 112 million years old. Early Cretaceous, Gething Formation. Peace River Country, British Columbia.

Trace Fossils

Trace fossils, or ichnites, include the preserved traces, burrows, and tracks of once-living organisms.  Fossil footprints provide important information about how animals moved, and interacted with their environments, and each other. These footprints reveal three small to medium-sized theropods walking on a mudflat and sinking up to their ankles.

Theropod trackway. 110 – 100 million years old. Early Cretaceous, Gates Formation. Grande Cache, Alberta.

Theropod trackway. 110 – 100 million years old. Early Cretaceous, Gates Formation. Grande Cache, Alberta.

Unaltered

In rare instances, fossils consist of the unchanged remains of organisms. Very cold temperatures can sometimes preserve specimens in an essentially unaltered state.  Unlike typical fossils, this walnut from the Miocene Epoch has not been turned into stone.

Juglands sp. 14 million years old. Miocene, Beaufort Formation. Northwest Territories.

Juglands sp. 14 million years old. Miocene, Beaufort Formation. Northwest Territories.

Skin Impression

Impressions of skin (and, in very rare circumstances, hair) can be preserved in rocks. A well-preserved hadrosaur was discovered at a Korite mine earlier this summer. The specimen contains a number of patches of preserved skin that our scientists are currently studying.

Skin impressions can give us new insights into what dinosaurs looked like when they were alive.

Skin impressions can give us new insights into what dinosaurs looked like when they were alive.

Amber

Amber is the fossilized resin, or sap, of ancient plants.  Occasionally, plant or animal remains become trapped in the resin and are preserved, often in great detail. Feathers were found in several pieces of amber in our collection, representing both dinosaurs and birds. Several of the feathers demonstrate that some birds had adaptations for flight and underwater diving during the Late Cretaceous.

Bird feather fragment in amber. 80 million years old. Late Cretaceous, Foremost Formation. Grassy Lake, Alberta.

Bird feather fragment in amber. 80 million years old. Late Cretaceous, Foremost Formation. Grassy Lake, Alberta.

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s