Preservation of Fossils

The principle of fossil succession introduced in Lab 3 states that fossil species occur within geological strata in a predictable sequence according to the evolution of life on Earth. Observations by many Earth scientists, biologists, anthropologists and others tell us that within local, regional, and the global stratigraphic columns there is a consistent progression of fossils from those that are the least complex (bacteria) up to recent life.

Preservation as a fossil is not a certainty. After death, the remains of most organisms are scavenged and consumed or otherwise degraded in the environment. The potential for the preservation of an organism as a fossil is greatly enhanced by two favorable conditions:

• the presence of durable internal hard parts (e.g. bones) or external shells, and;
• relatively rapid burial within a low-energy sedimentary environment.

Major Starting Materials for Fossils

Organisms can be entirely soft bodied, such as worms or jellyfish. Other organisms have harder parts within their bodies that act as support and anchor points for muscles (e.g. cartilage, bone, shells, exoskeletons). These harder parts are often better preserved.  Some organisms may have particular structures or parts of their bodies that separate easily from the organism’s main body and may be found preserved on their own separate from the rest of the body (e.g. teeth, feathers, scales).

1. Soft Organic Materials

1A. Animals: The flesh (skin, muscles, tendons, organs) of most animals is typically not preserved. Scavengers and bacteria will quickly degrade most soft parts. Rare localities such as the Burgess Shale preserved soft parts because the organisms were buried rapidly in cold, low oxygen conditions.

1B. Plants: Plants are commonly recognized as organisms that derive their energy from sunlight, but biologists and paleontologists make finer distinctions between algae, plants and fungi. Soft plants like mosses are composed of less resilient materials and, like animal soft parts, these are often easily degraded. Other plants have a greater proportion of more resilient materials in them which allow the plants to be more easily preserved.

2. Internal Skeletons

These provide support for muscles and movement, and are located inside the body.

2A. Bones: Bones are mostly composed of calcium phosphate. The centre of bones is porous (typically 30-90% is pore space) and this is where the living cells of the bone marrow are found. Marrow in humans produces our red blood cells and a variety of other cells, some of which form the bone itself. These pores are occasionally filled by other minerals after death and burial, which further strengthens the bone fossil.

2B. Cartilage: The skeletons of some of vertebrates (organisms with a spinal column) are composed of cartilage rather than bone. Cartilage is composed of proteins (collagen and elastin). In humans, cartilage makes up the shock-absorbing pads between our back vertebrae and inside our joints. The entire internal skeletons of sharks are cartilage, and it also appears in body parts of some invertebrates.

3. Exoskeletons

Exoskeletons are hard parts located on the outer part of the body that provide support for muscles and movement.

3A. Carbonate Shells: A shell is a hard exoskeleton capsule that completely or mostly surrounds an organism. Shells are most often made from the carbonate minerals aragonite, calcite or high-magnesium calcite. Aragonite is not very stable when fresh water is present and will alter to calcite. Calcite is a common mineral used in shells and preserves well. High-magnesium calcite contains more than 4% magnesium (typically 12 to 18%).

3B. Silicate Shells: Some creatures create their shells from silica rather than carbonate. In Lab 8 we will discuss microscopic plankton which have silicate shells.

3C. Chitin: The arthropods include animals such as crabs and all the insects. Although commonly called a “shell,” the outer hard body of a crab is actually an exoskeleton made of chitin. Chitin is composed of an organic compound similar to cellulose.

3D. Beta-Keratin: Reptiles like tortoises have shells made of the same protein-based material that makes claws, beaks, and human hair and fingernails. Reptilian skin contains beta-keratin which prevents desiccation and acts as a waterproofing agent.

4. External Support Frameworks

What are commonly known as corals are colonies made up of many small soft-bodied animals which gradually deposit calcium carbonates to form a hard framework to support themselves. These are not shells as they do not fully enclose the organism, but do offer a degree of protection into which the animal can retreat. Some corals produce frameworks with pores through which the individual coral animals can interact whereas other corals build solid frameworks. Modern corals use aragonite, whereas most earlier corals used calcite. We will see some sponges and other animals which make similar frameworks out of carbonates and organic materials like chitin.