9.2: Death to Fossil Transition

Decay of soft tissue

Figure \(\PageIndex{1}\): Otzi is a mummified man, who had been preserved in ice, and found high in the Alps between Austria and Italy.

When a living organism dies, soft tissues, as such muscles and internal organs, that make up the body of the organism will typically immediately begin to decay. Scavengers may take advantage of a feast. Bacteria and fungi will begin to break down the tissues to reuse available nutrients. Soft tissue decay happens fastest in the presence of warm, wet conditions with lots of hungry organisms around. Low temperatures, poor oxygen supply, and/or extremely high or low pH slow down rates of soft tissue decay, as these factors inhibit bacterial growth. Therefore, some of the best fossils that include unaltered soft tissues come from deposits that represent rapid burial which sealed the organism off from its environment. Other superb fossils including soft tissue remains are entombed in glacial ice. In some cases a thin film of carbon may be left behind as this rapid burial occurs, leaving a fossil preserved by carbonization.

Pre-burial decay of hard skeletal elements

For multicellular organisms with hard parts (shells, bones, teeth; collectively referred to as skeletal elements), it is the soft tissues that hold these hard pieces together. Once the skin, tendons, muscles, and other connective tissue decays away, there is little to keep the skeletal elements intact, and these shells and bones can disaggregate, becoming sedimentary particles.

Figure \(\PageIndex{2}\): Xenocrinus baeri fossil crinoids in fossiliferous limestone (Whitewater Formation, Upper Ordovician; northeastern Warren County, southwestern Ohio, USA)

Most of these skeletal elements are reworked by waves or other currents in the environments in which the organisms lived. If the environment in which the organism lived was prone to currents of running water, flooding, large storms, or turbidity currents, it is possible for the skeletal elements to be transported out of their original habitat.

In high energy environments, such as those with with active currents, it may become very easy for skeletal hard parts to undergo any (if not all) of the following physical processes prior to burial:

1. Disarticulation – Separation of valved shells or vertebrate skeletons into individual pieces.

2. Fragmentation – Physical breakage of shells or bone into smaller pieces.

3. Abrasion – Physical wearing away of shells or bone as they are jostled against each other or against particles of sediment in currents.

Generally, skeletal components that are thin, less dense, and elongate are more susceptible to physical breakage. Chunky, robust, dense skeletal elements are more likely to persist with minimal damage.

There are also some biological processes that can contribute to the physical weakening of skeletal components and increase the surface area for chemical processes to continue breaking down the hard parts, such as:

1. Boring – parasitic or predatory organisms create holes via chemical and physical means in shells or bone when the animal is alive. In marine settings, common borers include sponges and snails. After death, additional boring of shells and bones can occur by organisms that require hard substrates for growth.

2. Encrustation – organisms that need a hard surface on which to grow may populate the surface of the shells or bone (note for some shells, this may also occur while the shelled organism is still alive). Typical encrusting organisms in marine settings are sponges, bryozoans, tube worms, and barnacles.

Compare the three specimens below–click on the first two to rotate them and consider the differences between the first two well-preserved specimens and the bored and encrusted specimen.

Figure \(\PageIndex{3}\): Articulated Mercenaria sp. specimen

Figure \(\PageIndex{4}\): Disarticulated Mercenaria specimen

Figure \(\PageIndex{5}\): Sponge borings (Entobia made by the genus Cliona) and encrusters on a shell of the modern hard clam, Mercenaria mercenaria, from North Carolina. This encrustation and boring happened after the death of the clam when the shell was empty. (English Wikipedia [Public domain]; Photograph taken by Mark A. Wilson (Department of Geology, The College of Wooster), 2007.)

Post-burial decay of hard skeletal elements

After skeletal hard parts are buried, additional chemical changes will commonly occur to the shells or bone. Many of these processes are described in the next section on modes of preservation, and are related to the changing the mineralogy of the original hard parts via recrystallization, permineralization, replacement, or dissolution. These chemical changes occur due to groundwater seeping through the pores of the burial sediments, and increasing temperatures associated with deeper burial. As the fossil becomes more deeply buried, additional distortion and fragmentation of the material is possible due to the weight of the overlying sediment. Stretching and shearing of fossils due to applied tectonic stress may also occur.

Death to Fossil Transition - Quiz

Exercise \(\PageIndex{1}\)

Which of the following would easily be considered fossils?

a. Woolly mammoth bones

b. Civil War bullets

c. Beach shells

Answer

a. Woolly mammoth bones

Exercise \(\PageIndex{2}\)

Is this an example of biological, chemical, or physical weakening of a shell?

a. Chemical

b. Physical

c. Biological

Answer

c. Biological

Exercise \(\PageIndex{3}\)

Which of the following would NOT slow down soft tissue decay?

a. Low oxygen levels

b. Acidic water

c. High temperatures

Answer

c. High temperatures