Taphonomy & Preservation

TAPHONOMY & PRESERVATION

From Eldredge (1991)

INTRODUCTION

As fossils are the preserved remains of ancient organisms or their traces, understanding the process of preservation, and more importantly, being able to recognize and identify fossil remains after their discovery is an integral part of paleobiology. Protective cover (sediments) and stabilizing chemical environments are of prime importance in the preservation of once living organisms. Due to the process of aerobic decay and physical/chemical destruction, most animals leave no evidence of their existence.

In order to make a correct interpretation of taphonomic processes and mode of preservation, it is often necessary to have a prior knowledge of the structural features or morphology of original skeleton in addition to knowing its original mineralogical composition. This limitation should diminish as you become familiar with the various fossil groups throughout the semester.

TAPHONOMY

Taphonomy is the study of what happens to an organism after its death and until its discovery as a fossil. This includes decomposition, post-mortem transport, burial, compaction, and other chemical, biologic, or physical activity which affects the remains of the organism. Being able to recognize taphonomic processes that have taken place can often lead to a better understanding of paleoenvironments and even life-history of the once-living organism.

In addition, understanding which taphonomic processes a fossil occurrence has undergone, and to what degree, may have implication on interpreting the significance of the fossil deposit and clearer understanding of the biases in the sample.

An outline of the pathways affecting the preservation of once living organisms can be found in Figure 1 below. As discussed below, this encompasses both the processes of biostratinomy and diagenesis.

Figure 1 - The field of Taphonomy as it relates to steps in transformation from living organisms to fossils.

Modified from McRoberts (1998)

Processes that occur between the death of an organism and its subsequent burial in the sediment are termed biostratinomy. Generally, this includes the decomposition and scavenging of the animal's soft parts, and at least some amount of post-mortem transport. Such things as the amount of shell breakage and the concentration of shells in layers often indicate the level of water energy and post-mortem transport. For example, the shell-hash or coquina has experienced a significant amount of shell breakage and probably post-mortem transport suggesting deposition in high energy environments; whereas, the articulated plant remains are intact suggesting little or no post-mortem transport and deposition in a very low energy and oxygen-free environment. In Table 1 below are various taphonomic indicators and their environmental implications.

The physical and/or chemical effects after burial are called diagenesis. This is the realm in which dissolution, replacement, or recrystallization of original shell material occurs, as can the formation of molds and casts. A more detailed description of diagenesis with regards to fossil preservation in the next section.

Table 1

Summary of Taphonomic Indicators and TheirPaleoenvironmental Implications

TAPHONOMIC FEATURE
IMPLICATIONS

Abrasion

The wearing-down of skeletons owing to differential movement with respect to sediments is an indicator of environmental energy. Significant abrasion is most commonly found on skeletal material collected from beaches, or areas of strong currents or wave action.

Articulation

Multi-element skeletons are soon disarticulated after death. Articulated skeletons, then, indicate rapid burial or otherwise removing the skeleton from the effects of energy of the original environment.

Bioerosion

Bioerosion encompasses the many different corrosive processes by organisms. The most pervasive causes of degradation are boring and grazing. Bioerosion erases information from the fossil record, but it also leaves identifiable traces made by organisms on remaining hard skeletons or surfaces. Therefore, trace fossils produced by bioerosion add information on the diversity of ancient assemblages.

Dissolution

Skeletal remains commonly are in equilibrium with surrounding waters, but changes in chemical conditions can cause skeletons to dissolve. Dissolution represents fluctuation in temperature, pH or pCO2 in calcium carbonate skeletons. Siliceous skeletons also can dissolve because normal sea water is usually undersaturated with respect to silica.

Rounding

Broken edges of skeletons become rounded owing to dissolution and/or abrasion of exposed surfaces. Processes that control edge rounding probably include a combination of dissolution, abrasion, and bioerosion. Rounding gives an estimate of time since breakage.

Encrustation

The growth of hard skeleton substrates by other organisms is a common occurrence. Besides indicating exposure of the skeleton above the sediment-water interface, encrustation can specify a particular environment. Different patterns of encrustation, as well as different biota, occur in different environments.

Fragmentation

Breakage of skeletons is usually an indication of high energy resulting from wave action or current energy. Fragmentation also can be caused by other organisms through either predation or scavenging.

Orientation

After death, skeletal remains are moved by the transporting medium and oriented relative to their hydrodynamic properties. Fossil skeletons in life position indicate rapid burial, attachment to a firm substrate, or death of in-place infauna. Hard parts tend to orient long-axis parallel to unidirectional flow in current-dominated areas and perpendicular to wave crests on wave-dominated bottoms.

Size

After death and if not rapidly buried, a skeleton behaves as a sedimentary particle and is moved and sorted with respect to the carrying capacity of the flow of currents, waves, or tides. Size can, therefore, be an effective indicator of flow capacity in a hydraulic or wind-driven system.

From McRoberts (1998)

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FORMS OF PRESERVATION

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Table 1
Summary of Taphonomic Indicators and TheirPaleoenvironmental Implications
TAPHONOMIC FEATURE	IMPLICATIONS
Abrasion	The wearing-down of skeletons owing to differential movement with respect to sediments is an indicator of environmental energy. Significant abrasion is most commonly found on skeletal material collected from beaches, or areas of strong currents or wave action.
Articulation	Multi-element skeletons are soon disarticulated after death. Articulated skeletons, then, indicate rapid burial or otherwise removing the skeleton from the effects of energy of the original environment.
Bioerosion	Bioerosion encompasses the many different corrosive processes by organisms. The most pervasive causes of degradation are boring and grazing. Bioerosion erases information from the fossil record, but it also leaves identifiable traces made by organisms on remaining hard skeletons or surfaces. Therefore, trace fossils produced by bioerosion add information on the diversity of ancient assemblages.
Dissolution	Skeletal remains commonly are in equilibrium with surrounding waters, but changes in chemical conditions can cause skeletons to dissolve. Dissolution represents fluctuation in temperature, pH or pCO2 in calcium carbonate skeletons. Siliceous skeletons also can dissolve because normal sea water is usually undersaturated with respect to silica.
Rounding	Broken edges of skeletons become rounded owing to dissolution and/or abrasion of exposed surfaces. Processes that control edge rounding probably include a combination of dissolution, abrasion, and bioerosion. Rounding gives an estimate of time since breakage.
Encrustation	The growth of hard skeleton substrates by other organisms is a common occurrence. Besides indicating exposure of the skeleton above the sediment-water interface, encrustation can specify a particular environment. Different patterns of encrustation, as well as different biota, occur in different environments.
Fragmentation	Breakage of skeletons is usually an indication of high energy resulting from wave action or current energy. Fragmentation also can be caused by other organisms through either predation or scavenging.
Orientation	After death, skeletal remains are moved by the transporting medium and oriented relative to their hydrodynamic properties. Fossil skeletons in life position indicate rapid burial, attachment to a firm substrate, or death of in-place infauna. Hard parts tend to orient long-axis parallel to unidirectional flow in current-dominated areas and perpendicular to wave crests on wave-dominated bottoms.
Size	After death and if not rapidly buried, a skeleton behaves as a sedimentary particle and is moved and sorted with respect to the carrying capacity of the flow of currents, waves, or tides. Size can, therefore, be an effective indicator of flow capacity in a hydraulic or wind-driven system.