Invertebrate Paleontology Tutorial

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Invertebrate Paleontology Tutorial

Welcome to the Invertebrate Paleontology Tutorial Web Site

This web site was created to be a companion to the laboratory for GLY 363 - Invertebrate Paleontology.It is designed to provide the student enrolled in GLY 363 with an additional resource for reviewing laboratory materials. It is set up in a format which parallels the laboratory handouts. Most of the images contained within the web site are taken directly from specimens that are found on display for study in the paleontology laboratory.


Taphonomy & Preservation

Protista, Eubacteria, & Porifera




Cephalopoda, Gastropoda & other Molluscs

Bivalve Molluscs



Miscellaneous Fossil Groups

Trace Fossils

Geologic Time Scale



Links to other Paleontology Pages on the WWW

From Eldredge (1991)


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.


Taphonomyis 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 inFigure 1below. As discussed below, this encompasses both the processes ofbiostratinomyanddiagenesis.

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 termedbiostratinomy. 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. InTable 1below are various taphonomic indicators and their environmental implications.

The physical and/or chemical effects after burial are calleddiagenesis. 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


AbrasionThe 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.

ArticulationMulti-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.

BioerosionBioerosion 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.

DissolutionSkeletal 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.

RoundingBroken 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.

EncrustationThe 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.

FragmentationBreakage 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.

OrientationAfter 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.

SizeAfter 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.




This form of preservation is rare in most of the geologic column, but becomes more frequent in younger sedimentary rocks. Types of unaltered preservation where even the soft body parts are preserved include: (i) mummification, (ii) encasement in tar, (iii) encasement in amber, (iv) encasement in sediment, and (v) freezing. More frequently, however, only the hard skeletal material is preserved after removal of soft body parts.

Examples of unaltered preservation include the skeleton of a horseshoe crab,

whose shell is composed of interlocking plates and jointed appendages which quickly disarticulate after death; cockle bivalved molluscs,

whose outer-most shell layer has been removed by abrasion, yet the original shell material of the inner layers remains; an ammonoid

from the Cretaceous period in which you should note the pearly luster which is original aragonite shell material; and an insect encased in amber



This general class of preservation entails making "replicas" of the skeletal hard parts of organisms. In general, amoldis an impression in the sediment of a skeleton or shell. Once encased in lithified sediment, the dissolution of skeletal material leaves behind the impression or mold of original skeletal form. Thus, a mold is a "mirror image" of the original skeleton. Aninternal mold(sometimes called a steinkern) is the impression of the inside surface of skeletal hard parts. Anexternal moldis the impression of the outside surface of skeleton or bone. An example of both types of molds can be seen in this image of a trilobite


Acastis formed by the filling-in of a mold. It is thus a true replica (not a "mirror image") of the original skeleton or shell. By this definition, the cast one gets for a broken limb is not really a cast at all but an external mold.

A graphical representation of the formation of casts and molds is provided inFigure 2below.

Figure 2-Different diagenetic processes leading to different preservational styles in skeletal materials.

*Note that molds are produced directly as imprints of the shell and casts are produced from molds.

Modified from McRoberts (1998)


This common form of preservation involves chemical and/or physical alteration or replacement of original skeletal material. To properly identify replacement and recrystallization, one must know what the original constituents of the organism's skeleton were. These are provided in Figure 1.3.Replacementoccurs often by the filling in (by various minerals) of the void space after dissolution of original skeletal