Introduction

Ordinarily, only the hard parts of organisms are preserved (for example, only the shells of invertebrates, and only the bones and teeth of vertebrates). In most instances we must make inferences about fossil organisms using only these hard parts. Despite this challenge, we must try to understand the soft-part anatomy of fossil organisms so that we can better appreciate them as organisms that were once alive, that consumed food, breathed oxygen, interacted with their physical and biological environments, etc. Taphonomy is the science that studies the information that is lost between the death of an individual and its final discovery, and will be covered in the next lab.

This lab is designed to introduce you to some of the methods that paleontologists use to reconstruct fossil biology and ecology, and at the same time to acquaint you with some of the problems that are encountered after fossilization.

The following text should be studied, and referred to while examining the displays.

1.1.1 What is a fossil?

What is a fossil and what processes are required for their preservation? A fossil is any evidence of a once-living organism. This includes body fossils, casts, molds, footprints, trackways and feeding traces. This evidence of previous living organisms can then be used to study changes in life forms through time. This includes their evolution, ecology, functional morphology, growth and form, as well as their geographic distribution. Fossils provide us with our best link to the history of life.

1.1.2 How do we get fossils?

One of the keys to preservation is resistance. Either the conditions are mild enough (calm water, little oxygen) not to destroy much of the organism, or those parts that do get preserved are the most resistant to chemical and physical damage. Good examples of this are the shells of clams and the teeth of mammals. Both of these examples demonstrate that there is a preservational bias for hard parts compared to soft parts.

The nature of preservation is dependent upon the interaction of several factors. The composition of the organism and its structure play vital roles in how the body will react to the physical and chemical activities that normally break down or damage dead organisms. Intimately related to this is the sedimentary environment in which the organism lived. It will determine the type and intensity of the physical and chemical processes. These all contribute to the post-depositional changes (such as replacement, recrystallization, carbonization, the formation of casts, etc.) that take place during fossilization. And finally, numerical abundance will affect the nature of preservation by increasing or decreasing the chances of something being preserved, simply because of the sheer numbers or lack of certain organisms (this does make sense, if you think about it for awhile).

As mentioned above, the bias of hard parts over soft parts can provide considerable problems for paleontologists. Often, as is the case with most molluscs for example, much of the diagnostic information is in the soft part morphology, making it difficult to say certain specific things about organisms whose only record is in the hard parts. It is then necessary to draw upon recent analogues and extrapolate that information back to the fossil record. This can be dangerous if the past was not entirely like the present in environmental or ecological conditions. We call this the "pull of the Recent analogue" and it can be a serious problem if not recognized at the outset.

1.1.3 Types of fossils

There are many ways in which a record of an organisms can be preserved. Body fossils can occur in many ways, including: unaltered preservation, recrystallization, replacement, permineralization, carbonization, impressions, casts and internal molds.

Unaltered preservation implies the preservation of the original composition such as aragonite, calcite, chitin, cellulose, and calcium phosphate. Recrystallization means that the less stable hard part mineralogies are transformed through void time, temperature and pressure to more stable minerals. This is usually a destructive process, where much of the fine morphological detail (e.g. ribs on a clam shell) is lost. The most common form of recrystallization in the invertebrate record is the change from aragonite and/or Mg calcite to the more stable calcite form of CaCO3. In contrast to recrystallization, which is a rearrangement of the crystal lattice in which the chemical composition remains the same, replacement is an atom for atom substitution of a mineral's components with the elements composing the replacing mineral. Thus, pyritization, phosphotisation, silicification and dolomitization are all good examples of the replacement process. One should also note that contrary to recrystallization, replacement is usually NOT destructive; that is, you can see many of the original morphological details.

Permineralization is yet another mode of preservation, where pore-space is infilled by percolating fluids. The pore-space is usually the xylem and phloem (transport tissues) of woody tissue. Another name for this process is petrification.

Carbonization is often indicated by the shiny black texture of what appears to be an impression of an organism, often a plant leaf or crushed arthropod. This process is due to distillation. An organic film is formed as water is driven off. You can recognize carbonization easily by the shiny black or dark brown color.

The next three modes (impression, cast and internal mold) are often confused, but they are distinct both in patern and process. Impressions or external molds are nothing more that what is produced when something is pressed into soft sediment and that "impression" remains. You can recognize external molds because they show only external detail, and they are negative in relief. A cast on the other hand, is the sediment infilling of an external mold. It will also show only external features, but will be positive in relief, not negative like an external mold. Lastly, internal molds form when sediment infills a shell or skeleton, hardens, and the shell is worn away. What is left is a mold showing internal features and will most likely have positive relief.

1.2 Exercises
1.2.1 Skeletal mineralogies

Before determining how a particular fossil has been preserved, its important to know the organism's original skeletal mineralogy and mineralogy present in the fossil. This, for example, enables you to distinguish between recrystallization and replacement. The following display is designed to familiarize you with different types of mineralogies commonly found in fossils.

• 1. Aragonite. Aragonite (CaCO₃) is a form of calcium carbonate that is fairly unstable and commonly dissolves away. Skeletons made originally of aragonite are commonly recrystallized to calcite and preserved as molds. Aragonite is easy to recognize. It is usually (not always!) milky white and has no luster.
• 2. Calcite. Calcite (CaCO₃) is the more common form of calcium carbonate. It is more stable than aragonite and therefore does not dissolve as readily. Calcite usually has a grayish color and a slight vitreous (or glassy) luster when found as a skeletal mineral. It can be found as an original skeletal material, or as a recrystallization product.
• 3. Silica. Silica (SiO₂) is easy to distinguish from the carbonate minerals since it will not react with acid. Skeletons composed of this mineral will commonly have a brown, earthy color, with or without a vitreous luster, and can have a granular texture. Silica is rarely found as an original material and most commonly occurs as a replacement product.
• 4. Pyrite. Pyrite (FeS₂) or "fools' gold" is a golden colored mineral with a metallic luster and is therefore identified easily. It always appears as a replacement product.

• 1. Trace fossils. Unlike body fossils, where a portion of the actual organism or its skeleton is preserved, trace fossils are the remains of an organism's activity or behavior. Examples include tracks, trails, burrows, and borings. Trace fossils will be covered in detail in a later lab.
• 2. Artifacts and oddballs. These are samples that could be considered fossils, yet they do not fit formally with a true fossil's definition. Examples include tools used by ancient humans, coprolites, and gastroliths ("stomach stones").
• 3. Pseudofossils. Pseudofossils are unusual structures formed inorganically that, by chance, resemble body or trace fossils. Some classic examples include dendrites. These are inorganic precipitates of manganese oxide that were described originally as fossil algae.

• 1. Unaltered hardparts. Occasionally an organism's skeleton is preserved intact without any chemical alteration of the original mineralogy. This mode of preservation becomes increasingly rarer for fossils of older ages. Which skeletal mineralogies would you expect to have better chances of being preserved in this way?
• 2. Mold. Often, a fossil has been encased in sediment and that sediment becomes cemented, and the original skeletal material dissolves away completely leaving a void. The dissolution leaves behind an impression (or mold) of the skeletal remains.
• 3. Carbonization. Carbonization occurs when all organic volatiles are distilled away because of the effects of heat and/or pressure, leaving a carbon film remnant of the organism. This usually occurs with organisms rich in carbon that possess thin or no skeletal material. Coal is an example of carbonized plant debris.
• 4. Permineralization. Skeletal material can be quite porous. If the pores are filled in by foreign minerals that precipitate out of solution, the fossil is said to be permineralized. Petrified wood is an example of wood that has been permineralized by silica.
• 5. Replacement. This occurs when skeletal material is replaced, molecule by molecule, by some new alien material. This process occurs gradually over a long period of time as the original mineralogy dissolves away and a new mineral precipitates in its place. Examples include:
  • (1) silicification - where calcium carbonate is replaced by silica, and
  • (2) pyritization - where pyrite replaces calcium carbonate.
• 6. Recrystallization. This mode of preservation is probably the most difficult to understand and recognize. Recrystallization occurs when the original mineral crystals are altered in size and/or geometry over time, yet the chemical composition remains unchanged. An example is aragonite recrystallizing to calcite. This type of situation can be tricky to identify, so be sure to check with your TA.

Chapter 1: Questions
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