Chapter 6: Fossil Preservation

Learning Objectives

The goals of this chapter are to:

  • Explain different modes of fossil preservation
  • Identify the mode of preservation for fossils

6.1 Introduction

Everyone knows what a fossil is! So, how do you define this term? In this lab, we will define it to mean any evidence for the existence of prehistoric life. What is difficult to define in this definition is what is meant by prehistoric. Would you consider bodies preserved at Pompeii to be fossils or how about the remains of a frozen mastodon from the Pleistocene that was preserved well enough to be eaten? Some say anything older than 11,000 years is a fossil, but this part of our definition is a matter of semantics. A good place to learn more about fossils and fossilization is the Digital Atlas of Ancient Life.

Exercise 6.1 – Fossilization Probability

We start this chapter on how organisms become fossilized with a quick exercise. Figure 6.1 contains three different organisms.

Three images of living organisms; from left to right these show several worms in soil, giant kelp, and mussel shells.
Figure 6.1 – a) Worms, b) Giant kelp; c) Mussel shells. Image credit: a) Soil-Net, CC BY-NC-SA; b) NPS, Public Domain; c) Linnaea Mallette, Public Domain.
  1. Which organism in Figure 6.1 do you think has the highest chance to become a fossil and why?
  2. Which organism do you think has the highest chance to leave behind a trace fossil? ____________________

6.2 Types of Preservation

Fossils are preserved by three main methods: unaltered soft or hard parts, altered hard parts, and trace fossils. You already learned about trace fossils in Chapter 4. Taphonomy is the science of how organisms decay and become fossilized, or transition from the biosphere to the lithosphere.

Unaltered fossils are incredibly rare except as captured in amber, trapped in tar, dried out or frozen as a preserved wooly mammoth. Amber is the fossilized tree resin that can trap flowers, worms, insects as well as small amphibians and mammals. The father of one of the authors was part of a gold mining dredge operation that unearthed a wooly mammoth calf (nicknamed Effie) in Alaska; this was the first mummified mammoth remains discovered in North America. Even though it was buried about 21,300 years ago, it still consists of tissue and hair. Sometimes, only organic residue is left behind and is detected by molecular biochemical techniques. Earth’s oldest fossils are only preserved as complex organic molecules.

Soft-tissue is hard to preserve as it needs to have been buried in an oxygen-free, low energy sedimentary environment where bacterial decay cannot occur. Since these conditions are uncommon, the preservation of soft tissue rarely happens. Instead, common examples of unaltered fossils are skeletal material that has been preserved with little or no change. Many marine invertebrate fossils and microfossils were preserved in this manner. Paleontologists are now looking closer at fossils and beginning to recognize thin carbon layers in the rock around fossils as soft-tissue. Recently, a team led by Mark Norell, a paleontologist at the American Museum of Natural History in New York City identified a layer of carbon around dinosaur embryos formed over 200 million years ago that they think was a soft-egg shell!

Unaltered fossils contain minerals that were biologically produced; these include apatite (in bones and teeth and rarely in exoskeletons, hardness = 5), calcite (calcium carbonate found in many organisms such as shells, hardness = 3, fizzes in acid), aragonite (similar to calcite, but an unstable polymorph) and opal (a type of silica found in marine animals and plants, hardness = 7).  The hard parts (exoskeleton) of some insects and arthropods are made of chitin, a polysaccharide related to cellulose. So, if you can identify the minerals present in a fossil, you can distinguish if it is original material or altered.

Alteration of hard parts is much more common in fossils and happens when original skeletal material is either permineralized, recrystallized, replaced, carbonized, or dissolved (Table 6.1).

Table 6.1 – Common types of fossil preservation
Type of Preservation Example
Permineralization occurs in porous tissue such as bone and wood. In this type of preservation, minerals dissolved in water such as quartz, calcite or pyrite permeate the pore space and crystallize. The addition of these minerals results in denser and more durable fossils. The original bone or wood material may be preserved, or it may be replaced or recrystallized
Petrified wood from the Petrified Forest National Park, AZ
Figure 6.2 – Petrified wood from the Petrified Forest National Park, AZ. Image credit: Jon Sullivan, Public Domain.
Recrystallization involves a change in crystal structure, but not a change in mineral chemistry, similar to recrystallization in metamorphic rocks. For example, the mineral aragonite, a common mineral of many shells, sometimes changes to calcite, a more geologically stable form of the same chemical composition, CaCO3 (aka a polymorph). Typically, the overall size and shape of a recrystallized fossil does not vary substantially from the original unaltered specimen, but fine details may be lost.
Recrystallized, Silurian-age coral from Ohio
Figure 6.3 – Recrystallized, Silurian-age coral from Ohio. Image credit: James St. John, CC BY.
Replacement is the substitution of original skeletal material by a secondary mineral. For example, the calcite of an oyster shell may be replaced on a molecule-by-molecule basis by silica. Remarkably, the replaced fossil may retain some of the fine cellular detail present in the original even though its composition changed. In this type of fossilization, pore space is not filled and the fossils are not as dense. The most common replacement minerals are silica (quartz), pyrite, dolomite, and hematite. Replacement by pyrite creates some spectacular fossils, especially those hosted by black shales!
Middle Permain fossils replaced with silica from the Road Canyon Formation in Texas
Figure 6.4 – Middle Permain fossils replaced with silica from the Road Canyon Formation in Texas. Image credit: Wikimedia user Wilson44691, CC BY-SA.
Carbonization is a type of fossil preservation in which the organism is preserved as a residual, thin film of carbon instead of the original organic matter. Leaves, fish, and graptolites are commonly preserved in this way. Compression of the original organism results in thin layers of carbon. Carbonization can also result in the formation of coal.
Carbonization of Silurian-aged graptolites from Poland
Figure 6.5 – Carbonization of Silurian-aged graptolites from Poland. Image credit: James St. John, CC BY.
Molds and casts form when the original skeletal material dissolves. The organism leaves behind an impression in the sediment, called a mold, and if that impression fills with new sediment, it creates a cast. Casts are made from molds.
A mold (left) and cast (right) of a trilobite fossil
Figure 6.6 – A mold (left) and cast (right) of a trilobite fossil. Image credit: Roger Wellner.
Internal molds form when sediment fills the inside of a shell before it dissolves; this occurs inside of bivalves, snails, or skulls. Often times, people confuse casts and internal molds because both have positive relief. Internal molds preserve a 3-dimensional mold of the inside of the organism, whereas a cast is going to preserve the structure of the outermost part of the organism.
Dissolution of a gastropod that has left an internal mold of the organism
Figure 6.7 – Dissolution of a gastropod that has left an internal mold of the organism. Image credit: James St. John, CC BY.

Trace fossils, which we discussed in Chapter 4, are not really fossils but the evidence that organisms affected the sediment by burrowing, walking, or even leaving behind excrement or vomit. No kidding, there is fossil poop; this kind of trace fossil is called a “coprolite,” from the Greek word kopros, meaning dung. One last rare type of trace fossil are gastroliths, extremely smooth polished stones that aided digestion in animals and fossils such as dinosaurs and crocodilia. These are more highly polished than stream worn gravels. Gastroliths found in Jurassic sediments in Wyoming may have been carried by sauropods over 1600 kilometers from their source in Wisconsin.

Different parts of organisms compared to how they can be preserved as fossils
Figure 6.8 – Different parts of organisms compared to how they can be preserved as fossils. The green circles are common types of fossilization, the light green, stippled circles are less common, and the light green circles are uncommon to rare ways. This chart is modified from Ritter and Peterson (2015).
Types of fossilization including alteration, replacement that result in casts and molds.
Figure 6.9 – Types of fossilization including alteration and replacement of the original shell. Follow the arrows from box to box to see how different processes can result in molds and casts. The brown color is a sedimentary rock. The random pattern represents recrystallized carbonate and the stippled pattern represent secondary minerals such as silica or pyrite. Image credit: Shell showing growth lines and internal structure adapted from Casella et al., 2017 and fossilization processes adapted from from Ritter and Peterson (2015).

3-D Model Box 6.1 – Molds

This model shows a preserved fossil shell on the right (not a cast, original) and an external mold on the left of the ammonoid cephalopod Gunnarites sp. from the Cretaceous Lopez de Bertodano Formation of Snow Hill Island, Antarctica. The fossil specimen is from the collections of the Paleontological Research Institution, Ithaca, New York. The diameter of the specimen (not including surrounding rock) is approximately 9 cm.

Cephalopod: Gunnarites sp. (PRI 61543)
by Digital Atlas of Ancient Life
on Sketchfab

This is an example of an internal (1) and external (2) mold of the gastropod Cassidaria mirabilis from the Cretaceous of Snow Hill Island, Antarctica. The specimen is from the collections of the Paleontological Research Institution, Ithaca, New York, and is approximately 6 cm in length (not including surrounding rock).

If you ever get asked by a friend to help identify a fossil, watch out for pseudofossils, accidents of diagenesis that look like a fossil but are just weird sedimentary formations such as septarian nodules that are mistaken for reptile skin or turtle shells, concretions are mistaken for eggs, and manganese oxide dendrites that are mistaken for ferns or moss.

6.3 Handling of fossils

If you are taking this lab when teaching is face-to-face in a lab setting, you will be able to handle both real and replica specimens of fossils. While these may have been around for millions or billions of years and seem like they are now rocks, they need to be treated with respect. Some of the fossils that you may handle may be the only specimen of its kind in the collection.

If you’ve wondered how to start your own fossil collection, you can either go start finding your own or buy them. The price of fossils for sale ranges from cheap to outrageously expensive. In 2020, an anonymous collector bought a fossil Tyrannosaurus rex, nicknamed Stan, for $31.85 million. This specimen only had 188 bones and was one of the most complete of its species. You can also find inexpensive fossils such as fossilized snails from Morocco for only $0.30 each.

Some fossils are extremely fragile. Some delicate samples are prepared by air abrasion with talcum powder to remove the matrix. For some trilobite specimens, this takes thousands of hours to expose their delicate features.

Some fossils you will use may be easy to replace and others impossible. Others may be part of a faculty member’s personal collection. Only handle the specimens that your TA says you can.

The fossils will only be available for you examine during the lab session. There are similar examples as web images that your TA will give you a link to.

You are free to make sketches or photograph the specimens. If you do this, you may want to put a scale in the image such as a coin or ruler. This will help you remember the size of the object.

Some of the specimens will have labels or numbers written on them and others will not as they may be too fragile to even be written on. It is crucial that you put each specimen back in its proper box or location in a lab tray. Also, do not move any of the paper labels from the boxes. This will prevent confusion for other lab students.

Some of the larger specimens may be heavy especially those that are molds filled with sediment. Never try to scratch the specimens for hardness. Also, never use acid as a mineral test.

Finally, if you break or steal a specimen, you will be charged for its replacement.

Exercise 6.2 – Identifying Types of Fossil Preservation

Inspect the first set of samples and fill out the table with information about the presence of original biologic material, positive and negative relief, and mineral composition of the samples. Identify the mode of preservation of the fossils. Use the flowchart in Figure 6.10 to help.

Figure 6.10 – Flowchart for identifying the type of fossil preservation. Image credit: Virginia Sisson and Daniel Hauptvogel.
Table 6.1 – Worksheet for Exercise 6.2
Sample Original Material Present? Relief* Mineral Composition* Type of Preservation

*Note that you may not see relief or be able identify the mineral. Leave these blank if necessary.

Critical Thinking: Why is replacement the most common mode of preservation?

Exercise 6.3 – Thinking about Preservation

The way an organism can become fossilized depends on many things. Below are some examples to think about.

  1. Examine an external mold in your fossil collection. These commonly preserve details such as the veins in leaves or scales of fish.
    1. What is the grain size of the surrounding rock? ____________________
    2. Do you think these impressions could be preserved in coarse-grained sediment?
  2. Look at some examples of carbonization. In these, the dark matter is the remnant of organic carbon that was never oxidized (decayed). Under what conditions might this kind of preservation occur?
  3. Your fossil collection may have graptolites; an extinct planktonic, colonial organism that secreted an organic shell of chitin similar to your cellulose. These colonies are usually preserved as two-dimensional impressions, almost always black (indicating carbonization of the chitin).
    1. What type of rocks are best suited to finding graptolites?
    2. What were the burial conditions?
  4. Some bones and teeth can be preserved such as unaltered bones or shark teeth.
    1. How would you distinguish these from permineralized fossil bones?
    2. Can permineralized wood scratch glass?
  5. Now consider the wide range of sedimentary environments.
    1. What sedimentary environments are not suitable for preserving fossils?
    2. Which sedimentary environments are good for preserving fossils?
    3. Which depositional environments within continental and marine environments are best for preserving fossils? Explain.
    4. Can volcanic eruptions preserve fossils? Explain.
    5. How can the energy of the sedimentary environment affect preservation of fossils?
    6. Can you find fossils in metamorphic rocks? If so, what factors aid in their preservation?
  6. Critical Thinking: There are more than fossils in this lab exercise. Explain why this is.

Exercise 6.4 – Modes of Preservation in an Ancient Reef

During the time, 299 to 252 million years ago, an extensive reef system grew in west Texas at the edge of a small inland marine basin that extended over 26,000 km² (10,000 square miles). Now it is called the Delaware basin, home to a major oil field (Figure 6.11). This reef is now exposed in three mountain ranges; Apache, Guadalupe, and Glass Mountains. Elsewhere, the reef is now buried around the entire rim of the basin.

Map of exposed and unexposed Permian reef that encircled the Delaware Basin, an inland sea
Figure 6.11 – Map of exposed and unexposed Permian reef that encircled the Delaware Basin, an inland sea. Image credit: Adapted by Virginia Sisson from National Park Service.

Unlike modern coral reefs such as the Great Barrier reef of Australia or the reefs off the coast of Florida and Belize, it was constructed from s, algae, and lacy animals called . One magnificent exposure of this reef is El Capitan in the Guadalupe Mountains National Park. The reef is subdivided into three parts: back reef, reef, and fore reef. Each had its own unique ecosystem as well as lithology and preservation. The deep part of this basin reached depths of almost 800 meters (½ mile) and is where a lot of organic matter was deposited leaving black shales – the source of petroleum.

Schematic cross-section across a reef showing the back-reef, reef and fore reef as well as the marine basin
Figure 6.12 – Schematic cross-section across a reef showing the back reef, reef and fore reef as well as the marine basin. Image credit: Adapted by Virginia Sisson from National Park System.

The Delaware inland sea had a narrow outlet to the ocean much like the Black Sea today. After ~30 million years, the entrance got restricted and the basin started to dry up forming extensive deposits (Castille and Salado Formations). This created supersaturated, acidic brines that started to dissolve the underlying carbonate reef forming extensive caves and karst that you can now visit at Carlsbad Caverns National Park and Lechuguilla Cave – 8th longest explored cave in the world at ~220 km or 138 miles long. These brines also dissolved the silica-rich sponges that formed the reef and affected the fossil preservation in parts of this Permian reef system.

Google Street View in Google Maps has recorded many of the trails in Gaudalupe Mountains National Park. Start at either McKittrick Canyon or the trail to Guadalupe Peak and drag the orange person icon onto one of the trails to see views the massive limestone reef.

The stratigraphy of this basin is complicated as not only does it vary with time but with position in the reef. According to recent sequence stratigraphic analysis, there were up to six transgressive to regressive sequences in this basin (Kerans and Kempter, 2002). Figure 6.13 gives a simplified stratigraphy for the basin during the Permian.

Simplified stratigraphy for the Delaware Basin
Figure 6.13 – Simplified stratigraphy for the Delaware Basin. Image credit: Simplified by Virginia Sisson from Kerans and Kempter (2002).

Fossils in the Capitan Formation of the Glass Mountains are uniquely preserved (see Figure 6.4). Paleontologists found that it is easy to dissolve away the host carbonate in weak acid and leave behind spectacular specimens.

Fossils from the Capitan Formation of the Glass Mountains
Figure 6.14 – Fossils from the Capitan Formation of the Glass Mountains. a) text here; b) text here; c) text here; d) text here. Image credit: a) Wikimedia user Wilson44691, CC0 Public Domain; b) text here; c) Wikimedia user Wilson44691, CC0 Public Domain; d) text here
  1. The mineral in these fossils is harder than glass and does not fizz as it is no longer a carbonate. Sometimes this mineral is just a coating and other times the entire fossil is this new mineral.
    1. What is the mineral? ____________________
    2. What is the mode of preservation for these fossils? ____________________
    3. Were fluids involved in their preservation? If so, what was their composition?
    4. Why do you think this type of preservation is found in this one stratigraphic unit.
    5. Which part of the reef were these fossils found? Back reef, reef, fore reef or basin? ____________________
      Fossil jaw of a Helicoprion ferrieri
      Figure 6.15 – Fossilized jaw with teeth of a Helicoprion ferri. The animal associated with this ~35 cm jaw is estimated to be 4 m (~12′) long. They could reach up to 10 m in length.
  2. Elsewhere in the Skinner Ranch Formation of the Glass Mountains, fossils include this amazing saw-toothed whorl of teeth from an extinct shark-like creature known as Helicoprion.
    1. What type of sediment is this fossil found in? ____________________
    2. What is this mode of preservation for this fossil? ____________________
    3. Where in the reef did Helicoprion live? Back reef, reef, fore reef, or basin? ____________________
      Four fossils from El Capitan reef in Guadalupe Mountain National Park
      Figure 6.16 Four fossils from El Capitan reef in Guadalupe Mountain National Park. A. Sponge (Porifera) B. Byrozoa C. Fragment of an ammonite. D. Fusilinids
  3. In the Guadalupe Mountains, you can find thick carbonate layers with many fossils such as in Figure 6.16:
    1. What is the mineral? ____________________
    2. What is the mode of preservation for these fossils? ____________________
    3. Were fluids involved in their preservation? If so, what was their composition?
    4. Why do you think this type of preservation is found in this one stratigraphic unit.
    5. Which part of the reef were these fossils found? Back reef, reef, fore reef or basin? ____________________
  4. Critical Thinking: Summarize your observations about the modes of preservation in different parts of the Permian reef system. Can you explain why the preservation is the same or different around the ancient reef?


Exercise Contributions

Virginia Sisson and Daniel Hauptvogel


Casella, L.A., Griesshaber, E., Yin, X., Ziegler, A., Mavromatis, V., Müller, D., Ritter, A.-C., Hippler, D., HarperE.M/, Dietzel, M., Immenhauser, A., Schöne, B.R., Angiolini, L., and Schmahl, W.W., 2017, Biogeosciences, 14, 1461–1492,  doi:10.5194/bg-14-1461-2017.

Kerans, C., and Kempter, K., 2002, Hierarchical stratigraphic analysis of a carbonate platform, Permian of the Guadalupe Mountains: The University of Texas at Austin, Bureau of Economic Geology (American Association of Petroleum Geologists/Datapages Discovery Series No. 5), CD-ROM.

Norell, M.A., Weimann, J., Fabbri, M., Yu, C., Marsicano, C.A., Moore-Nall, A., Varricchio, D.J., Pol, D., and Zelinitsky, D.A., 2020,  The first dinosaur egg was soft. Nature, 583, 406-410, Published online June 17, 2020. doi: 10.1038/s41586-020-2412-8

Ritter, S., and Peterson, M., 2015, Interpreting Earth History: A Manual in Historical Geology, Eighth Edition, Waveland Press Inc., 291 pp.


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