LAB 1

Chapter 1: Fossilization and Preservation

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 organism 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, by 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 pattern 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 the 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.

1.2.2 Other types of fossils

• 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.

1.2.3 Modes of preservation

• 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.

1.2.4 Questions

1. This question is designed to make you think about the biological information that is lost during fossilization.

(a) Look at the two specimens of Conus (one is Recent, the other is Pliocene); both have a very similar shaped shell. Do you think it fair to assume the soft-part anatomy of these two species was the same? Why or why not? Do you think they had similar life modes? (ask a TA to explain how cone snails “make their living”!)

(b) Compare the Conus shells in question 1a to the other two gastropod specimens labeled here. Do you think that the soft-part anatomy of these 2 snails was the same as that of Conus? Is there a difference in the life mode between the three?

(c) The specimen labeled here is not a gastropod, but the skeleton of a polychaete worm, yet it looks very much like the vermicularid in question 1b. How could this worm’s anatomy and life mode compare with the snails seen previously?

2. What portions of this pickled inarticulate brachiopod (Lingula preserved in a glass bottle) would you expect to be preserved? What other kind of information would be lost through fossilization?

3. Using the displays and notes for reference, examine the numbered specimens and indicate for each one whether it is a body fossil, trace fossil, or pseudofossil. If it is a body fossil, indicate (1) the original skeletal composition, and (2) the mode of preservation (UA = unaltered preservation; RC = recrystallization; RP = replacement; P = permineralization; C = carbonization; I = impression).

(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

(i)

(j)

LAB 1 continued

Chapter 2: Taphonomy

2.1 Introduction

Taphonomy is the study of postmortem processes on once-living organisms. In addition to determining the type and intensity of the processes and their role in preservation, taphonomy is a way to detect bias in the fossil record. For example, in a hypothetical fossil assemblage of shells certain questions can be asked, such as:

• 1. Is the assemblage a true representation of the original assemblage?
• 2. Was any material lost during the fossilization process?
• 3. Did the material condense?
• 4. How much time is represented in the rocks?
• 5. How can we tell what has happened to the shells from the time of the death of the organisms to their burial and ultimate fossilization?
• 6. What can these preservational features tell us about the depositional environment?

These are just some of the questions one could begin to ask about any assemblage of fossilized material. Indeed, it is documented that the relative abundance of species in a fossil assemblage may not be an accurate reflection of the relative abundances in the original assemblage of living populations.

2.1.1 Taphonomic processes

There are three major categories of taphonomic processes of alteration and destruction: physical processes, chemical processes and biological processes (See Figure 2.1). Physical processes involve the mechanical breakdown of organic material via water an/or wind action (storms are an excellent example of a physical process). Chemical processes include any alteration of a material's mineralogy (such as that discussed in Chapter 1), as well as any leaching of material by the surrounding water or air. Finally, biological processes, such as sponge or algal borings, can help to alter and eventually destroy potential fossil material. All three types of processes can act in concert at various amplitudes in any given situation. It is a taphonomist's job to look at the intensity and interactions of these processes and their effects on a fossil assemblage.

2.1.2 Fossil concentrations

Fossils can be concentrated in two major ways, first by physical processes mentioned above, such as storms and currents, or winnowing and deflation. Fossils are also concentrated by aggradation, which is a biological process in that it is the piling up of LIVE individuals, such as those found in oyster beds or coral reefs.

2.1.3 Konservat-Lagerstatten

This term was coined by German paleontologists. It means simply an exceptional preservation in the fossil record. Konservat-Lagerstatten represent a preservational endmember in the spectrum of fossilization. Not only are most of the hard and soft parts preserved, the assemblages in these types of deposits are probably the closest approximation to the abundance and diversity of the original assemblage. For Konservat-Lagerstatten to form, all taphonomic processes must be minimized. That is, physical, chemical and biological destruction must be kept to a minimum.

Some of the world's most famous fossil deposits happen to be Konservat-Lagerstatten. Faunas such as the Mazon Creek (Illinois), Solnhofen Limestone (Germany), La Brea (tar pits in Southern California; "La Brea tar pits" is a redundant name), insects and others in amber (the Baltic states, Dominican Republic), and Burgess Shale (Canada) are all good examples. The Burgess Shale is located in the Canadian Rockies (British Columbia). The shales and its fossils are dark black in color, suggesting anaerobic conditions (no oxygen) and the fine-grained nature of the sediment indicates quiet water deposition, because there is no disturbance from wave action or burrowing organisms in the sediment. The Solnhofen is also very fine grained. The complete skeletons (e.g. Archaeopteryx) preserved in the limestone indicate very quiet waters too.

Although these deposits give us some of our most spectacular fossil deposits, they are important for many other reasons. First of all, they represent a "snapshot" in time, because of probable rapid burial. Secondly, they provide previously unknown anatomical details that can be important from a systematic (evolutionary) point of view. They also can provide an additional test for environmental and diagenetic boundary conditions. And finally, the excellent time resolution may allow true biotic diversity for an assemblage to be observed. This may be the closest that paleoecologists can come to the conditions of modern ecology.

Figure 2.1 Processes of breakage and diagenesis of fossils. Dead organisms may be (a) disarticulated; or (b) fragmented by scavenging or transport; (c) abraded by physical movement; (d) bioeroded by borers; or (e) corroded and dissolved by solutions in the sediment. After burial, specimens may be (f) flattened by the weight of sediment above; or (g) various forms of chemical diagensis, such as the replacement of aragonite by calcite may take place (from Benton 1997).

2.2 Exercises

2.2.1 Taphonomic grades

Some sedimentary environments are better than others when it comes to preserving fossils. The high energy conditions of a river channel or beach may grind and abrade bones or shells so that they are unidentifiable after only a short period of time. The quiet waters of swamps and lagoons, on the other hand, may permit the preservation of the delicate features of many hard parts.

The purpose of this exercise is to illustrate how the preservational condition of fossils (i.e. taphonomic grade) can vary from sample to sample, how taphonomic grades can be recognized and analyzed, and how that variation can be used to interpret the ancient environment of deposition. The material: The shells are specimens of the species Chione fluctifraga from the northern Gulf of California. They are all Recent in age. Chione fluctifraga lives buried a few centimeters below the surface of the sediment in the intertidal zone and in the shallow subtidal. After death, the shells may be eroded out of the sediment by waves and currents and then abraded and worn.

Taphonomic grades: It's easy to pick out some shells that show well preserved surface sculpture and growth lines; other shells have had these surface features almost completely worn away; others are in an intermediate condition.

2.2.2 Questions

For the sake of this exercise, we recognize three "taphonomic grades":

• Good (1): Well preserved shells that show growth lines and ribs in their original or near-original condition.
• Fair (2): Shells in which the surface features are somewhat worn, but the growth lines and ribs are still seen.
• Poor (3): Shells in which most of the surface features are worn away and much of the shell's external surface is smooth and polished.

Examples of these three taphonomic grades are shown in the Reference collection. Look at these shells to familiarize yourself with the three taphonomic grades. You may need to refer to this reference collection later when analyzing the two samples.

Analysis: Compare the taphonomic condition of Sample A and Sample B. Three examples of Sample A are available; three of Sample B are available. Analyze one of each in the following way:

1. For each sample, tabulate the number and proportion of shells in each of the three taphonomic grades described above and illustrated in the reference collection. You might find, for example, that in one sample 30% of the shells are in "good" condition, 30% in "fair" condition, and 40% in "poor" condition. The other sample may have different proportions.

2. Plot, on the triangular diagram provided, the location of Sample A and Sample B.

3. Describe briefly how the two samples differ with respect to their taphonomic condition.

4. Discuss briefly why the taphonomic condition of the shells in the two samples might be different.

LAB 1 continued

Chapter 3: Ichnology: Trace Fossils

3.1 Introduction

Trace fossils or ichnofossils represent the effects of organismal activity upon or in the substrate. Tracks, trails and the like are the most commonly encountered traces. A distinction can therefore be made between body fossils, which are actual remains of organisms, and trace fossils that represent an indication of an organism's behavioral activity.

Trace fossils, though not preserving the body or necessarily the morphology of the original organism, do have certain advantages over body fossils. In general:

• Trace fossils are often preserved in environments that are hostile to the preservation of body fossils (e.g. shallow, high energy environments, shallow marine sandstones, and deep marine shales).
• Trace fossils are generally not affected by diagenesis, and may actually be enhanced (visually) by diagenetic processes.
• Trace fossils are not transported, and are thus good indicators of the original sedimentary environment.

(Exceptions: Feeding damage on body fossils (like damage on bones, coprolites and leaves) are also subject to the same taphonomic processes that affect the body fossil. Sometimes these traces actually facilitate the degredation of the body fossil.)

Trace fossils may be preserved in a number of reliefs. They may be preserved in actual 3-dimensional relief, within sediment or sometimes the traces become filled in by a more resistant mineral and are subsequently eroded out of the surrounding sediment in full relief. More often, there is partial preservation caused by the movement of the tracemaker in and out of the depositional interface. These semireliefs may occur on the upper surface of a bed (concave epireliefs, or their casts, convex hyporeliefs), or on the underside of a bed (concave hyporelief). What would you call a ridge or hill of sediment, obvious on the surface of a bed, made by an infaunal burrower (see Figure 3.1).

3.1.1 Terminology

Listed below are a number of terms used in the description of trace fossils. Become familiar with them. Also listed are some important ichnogenera. Examples of which will be available in the lab.

Terms		Important Ichnogenera
ichnology ichnofossil ichnogenera ichnospecies cubichnia fodichnia repichnia domichnia pascichnia fugichnia	epichnia endichnia hypichnia spreite coprolites gastroliths stromatolites	Arthrophycus Asteriactites Chondrites Cruziana Rusophycus Skolithos Zoophycos

Figure 3.1 Types of preservation of trace fossils. The dark stippling indicates mud; the light stippling indicates silt or fine sand (Redrawn form Prothero, 1998; adapted from Seilacher, 1964)

3.1.2 Types of trace fossils

The major categories of trace fossil ethological classes are described below. If examples are available, you should go through this section while examining the relevant specimens. Also, refer to Table 3.1 and Figure 3.2 for interactive summaries of these categories.

• Repichnia. These are crawling or walking traces; this group includes any trace that was made during locomotion. Included in this category are examples of amphibian, reptilian and mammalian footprints. Cruziana is an example of a crawling trace made by a trilobite; note the scratch marks made by the trilobite appendages.
• Fodichnia. These are feeding structures, usually infaunal burrows made by deposit feeders that systematically mine the sediment for food. A typical feature found associated with these types of traces is called spreiten (be sure that you understand how this feature is formed, and its significance). Other fodichnial traces consist of complex burrows that form branching networks where the organism has tunneled systematically below the substrate (e.g. Chondrites).
• Domichnia. Domichnial traces are burrows used principally for dwelling as opposed to feeding. These types of burrows may be oriented vertically (e.g. Skolithos), or are commonly U-shaped.
• Cubichnia. This group of behavioral traces includes resting or nestling traces; places where organisms rested temporarily on the substrate. Examples are common impressions left by sea stars and trilobites (Rusophycus).
• Pascichnia. These types of traces are made by grazing herbivores, usually at the sediment/water interface. Nereites is a systematic sinuous trail made by deep water gastropods.
• Hard substrate traces. All the above categories are associated with soft substrates. Traces can also be made on hard substrates (i.e. on shells or lithified surfaces) and are usually called borings. Examples are the small boring left behind by clionid sponges boring clam shells, the large holes left by endolithic bivalves, and the predatory drill holes of gastropods and octopods. Unfortunately, hard substrate traces are not generally included in the above ethological classification scheme.
• Stromatolites. Included in this section are the stromatolites. Make sure that you understand how stromatolites are formed, that you can recognize the different morphologies (SS, SH, LH, L), and that you understand their environmental significance.

Figure 3.2: Common ichnofacies and examples of the trace fossils that occur in them. 1. Koupichniurn (horseshoe crab tracks); 2. Isopodichnius; 3. borings of Polydora, a polychaete; 4. Entobia, clionid borings; 5. echinoid borings; 6. algal borings; 7. pholadid bivalve borings; 8. Diplocraterion; 9. unlined crab burrow; 10. Skolithos; 11. Thalassinoides; 12. Diplocraterion; 13. Ophiomorpha; 14. Arenicolites; 15. Phycodes; 16. Rhizocorallium; 17. Teichichnus; 18. Diplichnites (trilobite tracks); 19. Cruziana; 20. Rusophycus; 21. Ateriacites; 22. Zoophycos; 23. Lorenzinia; 24. Paleodictyon; 25. Taphrhelminthopsis; 26. Heminthoidia; 27. Spiroraphe; 28. Cosmoraphe. (Redrawn from Prothero, 1998, modified from Ekdale et al., 1984)

Table 3.l. Ethological classification of trace fossils. (Adapted from Frey, 1978)

Categories of Ichnofossils	Definition	Characteristic morphology
Resting traces (Cubichnia)	Shallow depressions made by animals	Troughlike relief, recording to some extent the that temporarily settle onto, or dig lateroventral morphology of the animal; on into, the substrate surface; emphasis structures isolated, ideally, but may intergrade.reclusion with crawling traces or escape structures
Crawling traces (Repichnia)	Trackways, surficial trails, and shallow borrows, emphasis on locomotion,	Linear or sinuous overall structures, some traces horizontal structures made by organisms traveling from one place to another; branched; footprints or continuous grooves, commonly annulated; complete form may be preserved or may appear as cleavage reliefs.
Grazing traces (Pascichnia)	Grooves, pits and furrows, many of them discontinuous, made by mobile deposit feeders at or near the substrate surface; emphasis on feeding	Unbranched, nonoverlapping, curved to tightly coiled patterns or delicately constructed spreiten dominate; patterns reflect maximum utilization of surficial feeding area; behavior analogous to "strip mining" complete form may be preserved.
Feeding traces (Fodichnia)	Temporary burrows constructed by deposit feeders; the structures may also provide shelter for the organisms- emphasis on feeding, behavior analogous to "underground mining"	Single, branched or unbranched, cylindrical to structures sinuous shafts or U-shaped burrows, or complex, parallel to concentric burrow repetitions (spreiten structures); walls not commonly lined, unless by mucus; oriented at various angles with respect to bedding; complete form may be preserved.
Dwelling traces (Domichnia)	Burrows or dwelling tubes providing more or less permanent domiciles, mostly for hemisessile suspension feeders, or in some cases, carnivores; emphasis on habitation	Simple, bifurcated, or U-shaped structures structures perpendicular or inclined at various angles to bedding, or branching burrow systems having vertical and horizontal components; walls typically lined; complete form may be preserved
Escape traces (Fugichnia)	Lebensspuren of various kinds modified or made anew by animals in direct response to substrate degradation or aggradation; emphasis on readjustment, animals upward or downward with respect to the original substrate surface; complete form may be preserved, especially in aggraded substrates	Vertically repetitive resting traces; biogenic structures laminae either in echelon or as nested funnels or chevrons; U-inU spreiten burrows; and other structures reflecting displacement of or equilibrium between relative substrate position and the configuration of contained traces

Fossils on other planets…

Work in groups of five for this question. Your group’s members are:

Your group is competing for a one million dollar grant from NASA to develop criteria for recognizing fossils on Mars and other planets that may have supported life in the past but no longer do so.

Your group must provide five criteria that would be useful for recognizing fossil life forms on Mars and other planets. At least three of these criteria must be ones that can either be applied by astronauts while they are exploring the planet, or applied by viewing images sent back to earth.

How would you distinguish between objects formed by life (organic) processes and objects formed by inorganic processes?

Each group competing for the grant must present its criteria – and the rationale for them before the entire group in 20 minutes.

List your criteria (and the rationale) below: