Lab 4 ≠ Fossils and Taphonomy

Introduction

Fossils are the remains of former organisms that are found embedded in rock. They may consist of the original organic or mineral materials that composed a particular organism, or, in many cases, different minerals may have replaced the original organic form. Paleontologists sometimes refer to the actual remains of organisms as ≥body fossils.≤

Trace Fossils (or ≥traces≤) are the remains of the former activity of organisms found embedded in rock. Footprints are a good example of trace fossils, since they represent the fact that an organism once walked on a sediment surface, yet the footprint itself is not physically part of an organism. The discipline concerned with the study of trace fossils is called, ichnology.

Taphonomy is the study of the processes involved in fossilization.

Purpose

In this exercise, we examine a wide variety of fossils and trace fossils. We also learn about a range of minerals that commonly form fossils.

Body Fossils

Classification

Although there is no universival system for classifying different types of fossil preservation, paleontologists do recognize a few styles of preservation type. These include casts & molds, impressions, compression & compactions, and petrifactions.

Cast and Mold.ãThis is a very common form of preservation and occurs when sediment infills the space inside a shell or other skeletal fossil. The terms mean essentially what they do in common English usage. A cast, then is the lithified sediment that fills in a void space defined by its outer bounding surface, a mold. If the original shell material is preserved, we still refer to the type as a cast/mold, but often the original material has eroded away leaving either the infilled cast, the external impression of the mold, or both. Casts are most typically comprised of sandy material and, therefore, because of the coarseness of the grains, does not always preseve fine surface details useful for identification and biological classification. In some sediments, however, a layer of finer grained silts and clays may form a fairly detailed reproduction of the minute details of shell shape.

[Paleozoic brachiopods, Neogene infilled oysters?]

Impressions.ãImpressions are also a very common fossil form. In this case, the original material is gone and we are left with only the imprint, impression, of the original structure. An impression is basically fla. It is typically used to describe plant remains, particularly leaves. Various minerals can form in the narrow voids left by impressions. These are usually different forms of clays which are difficult to identify.

[Glossopteris, Neuropteris from Naragansett Basin]

Compressions & Compactions: from Peat to Coal.ãIf the original organic material is retained during diagenesis, accumulations of plant debris can form compressions (sometimes called compactions) which ultimately can lead to coal. There is a sequence of organic accumulations that runds from slightly altered accumulations of peat to lignite (also known as brown coal) to bitumenous ≥soft≤ coal to anthracite ≥hard≤coal. These substances differ in the degree to which they have been altered by temperature and pressure, coal petrologists can often determine the temperature at which a particular coal has formed.

[peat, coal samples]

Plant debris that accumulates in a sedimetary basin can for organic-rich layers that do not constitute peat or coal, but are simply ≥organic-rich.≤ Oil shale can be considered a form like this, but in the case of petroleum, the organic matter is usually formeed by the breakdown of fats and hydrocarbons found in algae that become trapped in sediment. Oil is mobile and will move through pore spaces in coarser rock, eventually accumulating in the pore spaces of a resevoir host rock.

[Green River Shale sample]

Mineralization

Fossils are preserved in many different ways, depending upon the physical conditions of burial, the chemistry of the pore waters in surrounding sediments and changes that take place during diagenesis (=processes that take place as sediments are converted to rock under conditions of elevated temperature and pressure). In addition the type of starting material is important as well. Thus, many of the most common fossils are formed by the original permineralized shells of shallow marine invertebrate animals. These are often preserved as either aragonite or calcite (both are CaCO₃).

Petrifactions.ãDepending upon certain conditions of burial and subsequent movement of surrounding waters carring dissolved minerals, some fossils are formed by mineral substitution, or ≥replacement.≤ This method of fossilization results in what we call, ≥petrifactions.≤ A typical example is petrified wood, like that found in the Petrified Forest National Park in Arizona. In this case of petrified wood, the permineralizing agent is dissolved silica (SiO₂) which precipitates first as opal (SiO₂ÄnH2O), faithfully preserving the interior spaces of the woody cells. Opal, which has a non-crystalline amorphous structure, is often converted in time to chert (=flint) which is a cryptocrystalline (=very, very small crystals) form of quartz SiO₂.

Because the mineral replacement process proceeds at the molecular level, this form of fossilication can preserve extremely fine detail, even down to sub-cellular levels. (There is debate in the literature about the preservation of nucleii in eukaryotic dells). Because of this ability for preservation of fine detail, petrified cherts are the most common form of preservation of microfossils in the Precambrian.

Concretions.ã Concretions vary in their composition and origin. They can be rounded, oblong or irregularly-shaped. They are composed of secondarily-deposited minerals which differ from the surrounding matrix. Siderite (FeCO₃) and chert (SiO₂) are often concretion-formers. Concretions sometimes form around a nucleating site, which can be a fossil. Mazon Creek nodules, composed of siderite, are very common form of concretion from the Carboniferous of the Mid-western US that preserve a variety of rare animals and common plant remains such as leaves.

Sometimes concretions themselves can mimic organic forms. These are referred to as ≥pseudofossils.≤

[Mazon Creek nodules]

Amber.ãIn rare cases, organisms become entombed in resin which hardens to form a substance called amber. Amber is quite impervious to gasses and liquids and acts to completely seal off its contents from outside chemicals which might cause degradation. The idea behind Jurassic Park is perhaps not so far-fetched as it seems!. Examine the larger piece of amber on display and try to fine the spider that is preserved there. This is a rare specimen so please handle with care.

Primary accumulates.ãSome rocks are formed by the accumulation of the tests or shells of microorganisms. The Numullitic Limestones that capped the great pyramids of Ghizeh are one such roch. It is composed of the shells of a unicellular protoctist, foraminifers. Other examples include diatomaceous earth, composed entirely of the opaline frustrules of unicellular algae, diatoms, and chalk, a similar accumulation of coccoliths, the calcareous plates of coccolithophorids, a group pf unicellular planktonic algae.

Trace Fossils

The study of trace fossils, or ichnology, has become increasingly important in recentyears as interes in the exploration of other planets in the solar system begins to take place. In general, we think of trace fossils as signs of the activity of former organisms, rather than the actual remains of theose organisms. Most traces are simply burrows left by animals that either lived or moved through muddy and sandy substrates in search of food. Sometines we see traces as discrete footprints or crawling marks made by the legs of benthic (bottom-dwelling) invertebrates, such as trilobites.

Although not always categorized as trace fossils, stromatolites, microbial laminites and other microbial-sedimentary structures can be considered as a kind of trace fossil. Such structures are typically formed by the interaction of microbial growth with sedimentary deposition. As filaments of cyanobacteria grow toward the light, for example, the sticky, mucilagenous sheaths that surround these bacteria, trap and bind sediment into discrete structures. If the profile shape of such biosedimentary complexes is convex upward, we refer to them as stromatolites. If they are relatively flat or wavy-bedded, we call them microbial laminites. In order terminology, these are often referred to as cryptalgal structures.

Tracks & burrows.ãBenthic marine organisms can live directly upon the botton surface (epifaunal) or burrow within the sediment (infaunal). Epifaunal activity in soft substrates will often form trails or tracks as the bottom dwellers move around. Such trails are preserved sub-parallel to bedding planes. Cruziana, the trail of trilobites, is one such example. The large Beaconites specimen is another example. Infaunal burrowers that do not move from one place are sessile. They form burrows that crosscut bedding planes. Monocraterion is one such example.

Microlaminites & Stromatolites.ãStromatolites typically do not preserve the actual remains of the cyanobacterial that formed them. For this reason, they are perhaps most correctly thought of as trace fossils. Theoretically, the laminae that compose these sructures are formed daily (diurnal) as cyanobacteria respond to cycles of light and dark, but since lamination is also dependant upon sediment supply, diurnal lamination is rarely realized.

Pseudofossils

Pseudofossils are sedimentary structures that mimic organic forms. There is no general categorization of such objects, but there are several types that are typically confused with that real thing.

Dendrites.ãDendrites are metal oxide precipitates (usually some form of Fe, Mg or Mn ≠ oxides) that appear like leaves or minerature trees (hence the name). Thy are often found associated with sandstones wherethey form on fracture surfaces or bedding planes. They are formed when the above siderophile (=similarto iron[Fe]) elements dissolved in percolating groundwaters precipitate onto fracture surfaces. The fractal patterns formed by this crystal growth have been modelled mathematically.

Concretions. ≠ Concretions, as we have already seen with the Mazon Creek nodules, are typically ovoid to subspherical in shape. They occur for various reason ind in different circumstances. Generally, they are formed in place by secondary mineralization (cementation) of minerals brought in through the mighration of water moving through the sediment. Concretions that are the size of a baseball or smaller are often referred to as nodules.

Travertine & Geyserite.ã Hot, mineral-rich waters in the splash zones and marginal areas around thermal pools and geysers, can form a wide array of odd and interestingly shaped features. As these areas are alternately wetted and dry out, silica (SiO₂ÄH₂O) Calcium carbonate (CaCO₃) and other rarer minerals can precipitate out forming features that often mimimic stromatolites. In fact, in many of the thermal areas, various bacterial are living on such syrfaces and will, ultimately becomed entombed in such structures.

Geyserite is the mineral/rock name given to these features when they are composed of silica (SiO₂ÄH₂O). Sinter refers to the indifidual bits and balls of silica that make up the geyseite. Travertine is similarto geyserite, but is composed of calcium carbonate (CaCO₃).

Cone-in-cone structures.ãThese features are rarely confused with anything biological, but they are quite interesting and are very different from typical sedimentary rock. Some cone-in-cone structures were formed by high pressure shock waves as they move through the substrate. These have been used as evidenceof former meteorite impacts, since the rapid high pressure bursts needed to form them rarely occuras part of normal geologic processes. Some other forms of cone-in-cone structures form in a completely different manner, theough the dissolution of limestone in carbonate-rich beds.

Exercises

Look through the 6 ≥unknown≤ specimens we have pullled out. Write a brief description of each one and include in that description your guess as to

a. what kind of preservation is represented,

b. what mineral are involved in the fossilization,

c. what is the rock type or matrix in which the fossil was found, and,

d. what can you say about the kind of setting each fossil was found in, e.g. can you determine whether your sample formed in afreshwater vs. marine deposit?