Earth History: Paleontology, part 2

Alessandro Grippo, Ph.D.
THE NATURE OF THE FOSSIL RECORD

part 2: Modes of Fossilization and Taphonomy

Last Updated • September 14, 2015

Organisms can be turned into fossils upon death in several different ways
The preservation level can range from most complete (when we find fossil parts)
to least complete (when we only find indirect traces of organisms.

In the following list, preservation modalities near the top are the most complete but they are also relatively uncommon.
The ones towards the bottom of the list are instead less complete but a bit more common.

Freezing
Under exceptional circumstances, it is possible to preserve fossils in ice without virtually any sign of decay.
An example is given by the wolly mammoths occasionally found frozen in the permafrost regions of Siberia (Russia), and other parts of the world. While these fossils are recent (only a few thousands of years old), they have provided us with a lot of information through DNA samples and materials from their stomach and intestine.
Preservation in amber
Small organisms such as spider and insects can become trapped in very viscous resins released by a variety of different trees.
When the resin hardens into a solid, these organisms are preserved, together with bubbles of air, within a yellow-orange transparent material, called amber.
Not only can paleontologists get a lot of information from insects fossilized in amber, but geologists and paleoclimatologists use the trapped air bubbles to study the composition of Earth's past atmosphere.
Carbonization
Under conditions of high heat and pressure, distillation of organic matter may occur.
Distillation removes hydrogen and oxygen from organic matter, while leaving behind carbon.

C₆H₁₂O₆ (organic matter) ————> 6C (carbon) + 6H₂O (water)
As a consequence, the soft parts of organisms can be preserved as very thin carbon films.
We lose the original chemical signature of the animal or plant in question, but we might get in exchange very fine details of its soft tissues.
Permineralization
We have seen earlier (part 1) that the organic matrix present in skeletal materials can effectively decrease the chances of skeletal preservation. When skeletal materials are buried within the sediment, they may be affected by pore water that percolates from the surface. Pore water can dissolve skeletal elements but it might also carry ions in solution; these ions, in the right conditions, precipitate in spaces within the bones, or the shells, or the teeth. The ions, usually silica, phosphate and pyrite (FeS₂) harden the remains, favoring the preservation of very fine details such as shell layers, pores, and growth bands in the skeletal elements.
Petrifaction
When the conversion of organic material to mineral material is total, that is when all of the original organic materials have been replaced by a mineral, permineralization gives way to petrifaction. A famous example is given by the petrified logs of the Painted Desert, in Arizona.
Replacement
Similar to the previous two modalities of fossilization is replacement: in this case the skeletal material is replaced very slowly, often molecule-by-molecule, providing the fossil with a very high detail level. Again, the materials involved are mostly silica, phosphate and pyrite.
Recrystallization
Certain skeletal materials become unstable under conditions of higher temperatures and pressures, conditions that are commonly reached after burial. The most common examples are provided by aragonite, turning into calcite, and by amorphous silica, turning into quartz: in both cases the chemical composition does not change, but the new minerals recrystallize in a different form, destroying the original structures. So when you look at a recrystallized fossil, the exterior form of the fossil may still look as before but the fine-scale details are totally lost.
Molds and Casts
A mold is a negative impression of hard parts. If the skeletal elements have been dissolved completely by water, a replica of the outer parts (external mold) can be preserved in the surrounding sediment, particularly if this is very-fine grained (the smaller the size of sediment, the more likely we can record the detail of our fossil).
If the skeletal elements do not get dissolved, but sediments or minerals fill them from the inside, what forms is an internal mold, or steinkern. In this case, fine details can be preserved.
An external mold can remain in the rock as an empty space, or it can later be filled by other, different materials (such as minerals or sediment itself) to obtain a positive cast. Sometimes casts are made on purpose by paleontologists through the injection of an epoxy resin in a mold. The newly obtained cast would thus allow the study of the fossil.
Trace Fossils
While all of the previous modes of fossilization refer to fossils of an organism (body fossils), trace fossils simply preserve traces of their activity, such as tracks, burrows, footprints, etc. A trace fossil often does not allow to tell precisely what organism left a trace. Still, in several occasions it is possible to make behavioral observations. For instance, the discovery of dinosaurs' parallel trackways would imply that these creatures, albeit unknown in detail, traveled in groups.

pseudofossils and artifacts

Last Updated • September 14, 2015

Pseudofossils
It is not uncommon to find inorganic structures that look like biological remains. These are known as pseudofossils.
The presence of pseudofossils in the geological record can sometime trick the unexperienced eye. A classic example is given by manganese dendrites, that might seem at first a plant (fern) fossil.

Manganese dendrites are delicate crystals, usually formed by different kinds of manganese oxides, that often grow on a rock surface following a fractal pattern, either between layers (along bedding planes) or in rock fractures. The name "dendrite" refers to the similarity between this pseudofossil and the pattern of a branching tree.

While manganese dendrites are not uncommon, some of the most beautiful examples come from the Plattenkalk, or Lithographic Limestone of Solnhofen, in Germany. This limestone is also one of the greatest examples of Lagerstätten (see below).

Manganese dendrites from the Jurassic Solnhofen Limestone from Bavaria, Germany

Artifacts
It has also been observed that some artifacts are generated during the processes that lead to preservation of fossils. This is particularly true in the study of microbes and bacteria (remember that knowing about microbial life is essential in order to understand early life during the Archean and the Proterozoic). In these cases, distinguishing the real fossil from an artifact can be very difficult. It is up to the expertise of paleontologists to use all of their knowledge to discard pseudofossils and artifacts.

taphonomy

Last Updated • September 14, 2015

Fossils are most often preserved within sediments and sedimentary rocks. Both sediments and sedimentary rocks reflect a particular environment of deposition and most of the times a fossil animal or plant is characteristic of that specific ancient environment.

By using the principle of uniformitarianism, together with an understanding of contemporary living organisms (where they live and how they relate to other organisms in the same environment) it is possible to increase our capacity of interpretation of ancient sedimentary rocks, ancient environments and ancient geography (without ever forgetting about the time significance of fossils).

Still, the kind of information we get from fossils is different, in both quality and quantity, from what we would get if those same organisms were observed during life, in their own environment. If we understand what these differences are, and how they develop after the preservation of the fossil we have a tool that allows us to better understand the fossil record.

This is the field of taphonomy, which also includes two broad aspects related to fossilization:

Biostratinomy (the study of what happens to a dead organism before burial)

Diagenesis (the study of what happens to a dead organism after burial)

the bias of the fossil record

Last Updated • September 14, 2015

We have seen that the processes of fossilization are different for different organisms and can be different even for the same organisms if different conditions exist.

One thing we can say with certainty is that, in general, it is easier to preserve fossils of marine animals that have hard, mineralized parts. This bias in preservation would lead to a distorted reconstruction of ancient life at any given time.

As an example, the following table shows the percentages of living animal species (not individuals) alive today. Insects species (arthropods) are clearly the dominant ones in today's world. Still, the percentage of fossil insects in the geologic record is actually less than 1%. This dramatic drop is caused by the fact that the insects' external skeletons (exoskeletons) can not withstand degradation after death, thus decreasing their preservation potential.

Protozoans	2.5%	Porifera	< 0.1%
Cnidarians	0.8%	Mollusks	7.0%
Bryozoans	< 0.1%	Arthropods	82.0%
Brachiopods	< 0.1%	Echinoderms	< 0.1%
Vertebrates	3.0%	All others	5.0%

Percentage of different living animal species (from J.D. Cooper et al., 1986, A trip through time, 2^nd ed., Macmillan, New York)

It has also been shown that even animals with hard parts can add a great bias to the fossil record if the physical and chemical conditions of the environment in which they die are different: experiments where natural mechanical destruction processes (such as, for instance, wave action) have been duplicated in the laboratory have shown that while animals with less resistant shells had their remains destroyed, others were barely affected. These last ones then came to be relatively much more abundant, thus significantly altering the fossil record.

lagerstätten

Last Updated • September 14, 2015

A fossil Lagerstätten is a deposit of exceptionally high quality, with exquisite preservation of both organic and skeletal materials. A Lagerstätten can provide information on otherwise lost features, such as tissues from land plants, feathers from early birds, skin textures, limbs of arthropods, color patterns, and even embryos.

An exceptionally well preserved Lagerstätten can also illuminate past events during critical periods of time, as in the case of the Burgess Shale of British Columbia (northwestern Canada), where an extraordinary fossil assemblage of invertebrates was preserved during a very early phase of animal diversification.

A quick, incomplete list of world's Lagerstätten include:

The Middle Cambrian Burgess Shale of British Columbia, Canada
The Cambrian Chengjiang Fauna of Yunnan, China
The Cambrian fossils from Sirius Passet from Greenland, Denmark
The Pennsylvanian Mazon Creek Beds of Illinois
The Devonian Hunsrueck Shale of Germany
The Triassic Besano - Monte San Giorgio anoxic deposits, from Varese, Italy, & Lugano, Switzerland
The Middle Jurassic Posidonienschiefer of Holzmaden, Germany
The Jurassic Solnhofen Limestone of Bavaria, Germany
The Eocene Green River Formation at Fossil Butte, in Kemmerer, Wyoming
The Eocene deposit at Messel, Germany
The Eocene Bolca Lagerstätten, from Bolca, Verona, Italy

The delicate wing and body structures of a dragonfly are exceptionally preserved in the Eocene Green River Formation Lagerstätten

Fossil preservation is at its best in a Lagerstätten. Most of the deposits at the localities listed above formed::

In environments where rapid burial occurred and anoxic conditions existed. Both factors reduce the amount of available oxygen, and lack of oxygen prevents scavenging and decomposition.
In quiet, low-energy environments: lack of reworking currents favors the preservation of the fossils
In areas where little or no diagenetic alteration occurred after burial, so that fossils would not be destroyed.

how the fossil record changes over time

Last Updated • September 14, 2015

There is a straightforward correspondence between the amount of preserved sediment and its age,
and the same is true also for the number of fossil species known from a period of geologic time

But it is also evident that the nature of the fossil record has changed over long time scales, for instance in terms of abundance of species, skeletal composition, kind and distribution of life forms:

Bioturbation
Bioturbation, or the disturbance of sediment after deposition, has clearly increased over the past 500 million years of animal life. This increase, which first started in the Lower Cambrian, and then increased during the Middle-Upper Ordovician, marks the evolution and the diversification of several different animal groups that lived on the ocean bottom, either directly on the sediment or within it.
Skeletal Mineralogy
Different minerals had different importance over the history of life during geological time. The most important variation, in terms of the quality of the fossil record, is that of the relative percentage in calcite and aragonite in organisms' shells.
When the ion Mg²⁺ is more abundant in ocean waters, aragonite is preferentially used as a building materials by organisms; or, organisms that use aragonite instead of calcite would flourish during these times. The opposite would be true when the ion Mg²⁺ is less abundant: species that make their shell out of calcite can then expand and diversify.
The abundance of the ion Mg²⁺ seems to be a function of how fast tectonic plates spread at a mid-ocean ridge, and these spreading rates seem to have varied a few times in the past, in such a way that the oceans were either dominated by aragonite-producing organisms or calcite-producing organisms.
We have seen before that aragonite tends to recrystallize into calcite at high temperatures (around 300°C) and pressures.
During this process, when the newly formed calcite crystals grow, they destroy whatever previous structure we might have had in our fossil. As a consequence, because the original aragonitic structures are lost, it is almost always possible to tell if the calcite we find is an aragonite replacement rather than original.
Geographical and Environmental Distribution of Fossiliferous Rocks
Another factor responsible for changes in the nature of the fossil record over time is related to the physical environment where the fossils have been deposited.
Most of the fossil record, even on land, is marine in origin, so most research is done on marine sediments.
Deep marine sediment can today be sampled directly from the ocean bottom.
The problem is that the ocean bottom is nowhere older than about 200 million years, because of oceanic crust subduction.
Sometimes, older deep marine sediments can be scraped up onto continents at convergent boundaries, and that would be our only record, actually very sparse indeed, of Paleozoic oceans.
Most of what we know about ancient marine life actually comes from shallow water deposits.
These sediments formed when the oceans invaded continental shelves and continental lowlands during times of high sea level.
These sediments have been later exposed at the surface either by a drop in absolute sea level (that is, less water in the ocean, such as for instance during a glaciation), or by tectonic uplift (that is, a quick rise of parts of a continent).
When we look at fossils of land organisms, that is fossils buried in terrestrial deposits, the most common findings occur in ancient coastal lowland environments. Mountain areas are subject to erosion (the record for those areas increases when we approach present-day deposits) and thus not very important.

References:

Vittorio Vialli, Notes from the Paleontology Lessons, Pitagora, Bologna
Michael Foote and Arnold I. Miller, Principles of Paleontology, Freeman, New York
Donald R. Prothero, Bringing Fossils to Life, An Introduction to Paleobiology, McGraw Hill, New York