Historical Geology: Paleontology, part 1

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

part 1: Paleontology, Fossils, and their Preservation

Last Updated • March 1, 2016

Paleontology is the study of ancient life in the broadest sense. Scientific information for the study of ancient life comes from fossils. A fossil is any remain of ancient organisms, or any evidence of their activity (a trace fossil).

(What does "fossil" mean? The word fossil comes from the Latin fodere, to dig: fossilis means "something that can be dug"; in the past it used to be referred to anything extracted from underground, including minerals and human artifacts, but today its meaning is restricted - as stated above - only to remains of ancient life).

The fossil record is far from being complete or perfect: most of the time fossils are missing, not preserved, and we speak of the incompleteness of the fossil record. That is, we can never have all of the fossils from all of the individuals from all of the species that ever existed.

The preservation of the remains of an organism after his death is not very common. If fossils are not preserved, their absence can originate a gap in the fossil record. When a gap exists (for a particular time interval and a certain location) scientists wonder if the fossil absence is true (that is, the organism in question actually never lived there at that time) or if it is preservational (that is, the organism lived there, but its remains were not preserved in the record).

A way to tell is by using what is called a taphonomic control: Taphonomy is the study of the fossilization processes.
Imagine two organisms, A and B, who are known to live in close association one with the other. If after death one fossil species, A, is missing from the fossil record, but the other, B (which is our taphonomic control) is present, we then know that the conditions for preservation existed, so very likely that missing species was really not living in that area at that time.

macrofossils and microfossils

Last Updated • March 1, 2016

MACROFOSSILS

Large fossils that are visible without the aid of microscopes or magnification are called Macrofossils. In the past, paleontological expeditions were aimed at the collection, observation and description of big, visible and easily identifiable fossils. Many of these fossils are now preserved in museums, and most fossils in museums are actually macrofossils.

Macrofossils in general represent organisms that lived in a variety of different environments, marine or terrestrial.

Benthic organisms (or benthos) are those that live at the ocean bottom, while pelagic organisms are those that either swim (nekton) or float (plankton) in ocean waters.

(from the United States Geological Survey, via SEPM, Society for Sedimentary Geology)

Two examples of macrofossils:
above, Tyrannosaurus rex, Late Cretaceous, from Cowley, Alberta, Canada (at the Royal Tyrrel Museum, Drumheller, Alberta, Canada)
below, Mastodon, Pleistocene, from La Brea Tar Pits, Los Angeles, California (at the George C. Page Museum, Los Angeles, California)

Both pictures: © Alessandro Grippo

MICROFOSSILS

With the development of more advanced observational tools, microscopes in particular, it became possible to study Microfossils, fossils of organisms that would not be visible to the naked eye. Microfossils are more common, more widespread, more likely to remain intact over time and more likely to provide a huge amount of information. Think for instance of what could happen when drilling or even simply excavating: you can easily break a big, macroscopic bone, but a microfossil would remain intact even in fragments the size of sand.

Micropaleontology has become in time a discipline of its own. It can help solve a variety of basic geological and biological problems, and is a precious tool for correlation and dating in the oil industry.

above, Florilus chesapeakensis, Miocene, from Randle Cliff Beach
below, a paleontologist at work with microfossils at the George C. Page Museum, Los Angeles, California

above: from the blog Fossils and Other Living Things
below: © Alessandro Grippo

preservation of fossils

Last Updated • March 1, 2016

As a general rule, the hard, mineralized parts (bones, shells, teeth, etc.) of an organism tend to be preserved more easily than its soft tissues, which are subject to rapid microbial decay in presence of O₂.

Skeletal materials often contain also an organic matrix which undergoes rapid decay after death. This may hinder durability and preservation of the hard parts too. It is also possible that, under unusual circumstances such as, for instance, lack of O₂ (anoxic conditions) in the environment of deposition, the soft parts of an organism can be preserved.

The chances of fossilization can be reduced

by biological activity

by the physical elements (weathering)

Biological activity is caused, among other things, by the arrival of scavengers, by the decay of organic tissues, by the use of hard parts as substrates by other organisms.
All these activities may occur soon right after death, thus quickly reducing the chances of fossilization.
Removal of the remains to an area of lower biological activity can instead enhance preservation.

Some kinds of biological activities can actually work the other way around, increasing the chance of preservation:
- encrusting organisms may protect shells from dissolution in ocean water
- certain bacteria may change ocean water chemistry in the area immediately around the dead body they are feeding on, favoring its preservation
- certain burrowing animals can actually bury skeletal material at depth, thus preserving it from decay

Loss of skeletal material can also occur because of the action of physical elements (weathering processes)
such as wind and freeze-thaw cycles in subaerial environments, and current and wave action in subaqueous environments.
While the wind action is mainly related to the materials that the wind carries (abrasion, caused by sand and dust), the force of the water (hydraulic action, the physical push of running water) is often a factor in itself that adds to abrasion.

There are several materials that are produced by an organism during its life, and could potentially be preserved in the fossil record.
The most important ones are carbonates, phosphates, silicates and a variety of organic compounds:

Carbonates include CaCO₃ (calcium carbonate), secreted by a variety of different organisms, either as calcite or aragonite.
Aragonite is less stable than calcite and over time will either dissolve in water or turn into calcite (at around 300°C)
Phosphates include calcium phosphate, one form of which is apatite, Ca₅(PO₄, CO₃)₃(F, OH, Cl).
Phosphates are important components of the teeth and bones of vertebrates and of a few other organisms.
Hydrous Silica, or Opal, SiO₂*H₂O, is important in sponges, diatoms and radiolarians.
Organic compounds include:
- Chitin, a major component of Arthropods' cuticles and Fungi
- Cellulose and other Polysaccharides, a major components of cell walls in Algae and Plants
- Lignin, a component of tissues in Vascular Plants
- Collagen, a dominant component of the connective tissues of animals
- Keratin, a protein constituent of horns, claws, bills, and feathers

This table shows different groups of organisms that are important in Paleontology
and the most important Inorganic and Organic Components they produce


	major component					minor component


	Inorganic constituents				Organic constituents
Group	Carbonates	Phosphates	Silica	Iron Oxides	Chitin	Cellulose	Lignin	Collagen	Keratin
Prokaryotes
Algae
Plants
Unicellular eukaryotes
Fungi
Porifera
Cnidaria
Bryozoa
Brachiopoda
Mollusca
Annelida
Arthropoda
Echinodermata
Chordata

Adapted by Alessandro Grippo from:
Towe, K.M., (1987) Fossil Preservation. In: R.S. Boardman, A.H. Cheetham, and A.J. Rowell (eds.), Fossil Invertebrates.
Palo Alto, California, Blackwell Scientific Publications, pp. 36-41.

What about soft tissues, or the DNA molecule for example?
Soft tissues are lost very quickly after death in presence of oxygen. Often, when we discuss the preservation of organic compounds one of the first molecules that comes to mind, given also its presence in contemporary pop culture, is that of DNA. In reality, the DNA molecule is not very stable, and decays very quickly right after death of an organism. When the DNA molecule is found in a fossil, it always belongs to a very young (less than 1 my old) organism, and special preservation conditions, such as either desiccation or freezing, have occured. As a consequence, despite of what we hear in certain movies, the chance of finding dinosaur DNA are close to none.

References:
- Vittorio Vialli, Notes from the Paleontology Lessons, Pitagora, Bologna
- Michael Foote and Arnold I. Miller, Principles of Paleontology, Freeman, New York
- SEPM Strata, Society for Sedimentary Geology, Tulsa, Oklahoma