History of Life – Background Reading


Our solar system, including the Earth, formed about 4.54 billion years ago. Since then, the Earth has undergone many dramatic changes. The chemical composition of the atmosphere, the arrangement of the continents, and the global climate have all changed, and continue to change, over time. These factors, and others, have determined the evolutionary history and diversity of life on Earth.

The immense length of Earth’s history is difficult for us to grasp. Thousands of years, let alone millions or billions, are simply outside the range of human experience. Geologic timescales in textbooks are useful reference points, but often compress deep time due to space limitations, which can give a misleading impression of Earth’s history.

Today, you will make a scaled timeline of Earth’s history and place key events on the timeline to help you better appreciate the long, slow processes of change that have led to the Earth we know today.

In addition, we will consider how a paleontologist might study the history of life on Earth using the fossil record. 

The Fossil Record

Fossils are evidence of ancient life. They exist in many forms and are extremely useful in helping understand the history of life.  Some types of fossils include:

  • Compressions/impressions: formed when an organism is compressed in a sedimentary layer.  The remnants of the organism may be present as a film of carbon in the resulting sedimentary rock.  Loss of all organic matter may result in an impression, left after the decomposition of the organism.  If a cavity was left by the decomposing organism, this may be filled secondarily to form a cast of the organism.
  • Permineralizations: formed when a mineral solution infiltrates the tissues of a dead organism and hardens. This may result in the preservation of the organism in fine (often cellular) detail.
  • Microfossils: formed when microscopic structures (such as unicellular organisms or pollen/spores) are preserved.
  • Trace fossils: When evidence of organisms (but not the organism) is preserved.  These might include burrows, tracks, or residual organic molecules that have been preserved.

Like extant organisms, fossils are named and classified using the binomial system and the taxonomical hierarchy.  Because of the fragmented nature of many fossil specimens classification can be problematic.  Large organisms are rarely found intact, so specimens of different parts of the organism are often placed in provisional “form” or “organ” genera until they can be shown to be connected (for example, some fossil plants were first described with separate names for roots, stems, leaves, and reproductive structures).

Coal Balls

Coal balls are permineralized portions of coal-forming plant peats known from Upper Carboniferous (Pennsylvanian) coal seams in Europe and the Midwestern US.  These coal seams are dated (for the most part) at between 300 and 320 million years old.  Coal balls range in size from a few centimeters to over 2 meters in diameter and are variable in shape, but often rounded.  They formed when peat in the “coal swamps” was infiltrated with calcium carbonate (calcite), magnesium carbonate, or less commonly iron sulfide (pyrite).  Preservation within the coal balls is often “exquisite,” showing cellular structure (and sometimes subcellular structures such as nuclei) of stems, leaves, roots, and other plant structures.  Study of the plant materials within coal balls has contributed greatly to understanding of plants during the Pennsylvanian period.

One of the most widespread techniques for studying coal balls is by making peels.  Coal balls can be cut into slabs using lapidary saws (i.e. slab saws).  Serial peels can be made from the cut surfaces.  As many as 500 peels per inch may be made, providing a thorough understanding of the anatomy of these ancient plants.

In this lab exercise, you will be making a coal ball peel.  Additional peels are available for examination.  Please work in groups of 4-5.

A youtube video summarizing this process is available here: Coal Ball Peels

An Additional Reference: Fossil Peat of the Illinois Basin

Trilobites [1]

The Trilobites are an extinct subphylum of the Arthropoda (the most diverse phylum on earth with nearly a million species described). Arthropoda also contains all fossil and living crustaceans, spiders, and insects as well as several other extinct groups. The trilobites were an extremely important and diverse type of marine invertebrate that lived during the Paleozoic Era. They were exclusively marine, and ranged in size from less than a centimeter to almost a meter. They were once one of the most successful of all animal groups and in certain fossil deposits, especially in the Cambrian, Ordovician, and Devonian periods, they were extremely abundant.

Trilobites are well represented in the fossil record because of their mineralized (usually calcium carbonate), sturdy exoskeleton, which would have been much thicker and stronger (and harder to break) than the shell of a modern crab. Further, being arthropods, they molted as they grew, such that every single trilobite was capable of leaving behind many, many skeletons that could become fossilized. 

The name trilobite refers to fact that their body is made up of three longitudinal (along the length of the body) sections: the central section, known as the axial lobe; and the two lobes on either side of the axial lobe, known as the pleural lobes. Trilobites are also separated into three sections from front to back known as tagmata: the cephalon, or head; the middle section made up of multiple segments known as the thorax; and the posterior section, or pygidium (plural = pygidia) 

For this exercise, you will look for real trilobite fossils in limestone shale. This shale comes from the Wheeler Shale formation of Millard County, Utah. We obtained it from the U-Dig Fossils private concession. It dates to about 507 mya.

[1] Adapted/remixed from Casey, M.M. and Lieberman, B.S. (2014) Introduction to the Trilobites: Morphology, Macroevolution and More, SERC. Retrieved Jan 2, 2018, from https://serc.carleton.edu/NAGTWorkshops/paleo/activities/79291.html