CPSP118G Fall Semester: Earth, Life & Time Colloquium

Fossils and the Worlds of the Past

Thomas R. Holtz, Jr.

Fossils: The physical traces of past life.

Or, more fully, a fossil is any remain of an ancient organism or its behavior preserved in the rock record.

(Derived from the Latin word "fossilium": that which is dug up. Originally used for anything found in the ground, but by the 19th Century had come to mean traces of past life.)

Fossils are the only direct evidence of past life, although indirect evidence exists in the form of the evolutionary and biogeographic distribution of modern organisms.

Two major types of fossils:

Trace fossils are, essentially, biologically-generated sedimentary structures. They include:

Preservation of trace fossils is just like other sedimentary structures: must have rapid burial, and preserved by lithification of the rock itself.

Body fossils: can be preserved in a variety of ways.

In general, only organisms with hard parts can be preserved: shells, bones & teeth, wood, etc.

For vertebrates (such as dinosaurs), body fossils are primarily bones and teeth


But the rest of the vertebrate is soft tissue (and in many organisms there are NO hard parts), and so these are only preserved in rare instances.

Bone (like shell and wood) is not solid material, but porous. Pore space is occupied by organic material in life. Upon death, organic material begins to decay.

In order for bones and teeth to become fossilized (turned into a fossil):

The study of burial and fossilization is called taphonomy. There are various modes of preservation after the bone is buried:

Different organisms have different potential for fossilization:

Geologists could use physical rock relationships to figure out sequence of events for any given spot. But how to tell when event in one part of the world happened relative to events in some other part of the world? Needed a new method of correlation. Rock type doesn't work, because the same environment will produce the same rock type regardless of relative or absolute time. William "Strata" Smith, however, discovered thatFossils, however, were useful:

Fossils allowed correlation from continent to continent. Only certain types of fossils (called index fossils) were useful for correlation. To be a good index fossil, the species should:

Using index fossils, geologists were able to correlate across Europe, and then to other continents. Created a global sequence of events (based on the sequence of (mostly European) formations and the succession of fossils) termed the Geologic Time Scale. Became a "calendar" for events in the ancient past: used to divide up time as well as rocks.

Geologic Column divided into a series of units: from largest to smallest Eons, Eras, Periods, Epochs, Ages:

Animal and plant fossils are mostly restricted to the last (most recent) Phanerozoic Eon ("visible life eon"). The Phanerozoic Eon is comprised of three Eras:

So fossils show us:

In other words, fossils help us understand Earth, Life & Time!

But WHY did those enivornments change? And HOW can we get at numerical time? We'll see that next week.

Last modified: 10 August 2007