Geology 143

Lecture #1

August 28, 2002

The Fossil Record of the History of Life

This course serves as a survey of the history of life on Earth. The Earth is approximately 4.6 billion years old, and has been inhabited for approximately 4 billion years (the oldest life forms being one-celled organisms, similar to modern bacteria). Given the vastness of potential topics in a course with so broad a scope as the history of life, certain aspects of the record (vertebrate fossils especially) will be emphasized at the expense of others.

Due to the immensity of geologic time, the vast majority (> 99%) of the species that ever existed are now extinct. The primary information concerning the history of life is thus derived from the study of fossils: indications of past life preserved in a geologic setting such as rock, sediment (loose sand, mud, or gravel), or (in rare instances) glacial ice.

There are three main kinds of fossils. The most biologically informative are body fossils, remains of past organisms and representations of their remains. Body fossil formation requires the death and burial of the organism involved, and thus body fossils resemble organisms (such as this pictured trilobite), or at least some portion of organism anatomy. Trace fossils are indications of past organism activity. An example of a rock bearing trace fossils shows the sinuous path of worm-like organisms grazing the surface of the sediment. Many trace fossils were once thought to represent fossilized seaweed, but seaweed is very difficult material to preserve, and comparisons with activities of modern organisms have allowed us to understand these markings as results of animal activity. The discoloration associated with many trace fossils is derived from chemical interaction between an organism's mucus and the sediment. Aside from locomotion, feeding (through associated disruption of the sediment) and defecation (fossilized feces are termed coprolites) are other behavioral activities represented by trace fossils. Chemical fossils are chemicals preserved in a geologic setting that are biogenic (derived from organisms or their activity). Pristane and phytane are examples of chemical fossils that are derived from the chemical breakdown of chlorophyll (used by plants and algae in photosynthesis).

Because they provide more information than trace and chemical fossils, body fossils are of primary interest to us. Although fossils are numerous, the fossil record is far from being a complete record of past life. The fossil record is unfortunately quite incomplete (most species in Earth history have no fossil record), and the record we do observe is strongly biased against certain groups and in favor of others. Fossilization generally requires burial (the chances for burial are negatively affected by biological predation, scavenging, decay, as well as physical disruption from winds, currents, and precipitation). Thus, a very important biasing factor is environment (habitat). Organisms living in an environment of active deposition of sediment (which includes many marine environments and river floodplains on the continents) are favored over those that live in environments of nondeposition or erosion (such as mountainous areas). The other major factors causing bias in the record concern differences in organism construction. Organisms (such as worms and plants) that lack mineralized hard parts are much less likely to be preserved than organisms that produce mineralized skeletons (shelly invertebrates such as clams and bony vertebrates such as mammals and dinosaurs). Some skeletal minerals (such as the calcium phosphate of vertebrate bones) are less likely to be dissolved in the burial environment than others (the calcium carbonate of clam shells being more susceptible to acidic groundwater than bone is). Organisms such as bats, birds, and fish, which have skeletons composed of a large number of delicate pieces are less likely to be preserved than organisms with fewer, more robust hard parts, such as snails and clams.