Natural History of Vertebrates
Lecture Notes
Fossils and Continental Drift (Chapter 7)
These notes are provided to help direct your study from the textbook. They are not designed to explain all aspects of the material in great detail; they are a supplement to the discussion in class and the textbook. If you were to study only these notes, you would not learn enough to do well in the course. These notes are also linked with the notes from Vertebrate Structure and Development (ZO 515).
List of Terms
Every time that you set down to study, the first thing that you should do is write down the
periods of the geologic time scale in order and the times that define the boundaries of these periods (see the inside front cover). It is important that you know these periods because as we
progress through the evolution of the vertebrates, times of occurrence will typically be given in
terms of these periods.
When examining the fossil record, some things to keep in mind.
- Organisms fossilized better in some environments
- grasslands, forests are very poor at producing fossils.
- swamps are better because of the anaerobic conditions
- areas with high siltation rates are good e.g. the floodplain of a river
- ocean bottoms are generally good because of the high siltation and reduced activity of
decomposers and scavengers
- reefs are generally good
Many organisms probably never produced fossils, thus the fossil record is better for some
habitats and essentially non-existent for other habitats.
- Older fossils are less common than recent fossils.
Degradation of fossil-bearing rocks has had longer to occur for older rocks and thus fewer old
fossil-bearing rocks exist today.
- Fossil record has gaps
- the vagaries of nature: which rocks are brought to the surface and which rocks are destroyed determines which rocks we might find
- Punctuated equilibrium leads to inconstant rates of evolution. During some periods, the rate of
change was so fast there was little chance of producing a fossil of the intermediate life forms. At other times,
there would be little to no change for a very long time, with a much greater chance of producing a fossil.
When attempting to describe the nature of some ancient habitat or the habits of a fossil animal, we use the principle of
uniformitarianism, which says that processes that occurred in the past are the same processes
that occur today. For example, the movement of glaciers causes striation in the rock as the
glacier passes over the rock. Similar striations found in an ancient rock would be assumed to be
caused by an ancient glacier. Another example, in modern taxa, flat, rounded teeth are associated
with organisms that crush their food; the same type of teeth found in a fossil are assumed to be
from an animal that also crushed its food.
Continental Drift is an important concept. Basically, the continental plates float on a bed of
molten lava and because of this the continents tend to drift around. However, much of the
movement is not random but directed by upwelling of molten lava to form new crust at a ridge
and the subduction of material from the crust into a subduction zone. For example, the North
American Plate is being pushed from the European Plate by upwelling of material at the Mid-Atlantic Ridge. The North American Plate is being reduced at its western edge as it flows into a
subduction zone along the California coast (figure 7.1). Thus the North American Plate moves a little west
away from European Plate each year and a little bit of the western edge passes under the Pacific
Plate (figure 7.1). Be sure to note the relative positions of the continental land masses during
the Cambrian through Devonian time periods. (figures 7.2, 7.3, and 7.4)
Last updated on 5 February 2008
Provide comments to Dwight Moore at mooredwi@emporia.edu
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