Fossils in Sedimentary Sequences

Figure Above: How fossils change in the sequence of sedimentary rocks. Can use the overlap of two fossils to constrain age. Some fossils spanned long times. Some species lived short timespans so they give a narrow age range of the strata. Fossils change up sections in a systematic way.

Sedimentary Sequences

Fossils change through time, which can be seen in the sequence of sedimentary rocks.

1. We can use the overlap of two fossils to constrain age. The two fossils together only occur in a narrow time range (I & G).
2. Some spanned long times and represented organisms that remained unchanged for long periods of time. These fossils are less useful for assigning ages than a fossil that lived for a short time period.
3. Some species only live a short time span, are widespread and abundant, and are easily identifiable. They are referred to as INDEX FOSSILS and are very useful for dating time periods.
4. Fossils change up sections in a systematic way. Changes in body design or shape help to distinguish one variety from another. Trilobite (A) is the main index fossil for early Paleozoic.

The principle of faunal succession, also known as the law of faunal succession, is based on the observation that sedimentary rock strata contain fossilized flora and fauna and that these fossils succeed each other vertically in a specific, reliable order that can be identified over wide horizontal distances. For example, a fossilized Neanderthal bone will never be found in the same stratum as a fossilized Megalosaurus because neanderthals and megalosaurus lived during different geological periods, separated by many millions of years.

This principle allows for strata to be identified and dated by the fossils found within. This principle, which received its name from the English geologist William Smith, is important in determining the relative age of rocks and strata. The fossil content of rocks, together with the law of superposition, helps to determine the time sequence in which sedimentary rocks were laid down. Theories of evolution explain the observed faunal and floral succession preserved in rocks, which are the facts on which the understanding of evolution is based.

Figure Above: How do we correlate sequences of rock? When you have two sequences of rock located in different but nearby regions you can correlate (match up) rock layers using the type of rock, sequence of rock, and fossils within the rocks as guiding principles.

1. Look at the rock sequences and determine similar patterns. For instance, if you have limestone with shale followed by another limestone sequence in both rocks. Look for units that appear in one and not the other. Is it reasonable to assume that deposition occurred in one area but not the other, or has there been some erosion? 2. Determine if the samefossils occur in the same types of layer.

Figure Above: Comparing partly overlapping sections. Observe how these rock sections correlate and how the thicknesses of some units vary. Using both sections, we can reconstruct a complete section.