Activity 93: Reading the Rocks Challenge - How can you determine which fossil layers are older, which are younger, and which are likely to be from an extinct species? Key Vocabulary: Law of Superposition Stratigraphic Column Drill Core

Have you ever seen rock layers on a cliff? In order to understand and interpret fossil evidence with more certainty, geologists and paleontologists have to look at evidence from layers of rock from thousands of locations Remember that every rock layer represents an “event” in geologic time Some layers are missing important evidence so we look at layers around the world

How do you think scientists figure out which layers in one location correspond in time which layers in another location? Read the Introduction on page 21.

Vocabulary: Law of Superposition - states that the top layer is the youngest layer, and the bottom layer is the oldest layer. Stratigraphic Column - shows the sequence of rock layers; they can be made by either looking at cliffs or at drill cores Drill Core - A cylindrical column of rock extracted from the Earth. Each core contains similar, but not identical sequence of alternating layers which contain one or more fossils within them.

Law of Superposition Non-scientific Scientific Wednesday Tuesday Monday

Procedure: Fern Fish Ammonite Trilobite Follow the procedure on page 22 DO NOT glue in SS 93.1 You do not need to exchange cores as is mentioned in Step 4, just rotate with your group. Once you have sketched each core, cut out the drill cores so that you can correlate them. When you have completed the activity, answer Analysis Questions 1-3 in your notebook, using complete sentences Ammonite Fern Trilobite Fish

Analysis Answers Difficulties matching evidence from drill cores included: No known age of layers on any of the core samples It was unknown how complete the layers were

Because each fossil is represented in the top layer of at least one of the drill cores, you can argue that all four of the species represented are still alive. You can also argue that because layer 1 in both core 3 and 4 are the youngest layers, and neither contains ammonites or trilobites, these tow species may be extinct. However, it is possible that fossils from these species could be found in recent layers in some other locality, or even alive somewhere on Earth.

From the evidence in these cores, it appears that trilobites and ammonites could have lived at the same time, as could have ammonites and ferns, and ferns and fish. But not necessarily ammonites and fish. Although these determinations are based on the observation that these pairs of fossils are found in the same layers, it must be inferred from this that the pairs of species might have lived at the very same times.