In one cliff, a geologist finds the layers in this order from the top: mammal bones, reptile bones…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
In one cliff, a geologist finds the layers in this order from the top: mammal bones, reptile bones, amphibian bones, fish bones and shells. In a second cliff, 40 km away, the layers from the top are fish bones, amphibian bones, mammal bones, reptile bones and shells, and the layers are bent into a steep fold.
- What does the first cliff tell her about the order in which the groups appeared?
- The second cliff has a different order. What is the most likely reason?
- How can she check which cliff shows the real order?
Need a reminder? The key ideaShow
Homologous organs have the same origin and different jobs (divergent evolution). Analogous organs have the same job and different origins (convergent evolution). Deeper rock layers are older. Radioactive carbon halves every years, so after half-lives is left.
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. (a) Deeper layers are older, so the groups appeared in this order: invertebrates, fish, amphibians, reptiles, mammals.
Step 2. (b) The rocks of the second cliff were moved after they formed. Folding or faulting tilts layers and can turn them over. Part of the sequence is also upside down.
Step 3. (c) She can compare many places. A real order shows up again and again in undisturbed layers, and the folded cliff does not agree with the others.
Answer: invertebrates, fish, amphibians, reptiles, mammals; the second cliff was disturbed by folding; compare many cliffs.
Watch out. Rule: a deeper layer is older only if the layers have not been turned over.
Discussion (0)
Log in to join the discussion.
Loading…