base. Two of the graphs that they generate are shown on the facing page.
"Now that we have these graphs of the fossils' lengths and widths," Karen says at the beginning of the next class period, "we can begin to talk about what these measurements mean. We see from one set of graphs that the fossils in the second group tend to be both wider and longer than those in the other group. What could that mean?"
"Maybe one group is older," volunteers one of the students.
"Maybe they're different kinds of fossils," says another.
"Let's think about that," says Karen. "How could their lengths and widths have made a difference to these organisms?"
"It could have something to do with the way they moved around."
"Or how they ate."
"That's good," says Karen. "Now, if you had dug up these fossils, you would have some additional information to work with, so let me give you some of that background. As I mentioned last week, these fossils are from marine animals known as brachiopods. When they die their shells are often buried in sediments and fossilized. What I know about the fossils you have is that they were taken from sediments that are about 400 million years old. But the two sets of fossils were separated in time by about 10 million years.
"Taking that information, I'd like you to do some research on brachiopods and develop some hypotheses about whether or not evolution has influenced their size. Here are some of the questions you can consider as you're writing up your arguments."
Karen hands out a sheet of paper containing the following questions:
What differences in structure and function might be represented in the length and width of the brachiopods? Could efficiency in burrowing or protection against predators have influenced their shapes?
Why might natural selection influence the lengths and widths of brachiopods?
What could account for changes in their dimensions?
The following week, Karen holds small conferences at which the students' papers are presented and discussed. She focuses students on their ability to ask skeptical questions, evaluate the use of evidence, assess the understanding of geological and biological concepts, and review aspects of scientific inquiries. During the discussions, students are directed to address the following questions: What evidence would you look for that might indicate these brachiopods were the same or different species? How could changes in their shapes have affected their ability to reproduce successfully? What would be the likely effects of other changes in the environment on the species?
The materials needed to carry out this investigation are available from Carolina Biological Supply Company, 2700 York Rd., Burlington, NC 27215. Phone: 1-800-334-5551. www.carolina.com