Our professor tells us that he has brought in a puzzle for us today, and as he speaks his fingers tap against a shell sitting in an unlabeled box. Does anyone know what this is? he asks. It looks like a clam or a mussel shell–a bivalve, with two sides of shells that clamp closely together. Specifically, it’s one half of a scallop shell, which we can tell from shell ridges shaped like corrugated cardboard and a small triangular base where the other half would attach. And, he mentions offhand, it’s about ten million years old.
His fingers wander over the edge of the shell with a tapping that varies between uncontrolled tremor and directed sensory input. On the shell are some lighter-colored growths, clusters of barnacles, themselves now shells. Our puzzle, he tells us, is to determine whether the barnacles grew on the scallop while it was alive or after it was dead: that is, were the two species part of an interacting community, or were they separated by that all powerful fourth dimension, time?
We pass the shell around, wondering at its familiarity. You could find shells washed up on the beach today that to our untutored eyes would be indistinguishable from this ancient specimen. We looked at colors, textures, and shape before handing it on to the next classmate, shaking our heads in failure to answer the proposed question.
The shell is handed back to him and when he asks for our decisions–and the reasoning to support them–silence descends. As he encourages us to share some response, any thought, he turns the shell over and over in his tapping fingers. Inside. Outside. Inside. Outside.
And there it is, the subtle obvious difference. My answer comes out as a question: The barnacles are only growing on one side of the shell? So they must have been growing at some point when the shell was still closed? I don’t know how long it takes for scallop shells to split apart after the organism inside dies. Maybe this tells us they lived at the same time, ten million years ago?
The fingers stop spinning the scallop. Yes, he says, and puts the ancient, fossilized community back into its box.