On the bottom, the giant clam "Inoceramus" left common fossilized shells in the Pierre Shale. This clam had a thick shell paved with "prisms" of calcite deposited at right angles to the surface, giving a pearly luster in life. Paleontologists suggest that its giant size was an adaptation for life in the murky bottom waters, where a correspondingly large gill area would have allowed the animal to cope with oxygen-depleted waters.