Choosing Index Fossils Diagram of 4 outcrops. Organism A is a spiral shaped shell, organism B is a trilobyte, organism C is a fan shaped shell, organism D is a tube shaped shell. Outcrop 1: bottom layer shows organism B; second layer shows organisms C and B; third layer shows organism A; top layer is empty. Outcrop 2: bottom layer shows organism C; second layer shows organisms B and C; third layer shows organisms A and C; top layer shows organism C. Outcrop 3: bottom layer is empty; second layer shows organisms B and C; third layer shows organism A; top layer shows organism B. Outcrop 4: bottom layer shows organism B; second layer shows organism C; third layer shows organisms A and B; top layer shows organism D. Answer the questions about the diagram. Organism appears in the least outcrops. Organisms are the oldest. Organism would make the best index fossil.