Ammonite is a species of shellfish that is used as an index fossil. It has a distinct spiral. Which statement most likely explains why other species of shellfish that existed around the same period of time are not considered index fossils?
A. Other shellfish did not contain the carbon-14 isotope
B. Other shellfish were much smaller in size than ammonites
C. Other shellfish were not as widespread as ammonites
D. Other shellfish did not have a rigid external body, like ammonites