An index fossil must be A. smaller than other fossils around it. B. larger than other fossils around it. C. older than 1 billion years. D. unique in its appearance.

D. unique in its appearance. Index fossils must have a unique form that is easy to identify without special equipment. Geologists make maps in the field as they gather data and specimens. If the index fossil is easy to identify without the use of a microscope or laboratory equipment, the field work can process more quickly.