Answer:A useful index fossil must be distinctive or easily recognizable, abundant, and have a wide geographic distribution and a short range through time. Index fossils are the basis for defining boundaries in the geologic time scale and for the correlation of strata.
A fossil is any preserved remains, impression, or trace of any once-living thing from a past geological age. A index fossil must have a short vertical range, wide geographic distribution and rapid evolutionary trends. Another term, Zone fossil is used when the fossil have all the characters stated above except wide geographical distribution, they are limited to a zone and can't be used for correlations of strata.
Explanation:
I really hope that helped. I don't know much about fossils though.
