Final answer:
The best index fossils are from species that lived briefly, were widespread and abundant, aiding in the dating and correlation of geological strata.
Step-by-step explanation:
The best index fossils are species who lived for a short time, were abundant, and were widely distributed. Index fossils are utilized for dating and correlating the strata in which they are found.
These fossils, from species like the Mesozoic-era Belemnites, provide important markers for geologic periods because they were widespread, easily recognizable, and existed briefly, making them reliable indicators of specific periods in earth history, such as the Cretaceous.