Help me please, it would mean a lot

Index fossils are fossils that only occurred during a specific time frame. Typically commonly occurring fossils that had a widespread geographic distribution such as brachiopods , trilobites, and ammonites make the best index fossils.

Most likely would be a beetle making the best index fossil.

What determines a fossil to be an index fossil ?

Index fossil, any animal or plant preserved in the rock record of the Earth that is characteristic of a particular span of geologic time or environment. A useful index fossil must be distinctive or easily recognizable, abundant, and have a wide geographic distribution and a short range through time.