(paleontology) index fossil

to show, to have somebody see / to display, to show, to demonstrate / to indicate, point out, to denote / to indicate (for a measuring device such as a clock to show a value)

important, significant

importance, consequence, significance

geological features

a certain one, particular one, specific one

specify, identify, determine

to fossilize

a fossil / petrifaction / (metaphor, insult) out-of-date person, thing

causing attention, showing, important