Hildoceras bifrons
... that the ammonite Hildoceras bifrons has been used as an index fossil to help identify the age of rocks?
... that the <a class="mw-redirect" href="./Ammonite" id="mwAqU" rel="mw:WikiLink" title="Ammonite">ammonite</a> <i id="mwAqY"><b id="mwAqc"><a href="./Hildoceras_bifrons" id="mwAqg" rel="mw:WikiLink" title="Hildoceras bifrons">Hildoceras bifrons</a></b></i> <i id="mwAqk">(pictured)</i> has been used as an <a class="mw-redirect" href="./Index_fossil" id="mwAqo" rel="mw:WikiLink" title="Index fossil">index fossil</a> to help identify the <a href="./Geologic_time_scale" id="mwAqs" rel="mw:WikiLink" title="Geologic time scale">age of rocks</a>?