▁The ▁foss il ised ▁teeth ▁or ▁elements ▁of ▁con od ont s ▁are ▁an ▁equally ▁useful ▁tool . ▁ ▁The ▁e ject a ▁from ▁vol cano es ▁and ▁bol ide ▁impact s ▁create ▁useful ▁markers , ▁as ▁different ▁vol can ic ▁er u ptions ▁and ▁impact s ▁produce ▁b eds ▁with ▁distinct ive ▁compos itions . ▁Mark er ▁hor iz ons ▁of ▁te ph ra ▁are ▁used ▁as ▁a ▁d ating ▁tool ▁in ▁arch ae ology , ▁since ▁the ▁dates ▁of ▁er u ptions ▁are ▁generally ▁well - est ab lished . ▁ ▁One ▁particular ▁bol ide ▁impact ▁ 6 6 ▁million ▁years ▁ago , ▁Ch ic x ul ub , ▁controvers ial ly ▁held ▁to ▁have ▁led ▁to ▁a ▁major ▁ext inction ▁event , ▁produced ▁an ▁ir id ium ▁anom aly ▁that ▁occurs ▁in ▁a ▁thin , ▁global ▁layer ▁of ▁cl ay ▁mark ing ▁the ▁C ret ace ous – P ale ogene ▁boundary . ▁I rid ium ▁layers ▁are ▁associated ▁with ▁bol ide ▁impact s ▁and ▁are ▁not ▁unique , ▁but ▁when ▁occurr ing ▁in ▁conj unction ▁with ▁the ▁ext inction ▁of ▁special ised ▁tropical ▁pl ank t ic ▁for amin if era ▁and ▁the ▁appearance ▁of ▁the ▁first ▁Dan ian ▁species , ▁signal ▁a ▁reliable ▁marker ▁horizon ▁for ▁the ▁C ret ace ous – P ale ogene ▁boundary . ▁ ▁F oss il ▁fa unal ▁and ▁fl oral ▁ass embl ages , ▁both ▁marine ▁and ▁terrest rial , ▁make ▁for ▁distinct ive ▁marker ▁hor iz ons . ▁Some ▁marker ▁units