The extinction came in two steps, at the start and the finish of the "Hirnantian" stage, which was the last stage of the Ordovician.
ð ɪ WORD_BOUNDARY ɛ k s t ɪ ŋ k ʃ ə n WORD_BOUNDARY k eɪ m WORD_BOUNDARY ɪ n WORD_BOUNDARY t uː WORD_BOUNDARY s t ɛ p s WORD_BOUNDARY æ t WORD_BOUNDARY ð ə WORD_BOUNDARY s t ɑ ɹ t WORD_BOUNDARY æ n d WORD_BOUNDARY ð ə WORD_BOUNDARY f ɪ n ɪ ʃ WORD_BOUNDARY ʌ v ð ə WORD_BOUNDARY h ɜː n æ n ʃ ə n WORD_BOUNDARY s t eɪ d̠ʒ WORD_BOUNDARY w ɪ t̠ʃ WORD_BOUNDARY w ʌ z ð ə WORD_BOUNDARY l æ s t WORD_BOUNDARY s t eɪ d̠ʒ WORD_BOUNDARY ʌ v ð ɪ WORD_BOUNDARY ɔ ɹ d ə v ɪ ʃ ə n WORD_BOUNDARY