Morphological and allozyme analyses have agreed that the common thresher is basal to the clade formed by the bigeye thresher and the pelagic thresher (A. pelagicus).[2][5] The closest relative of this species within the family may be a fourth, unrecognized thresher shark species off Baja California, reported from allozyme evidence by Blaise Eitner in 1995.[8] However, the existence of this fourth species has yet to be confirmed by other sources.[1][2]