Ammonoids repeatedly filled the same triangle in D-W plane after mass extinctions. (A) All of the ammonoid data used in the present study. Red points are genera before the FF (first) mass extinction, genera after FF are denoted by blue points. The green curve is W = 1/D. (B) Ammonoids before the FF extinction, together with a schematic arrow for the direction of evolution from ancestral taxa. (C) Genera between FF and DM mass extinctions fill out a triangle (obtained by applying the SISAL algorithm  on the dataset), surviving genera from the FF mass extinction are denoted by red bold points. (D) Ammonoids between DM and PT mass extinctions fill a triangle, surviving genera from the DM mass extinction are denoted by red bold points. (E) Ammonoids after the PT mass extinction fill a triangle, surviving genera from the PT mass extinction are denoted by red bold points. (F) Ammonoids from different periods, together, genera between FF and DM are denoted blue, DM to PT in red and post PT in green. The shell morphologies of the three archetypes at the vertices of the triangle are shown.