 The majority of ammonoid specimens, especially those of the Paleozoic era, are preserved only as internal molds; the outer shell (composed of aragonite) has been lost during the fossilization process.Only in these internal-mold specimens can the suture lines be observed; in life, the sutures would have been hidden by the outer shell.The ammonoids as a group continued through several major extinction events, although often only a few species survived.