 However, because the dimorphic sizes are so consistently found together, they are more likely an example of sexual dimorphism within the same species.The majority of ammonite species feature planispiral, flat-coiled shells, but other species feature nearly straight (as in baculites) shells.Still other species' shells are coiled helically, similar in appearance to some gastropods (e.g., "Turrilites" and "Bostrychoceras").