Sexual dimorphism as represented by the ammonite Yezoites frontierense (Cobban 1952)
Macroconch (female) 38mm on the left, microconch (male) 18mm on the right
Previously referred to Scaphites frontierense, presumably, the female was much larger to carry eggs. Many extant cephalopods (the Blanket Octopus for example) show the same relative (?) dimorphism with the males much smaller than the females.