Editing Ammonoidea (section)

=== Sexual dimorphism ===
[[Image:DiscoscaphitesirisCretaceous.jpg|thumb|''[[Discoscaphites|Discoscaphites iris]]'', Owl Creek Formation (Upper Cretaceous), Ripley, Mississippi, US]]

One feature found in shells of the modern ''[[Nautilus]]'' is the variation in the shape and size of the shell according to the [[sex]] of the animal, the shell of the male being slightly smaller and wider than that of the female. This [[sexual dimorphism]] is thought to be an explanation for the variation in size of certain ammonite shells of the same species, the larger shell (the ''macroconch'') being female, and the smaller shell (the ''microconch'') being male. This is thought to be because the female required a larger body size for egg production. A good example of this sexual variation is found in ''[[Bifericeras]]'' from the early part of the [[Jurassic]] period of [[Europe]].

Only recently has sexual variation in the shells of ammonites been recognized. The macroconch and microconch of one species were often previously mistaken for two closely related but different species occurring in the same rocks. However, because the dimorphic sizes are so consistently found together, they are more likely an example of sexual dimorphism within the same species.

Whorl width in the body chamber of many groups of ammonites, as expressed by the width:diameter ratio, is another sign of dimorphism. This character has been used to separate "male" (Largiventer conch "L") from "female" (Leviventer conch "l").<ref>{{cite book |doi=10.1007/978-1-4615-4837-9_23 |chapter=Whorl Width in the Body Chamber of Ammonites as a Sign of Dimorphism |title=Advancing Research on Living and Fossil Cephalopods |year=1999 |last1=Sarti |first1=Carlo |pages=315–332 |isbn=978-1-4613-7193-9 }}</ref>