Ammonite lappets - simmetrical lateral protrusions are located on the adult apertures of many Mesozoic ammonite microconchs (likely shells of male ammonites). Long lappets appeared in ammonoids in Early Jurassic simultaneously with prominent sexual dimorphism. Macroconch shells (likely shells of female ammonites) have no lappets. Many Jurassic and Cretaceous ammonoid taxa had lappets, whereas in several evolutionary lineages of ammonites the lappets gradually dissapeared (especially in Boreal and Subboreal ammonites). The lappets help paleontologists easily distinguish microconchs from macroconch shells. The lack of lappets in many Boreal taxa create a problem: it is very hard to distinguish micro- and macroconch shells in such a situation, because the size of small macroconchs and large microconchs can be nearly identical.
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