Definition of ammonite noun from the Oxford Advanced Learner's Dictionary

 

ammonite

 noun
noun
BrE BrE//ˈæmənaɪt//
 
; NAmE NAmE//ˈæmənaɪt//
 
 
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a fossil of a simple sea creature which no longer exists, and which was related to snails Word Origin mid 18th cent.: from modern Latin ammonites, from medieval Latin cornu Ammonis ‘horn of Ammon’, from the fossil's resemblance to the ram's horn associated with Jupiter Ammon, whose temple is found at Siwa in Egypt.
See the Oxford Advanced American Dictionary entry: ammonite