a fossil of a simple sea creature that no longer exists, and that was related to snails 菊石(已灭绝的头足动物,与螺近缘) Word Originmid 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.