菊石是一羣絕種海洋生物的總稱，是蛸亞綱動物如八爪魚、魷魚和墨魚的近親。牠們始見於約四億九百萬年前的泥盆紀，於白堊紀之末和恐龍同告絕種。水中礦物填滿菊石殘殼，繼而硬化，倒模成菊石化石。
Ammonoids are an extinct group of marine mollusc animals, closely related to living coleoids (octopuses, squid, and cuttlefish). They first appeared in the Devonian (circa 409 million years ago) and became extinct at the close of the Cretaceous along with the dinosaurs. These fossils were formed when water carried minerals into the mold left by the animal’s remains, which then hardened to make a natural stone cast of the Ammonoid.
