Plural of ammonoid; multiple extinct marine animals with coiled shells that are closely related to modern cephalopods.
Simply the plural form of ammonoid, formed by adding -s. The singular form came from the curved horn of Ammon, and the plural follows standard English pluralization rules.
Ammonoids dominated ocean ecosystems for nearly 200 million years until they went extinct 66 million years ago at the same time as the dinosaurs—they're evidence that mass extinction events reshape entire ocean communities, not just land animals!
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