[ am-uh-noid ]
/ ˈæm əˌnɔɪd /


any cephalopod mollusk of the extinct order Ammonoidea, from the Devonian to the Cretaceous periods, having a coiled, chambered shell.

Origin of ammonoid

1880–85; < New Latin Ammonoidea, equivalent to Ammon(ites) name of the order + -oidea -oidea; see ammonite2 Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019

Science definitions for ammonoid

[ ămə-noid′ ]

Any of various extinct cephalopods of the subclass Ammonoidea living from the Devonian to the Cretaceous Periods. Ammonoids had a symmetrical, coiled, chambered shell with angular sutures between the chambers. They are closely related to the nautiloids, including the modern-day chambered nautilus.
The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.