Ammonites are an extinct group of marine cephalopods that are more closely related to modern octopus and squid than the nautiloids that they closely resemble. Appearing as early as the Devonian period approximately 410 million years ago, ammonites survived in a variety of sizes and shapes as late as the Upper Cretaceous period, approximately 65 million years ago, making them an unusually successful and long lasting animal. Ammonites could be found in shapes and sizes from half an inch to over six feet across.
The name ammonite already existed in a slightly different form in antiquity: the Roman writer, scientist and occasional naturalist Pliny the Elder called ammonites cornua ammonis, meaning the horn of Amon, noting their resemblence to the curved horns of the Egyptian God Amon, who was often portrayed as a ram or with a ram’s head. The modern form of the word ammonite is attributed to French zoologist Jean Guillaume Bruguière (c.1750-1798) who coined the modern form in 1758. The English form comes directly and unchanged from French. See also post on amines and ammonia for other words derived from Amon.
Image of small ammonite by Colin-47. Image of large ammonite by Markus Schweiss. Image of Amon with horns and sundial by Rhys Davenport, all used with permission under a Creative Commons 3.0 license.
Personal note-it took almost 30 years to find a way to include the homotypical fifth declension noun cornu in a piece of writing. Thank you Mrs. Katherine Pettit, Latin teacher extraodinaire.