Ammonites are an extinct marine shelled mollusk animal belonging to the class of cephalopods related to today’s squids, octopus, cuttlefish and the nautilus. Ammonites moved in spurts using a method of jet propulsion by siphoning the ocean water into the inner chambers inside their shells and then pushing the water out powerfully through a tube structure called a siphuncle. These inner chambers held water and special gases which helped it descend deep down ocean depths or float upward to shallower depths by filling and releasing the gases and water in and out of the chambers.

The Douvilleiceras, mammilatum ammonite fossil shown below possessed well-defined growth patterns on its shell (sutures). Douvilleiceras’ knobs and spines are thought to be an indicator of a hostile environment. It lived during the Cretaceous Period (145 to 66 mya) and was unearthed in Madagascar (Albin Formation).

Ammonites lived during the Paleozoic and Mesozoic Eras, specifically from the Devonian period (around 419 million years ago) until the end of the Cretaceous period (about 66 million years ago). They were descendants of their cousin straight-shelled nautiloids, but the ammonite’s coiled shells allowed them more agility and maneuver ability within their oeanic environments than their predecessors.

The above fossil is a broken section off an ammonite’s shell belonging to the genus, Mortoniceras sp. It was found in Arkansas in a dried up riverbed within the limestone, Goodland Formation. The shell is characterized by deep keels and ribbing. It lived mainly during the Cretaceous Period (145 to 66 mya). As with all the ammonites, its fate was doomed side by side with the dinosaurs.

The siphuncle tube was also used to regulate buoyancy by controlling the amount of gas and water in the animal’s inner chambers. It lay along the rim of the ammonite underneath its shell.

Multi-purpose Tentacles

The ammonites were ocean predators grabbing their victims with precision and crushing them with their long, powerful tentacles. These tentacles contributed to another important function. They contained special sensors which facilitated their ability to navigate and locate prey in the vastness of the ocean.

Ammonites possessed large heads and are assumed to have been highly intelligent like their modern day cousins octopuses, squids, cuttlefish, nautilus etc. Scientist debate whether ammonites contained ink sacs for defense.

See two gigantic ammonite fossils from another article I have written (scroll to the bottom of article).

CLASSIFICATION

Scientific Name: Mortoniceras

Common Name: Ammonite

Kingdom: Animalia

Phylum: Mollusk (large diverse group of invertebrates possessing a shell, i.e. clams, snails, oysters, etc.)

Order: Ammonitida (characterized by thick, ribbed patterned shells)

Class: Cephalopod (means prominent head and tentacles, i.e. octopuses, cuttlefish, squids, nautilus)

Family: Brancoceratidae

Genus: Mortoniceras (characterized by deep keels, tubercules (knobs) and ribbing)

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