Amazing Ammonites

Ammonite Fossil (Douvilleiceras, mammilatum)

Douvilleiceras mammilatum was a marine cephalopod ammonite, which are ancestors of today’s chambered nautilus. It 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).

Cretaceous Period Oceanic Environment (145-66 million years ago) Artist Rendition
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Mortoniceras sp Ammonite Fossil

The above fossil is a broken off section from another 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.

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Mortoniceras Ammonite Fossil (Top View)
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.

A Complete Sample of Mortoniceras sp from Texas, Fort Worth Formation, Tarrant County
Function of Inner Chambers

Ammonites moved in spurts using a kind of jet propulsion by siphoning the ocean water into 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.

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

Inner Chambers of Ammonite Fossil

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


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)

Mortoniceras Ammonite Rendering Drawing

Manuiceras sp (Ammonite)

Ammonite Dufrenoy
Ammonite Fossil

Manuicera sp. lived in the ancient seas when dinosaurs were around. In general, the ammonite’s plethora peaked during the Cretaceous Period (145 to 66 million-years-ago). But according to the fossil records, their incredible long history began as early as 440 mya during the Silurian Period.

Manuicera sp ammonite was unearthed from a dried up riverbed in Arkansas, they have also been unearthed in Texas. Both of these U.S. states lie within the limestone, Goodland Formation where many other Cretaceous fossils have been discovered. Originally, I mistakenly identified the fossil as, Dufrenoy justinae, but that was when I was newer to the field with a less discerning eye and research skills. So there you go. 

A variety of ammonite forms, from Ernst Haeckel‘s 1904 Kunstformen der Natur (Art Forms of Nature)

The amazing illustration above shows how ammonites vary greatly in the ornamentation (surface relief) of their shells. Some may be smooth and relatively featureless, except for growth lines. In others, various patterns of spiral ridges and ribs or even spines are shown.

Ammonite fossils have a world wide distribution indicating the theory of continental drift and due to their abundance (estimated 10 thousand species) scientists use them as date markers for other fossils along the same rock layers.

Some varieties grew to gargantuan sizes, larger than semitruck tires. You can see this in a photo from another article I have written, (scroll to bottom).


Scientific Name: Manuiceras sp. Common Name: Ammonite

Kingdom: Animalia

Phylum: Mollusk (soft body of invertebrate animal encased in shell)

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

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

Family: Acanthoceratidae (possibly)

Genus: Manuiceras

Manuiceras sp Ammonite Rendering Drawing