#goniatida

All ammonoid animals, like Gonioloboceras, had a soft body that rested in the outermost, open end of a hard shell.

The rest of the shell of each animal had many inner walls that divided the structure into chambers. The borders between each lightning bolt in the photo are the remains of those inner walls.

As the ammonoid animal grew larger, it made more shell to accommodate its body. It also made more chambers that it could fill or empty in order to offset its increasing body weight.

When those chambers were full of water, the animal sank deeper into the ocean. When the animal replaced the water with air through a tube running along the outer border of the shell, the animal floated higher.

In that way, an ammonoid could swim around the ocean without flippers or fins, like its closest living relative, the nautilus, does today.

Specimen Number: BEG 5886

Sources:

Hewitt, R. A., A. Checa, G. E. G. Westermann, and P. M. Zaborski. “Chamber growth in ammonites inferred from color markings and naturally etched surfaces of Cretaceous vascoceratids from Nigeria.” Lethaia 24(1991):271-287.

Mutvei, H., and R. A. Reyment. “Buoyancy control and siphuncle function in ammonoids.” Palaeontology 16(1973):623-636.