Shelled Moluscs, related to Squid
Ammonites were a huge group of now extinct cephalopod mollusks, relatives of the squids, octopuses, and nautilus. They are in the subclass Ammonoidea of the class Cephalopoda.
They were named by Pliny the Elder (23-79 CE), who called them “Horns of Ammon,” Ammon being an Egyptian god pictured with curled rams’ horns.
The fossils are their curled shells. Like the nautilus, ammonite shells have interior dividers between chambers. The dividers in most species were very complicated in their construction. The curves and shapes in them acted to increase their strength, and the strength of the shell as a whole.
This made them more resistant to predators like mosasaurs, and to water pressure at greater depths. Many shells have been found with the teeth marks of mosasaurs (marine predatory reptiles).
Like the nautilus, the ammonites were able to move gas and liquid around among the chambers to allow them to float in the water column.
Ammonites were predators, and probably also were able to “jet” by holding and pushing water out of their bodies, like their modern relatives.
There is some evidence that ammonites had ink sacs like modern cephalopods, but it is far from settled. Most people who study them say they did not.
Also like the nautilus, the animals would have tentacles and a head that would stick out of the opening in the shell. The rest of the body was in the first few segments of the shell.