Ammonite and Orthoceras

Ammonite

Ammonites first appeared 450 million years ago and are related to the squid and octopus. These ocean-dwelling mollusks were cephalopods and are known for their distinct spiral shell. Ammonites were born with their shells and these shells grew as they grew by adding new chambers within it. As the new chambers were added, the ammonite moved their body into the new chamber and sealed off the old one with walls called septa. This process helped to make the shells buoyant which allowed the ammonite to move more easily through the ocean waters.

photo of an ammonite [photo credit: Natural History Museum]

Orthoceras

The Orthoceras is an ancient mollusk that lived during the Ordovician period (370 to 488 million years ago). The name Orthoceras means ‘straight horn’. They ranged in size from 2.5 centimeters to 4 meters and were distinctive by their long, straight, conical shells and long tentacles. The tentacles were used to move the mollusk by jet propulsion. Having lived in the deep ocean ecosystems, they ate smaller Orthocerida species, invertebrates, and fish.

photo of an ammonite [photo credit: Natural History Museum]