Orthoceras and Ammonite Fossil Plate. Measures 18″ x 13″ Orthoceras and Ammonites are primitive members of the phylum cephalopoda and are the ancestral cousin of the modern squid or octopus. The word orthoceras means “straight horn” and refers to its shape. All orthoceras have chambers separated by walls called septa. They also have a tube that runs down the center of the fossil called a siphuncle. It is believed that this tube was used to carry gas into the chambers to control the buoyancy of the shell. The word ammonite is derived from the name of the Geek God “Ammon” that appeared with rams horns on his head. The Ammonite is a distant cousin of the chambered Nautilus. These fossils from Morocco are approximately 400 million years old.