Orthoceras Plate. Measures 17″ x 14″ These primitive members of the phylum cephalopoda are similar to the ammonites with a straight shell. Orthoceras 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. These orthoceras from Morocco are approximately 400 million years old.