Late Early Cretaceous. Coober Pedy Formation. Coober Pedy, South Australia. Belemnites are the ancient relatives of modern squids, and looked much like squids do today. The difference between squids and belemnites can be seen in their internal stuctures. Belemnites have a delicate structure within them called a phragmacone, this structure was used to regulate the animal’s buoyancy. Another structure called a guard protected the phragmacone and also counterbalanced it, making swimming easier. This fossil is a belemnite guard, and interestingly it has been opalized.