You will have noticed that this fossil Lamellibranch shell is shut. It doesn't take much investigation to find out that most Lamellibranchs open quite quickly after they die. The reason is simple - in life, their two halves are held shut by a muscle attached to both halves inside the shell.
While the shellfish is alive, the muscles pull on each half of the shell to keep the shell shut. On the outside of the shell, there are elastic ligaments, which act as a doorspring. When the muscle inside relaxes, the shell will automatically open. Some shells can open and shut rapidly, to push water out quickly, and therefore have a jet propulsion unit. One scallop shell jumps around the ocean floor this way. Some shells open just a fraction to allow water in which they filter and extract nutrients from.
Due to this intriguing construction, when the Lamellibranch shell dies, the muscle inside automatically relaxes. The tension from the outside ligament, which wraps around the outer hinge, will then automatically open the shell.
It's not long before the muscle inside decays and the shell arrives at its butterfly appearance. Soon after, the outside ligament will decay, the shell breaks into two halves, and usually ends up smashed apart by further wave action, and is finally ground into sand sized shell particles.
The reason for this lengthy explanation is simple - whenever you find rock layers containing fossil Lamellibranch shells that are shut, you know they were buried alive. If they remained shut, then they were not only buried alive, they were also buried deep enough and the mud compressed so quikly, they could not open even after they had died.
Some Lamellibranchs dig into the sea-floor while they are alive and bury themselves in the mud, usually in a vertical position, to hide from scavengers. An interesting feature of many fossil Lamellibranch beds, is that the closed shells are usually found lying in the horizontal position.
This can only happen after the living shells have been washed into place by a fast current, then buried quickly before they died and opened. So a closed Lamellibranch fossil is excellent evidence that both fossils and rocks do not take a long time to form. If they did, all Lamellibranchs in the fossil record would, at the best, be open shells, and at worst, smashed by wave action and ground to shell grit long before they were buried and fossilised.
Our Aussi fossil shell also has one rare and precious point. After rapid burial "down under", much of it has been replaced by hydrated silica in the form of opal-not a common feature at all, but one that gives this Australian clam much increased value - it's opal and it's rare!