Curious Minds Did You Know? Coal is a fossil fuel formed from prehistoric vegetation that has decayed into peat and then been subjected to intense heat and pressure over millions of years.
Bivalves appear in the fossil record first in the early Cambrian Period more than 500 million years ago.
Bivalves as a group have no head and they lack some usual molluscan organs. The shell of a bivalve is composed of calcium carbonate, and consists of two, usually similar, parts called valves.
These are joined along one edge (the hinge line) by a flexible ligament that, usually in conjunction with interlocking "teeth" on each of the valves, forms the hinge.
This arrangement allows the shell to be opened and closed without the two halves detaching.
