Answer:A evolution: the squid lost its other bones
Explanation:The pen, a stiff, rod-like structure found in the body of some species of squid, is believed to have evolved from the remnants of a once more extensive skeleton that the animal no longer needed. Over time, as the squid evolved, it lost many of its bones, including those in the tail, leaving only the pen as a remnant of its evolutionary history. This is known as a process of reduction, in which an organism reduces its overall skeletal structure as it adapts to its environment and changes in behavior or anatomy.