Short-spined sea urchin fossils were found directly above long-spined sea urchin fossils in a layer of rock. What do the differences between the two sets of sea urchin fossils suggest about them?

The longer spines developed as the shorter-spined sea urchins grew older.
The longer-spined sea urchins existed before the shorter-spined sea urchins.
The shorter-spined sea urchins stretched their spines to ward off predators.
The shorter spines did not protect the sea urchins well enough and they died off.