Short-spined sea urchin fossilswere 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?

It suggests that the longer spined sea urchins existed before the shorter spined sea urchins.

As the animals die, they get covered in layers. The lowest being the oldest. The difference is showing us a direct link between the two fossils. This means that the longer-spined sea urchins existed before the shorter-spined sea urchins. And evolved to have shorter spines rather than long ones.