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.
Homology is the term that used to describe the similarities in between biological structures or the organs in 2 or more distinct organism that comes from a common ancestors.
Thus, longer-spined sea urchins existed before the shorter-spined sea urchins.
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