Sea urchin fossils in one layer of rock have longer spines than the sea urchin fossils in the rock layer directly above them. What do the differences between the two sets of sea urchin fossils suggest about them?

A.) The longer-spined sea urchins may be related to the shorter-spined sea urchins.

B.)The longer spines developed as the shorter-spined sea urchins grew older.

C.)The shorter-spined sea urchins stretched their spines to ward off predators.

D.)The shorter spines did not protect the sea urchins well enough and they died off.

Question

Sea urchin fossils in one layer of rock have longer spines than the sea urchin fossils in the rock layer directly above them. What do the differences between the two sets of sea urchin fossils suggest about them?

A.) The longer-spined sea urchins may be related to the shorter-spined sea urchins.

B.)The longer spines developed as the shorter-spined sea urchins grew older.

C.)The shorter-spined sea urchins stretched their spines to ward off predators.

D.)The shorter spines did not protect the sea urchins well enough and they died off.

anonymous · Answer

i'm thinking that it's A but i'm not sure, any help?

blues · Answer

First of all, thank you for posting your hunch as well as the question!

I don't think the question gives you quite enough info to differentiate between the possible responses. As you suggest, A is certainly a possibility: the shorter spined sea urchins are probably related to the longer spined sea urchins, but I am going to go with D.

Because there are no long spined sea urchins in the older layer and no short spined sea urchins in the newer layer, I think that long spined sea urchins evolved from short spined sea urchins - that is, natural selection acted on short spined sea urchins in such a way that only their longer spined offspring survived long enough to pass on their genetic material.