What are the speakers mainly discussing?

W: Dr. Manning, do you have a few minutes? I’d like to ask you about this fossil I found.
M: Sure. What have you got there?
W: Well, it’s some kind of shellfish. I thought it might be a brachiopod, but I am not sure. I found it when I went out hiking last weekend.
M: I think you might be right. It certainly appears to be a brachiopod. But it’s easy to confuse them with bivalve mollusks. Superficially they resemble one another. Both have two shells for example.
W: How can you tell them apart then?
M: Well, there are some minor differences in the shell. A lot of brachiopods have ridge shells like this one. There are other distinctions of course. Internally they are quite different. Mollusks and brachiopods belong to different animal groups, and they had been in competition with one another millions of years ago.
W: How old do you think this one is?
M: It’s hard to tell. The earliest brachiopods were from the Paleozoic era, 300 to 500 million years ago. They were quite successful then in terms of the number of species. It’s not out of the question that your fossil could be that old.
W: Wow, that’s amazing. It makes paleontology seem more interesting when you can hold it in your hand.
M: You know, that gives me an idea. Would you mind bringing this to our next class? The others might find it interesting too.
W: Well, I could. But if you don’t already have one in your collection, I thought you might as well keep it.
M: That’s very kind of you.