Thylacosmilus is a marsupial, and Smilodon is a mammal. Thylacosmilus and Smilodon both have saber teeth, but they do not share an ancestor that had saber teeth. What are saber teeth for these organisms?

Homologous structures are similar because they were inherited from a common ancestor. The question says Thylacosmilus and Smilodon did not share an ancestor with saber teeth, so their saber teeth evolved independently. Similar features that evolved independently for a similar function are analogous structures, making D the correct choice.

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