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?

The saber teeth represents a convergent evolution. This type of evolution is present when two organisms do not share a common ancestor having a specific trait however having a history that has selected for the same trait to evolve independently.