What does the presence of teeth, a vestigial structure in whale sharks, tell you about the whale shark's ancestor?

The whale shark forages for food at or near the surface of the ocean. Its large mouth is well adapted to filter feeding and contains more than 300 rows of small, pointed teeth in each jaw. Ichthyologists consider these teeth to be vestigial structures, and they do not play a role in feeding.

Explanation:

  this riddles' answer is library.

  the "leaves" are a reference to the books' pages that the library contains; the branches are a network of libraries that connect multiple libraries' collections, resources, and items in a system of a common database, integrating them; the branches' organization are a useful management tool for coordinating the multiple collections many libraries have.