How did Pliny the Elder identify the sources used in Natural History?

A novel feature of the Natural History was the meticulous care Pliny the Elder took in documenting his sources, citing more than 100 authors.1

He identified these sources primarily through Book I, which serves as a comprehensive summary of the remaining 36 books. In this opening book, Pliny listed the authors and, in many instances, the specific titles of the works from which he derived his material. This practice is particularly significant because many of the original texts he referenced have since been lost to history.1