The Dinotopia books are framed as ‘found manuscripts,’ a technique popular in gothic (in the literary sense) novels, usually as the subtitle “Manuscript Found in an Abandoned Castle,” “Manuscript Found in a Bottle,” etc. Lovecraft does this as well.
The frame for these books are the ‘found’ 19th century journals of shipwreck survivor Arthur Denison and his son. Gurney mocked-up the first journal, though whether this was simply for himself or for inspiration or as a model for an illustration I don’t know:
He also made a more polished version of the third journal for public exhibition:
Here is a staged tableau, and JG’s illustration of the same:
Here are some regrettably small images of the book, showing the spine and lock: