I read the book in the late 80's.
I believe that the huge "ship" was a pre-programmed vessel that was heading back to Earth with a collection of intelligent lizards/dinosaurs as the cargo. If I remember correctly, the ship created its own centrifugal gravity and was described as a test-tube of life from a former time on earth, or as an alternate developmental path to human development on earth. Sort of a "what-if" the reptilian life had been allowed to evolve instead of being wiped out.