I am trying to recall the title a fiction novel that involves a race of sentient dinosaurs that travel from the past into the future to when humanity is living underground, possibly due to apocalyptic events.
I recall it being mentioned that the reason the dinosaurs disappeared from the fossil record is that that is when they time traveled. Once in the same time as humanity, several of the underground cities are attacked with a metal corroding gas.
Two of the main characters are survivors of one of these attacks and the novel is mostly their life afterwards where they find dinosaurs on the surface as well as other humans who never lived underground. Additionally on the surface are cities built by the dinosaurs where they raise other dinosaurs such as triceratops and hadrosours much as humans do cattle.
Some other details: The dinosaur cities were described with walls made from a plastic. One of other humans stated his age in moons (i.e., 268 moons for about 22 years).
I had read this book at least 10 years ago. Any help is appreciated.