This is the third, and best, of the author's Time Quilt series. In the first book, which takes eighteen years before the events in this one, the prehistoric past and present were mixed up by time distortions. Huge areas of land in the modern era were replaced by that of the Cretaceous, land inhabited by dinosaurs and prehistoric life-forms, Likewise, entire human cities were transported back into the primitive past. Then, the time disruptions stopped--until now. Doorways are opening up, allowing more dinosaurs to come through. And some people travel back into the past and encounter the humans who've managed to survive in the Cretaceous. They also discover that there's another intelligent species, the "Inhumans", who are humanoids clearly evolved from dinosaurs. However, this time period in the Cretaceous is when the big asteroid strikes, that killed 90% of the life on the planet, 65 million years ago. It's a race against time to get back to the present. But another time gate has opened that leads to a completely different place--to a prehistoric Mars that supports life.
Enough of the back story and events are explained so I don't think a new reader necessarily needs to read the other two installments. However, if you're a fan of time travel and dinosaurs you'll eat this novel up. And it ends in such a way that you'll be looking forward to the inevitable sequel, which I'm sure will involve those dinosaur descendants on Mars. Highly recommended.