This is a book I read years ago - likely in the late 1960s or early 70s. It was a hardback, and I think (long time ago) the cover had a T-Rex strapped to the back of a rocketship in space.
I am sure most of the story went over my head at the time, but I think the plot was of aliens that somehow were stranded on earth in the time of the dinosaurs. T-Rex was vaguely similar to the aliens, so they put them through some forced evolution to make companion/servants. There was plotting by the T-Rex against their creators.
The book ended with the aliens managing (either through time dilation or time travel), to get to a "big crunch" of the universe collapsing into a black hole and a subsequent big bang. By being around during the big bang they were able to shape the new universe more to their liking, and they ended up back on Earth - and I wasn't sure if "our" earth was the new one universe or the old one.
I think that the author was east European or Russian, but now I don't know why I think that.
Despite reading it as a kid, it was definitely not young adult material, which is why I may have missed whole important elements of the book, but it has always stuck with me.