Grandiose and unnecessarily long reply
Herbivores
In a plains ecosystem the Sauropods would die off from lack of food because they couldn’t live off of such meager forest pickings and the lack of ferns (I don’t believe that the Sauropods would be hunted to extinction though that’s personnel opinion). Though if they evolved into smaller more compact Sauropods like Camarasaurus.lentus and adapted to plains life instead of forest grazing the newer Sauropods could thrive in this hypothetical place.
The Triceratops/Ankylosaurus would more than likely die off due to over hunting, improper habitat, and competition with the other herbivores and their inability to adapt quickly enough. While this is mostly personal opinion on my part I just don’t believe that the Ceratopsians/Ankylosaurus could adapt quickly enough to such an environment before they were out competed for food and over hunted into extinction.
The Hadrosaurs would adapt to the new environment with little to no trouble given our understanding of their original ecosystems and how adaptable they were I have no qualms with saying that the Hadrosaurs would be the most likely to survive out of all the species.
The Pachycephalosaurus would do extremely well in this environment given how it isn’t that far of an off shoot from their original habitat. They are too small to be out competed for food by the larger herbivores (though their ability to adapt is not a matter I know like most the rest of these animals). And I believe they wouldn’t be hunted to extinction though it is a possibility.
Well there isn’t really much to say about the stegosaurs, I’d say they fit in with what I said about the Hadrosaurs and the Pachycephalosaurus.
Carnivores
While the tyrannosaurs would survive for quite a while I'm torn on if they would actually survive or not once the Ankylosaurus and Ceratopsians died off, and while they could survive off the Hadrosaurs the population would steadily plummet after the heavily built herbivores died off because of the Tyrannosaurs specialty. I also don’t think that this environment would be conducive to the tyrannosaurs because it would leave them to open to hunt the Hadrosaurs. I also believe that in this circumstance the other predators have the ecological niches for such a habitat. So for now I’m going to say that the tyrannosaurs would thrive while the more heavily built herbivores survived, but once they died off the tyrannosaurs specialty would become their downfall, and eventually their extinction.
Now the Allosaurus on the other hand wouldn’t have the same problem as the tyrannosaurs because they were specifically niched for such an environment. The Allosaurus might even thrive more with the introduction of Hadrosaurs into their available pool of prey since their more than fast enough to keep up with the Hadrosaurs and would eventually create hunting strategies to take them down. Even though the tyrannosaurs would rule as long as the Ceratopsians and Ankylosaurus survived, when they went extinct so would the tyrannosaurs. So in this instance I’d say the Allosaurus would evolve and thrive in such a habitat with such large prey, and they would more than likely out survive the tyrannosaurs since they wouldn’t necessarily be in competition for the same food sources.
And last but not least the Velociraptor, the raptors would do incredibly well in such a habitat that so closely resembles the one they were adapted to, The available prey for the raptors would be astounding in such an environment and I’m not sure if the term thrive would do their survival capability’s in such an environment justice. Honestly I don’t really know what to say about the raptors except that if the Allosaurus were wiped out by the tyrannosaurs then after the tyrannosaurs died out the raptors would soon evolve into the super predators of these particular planes, we could even see species of raptor that rival the tyrannosaurs within a few million years.
