I realized that although certain ecological niches were dominated by Dinosaurs on the continents, the numerous small islands, no doubt many due to the increase volcanism, could open up those ecological niches to mammals.

 

It could have been possible that for the 175MY reign of the Dinosaurs could have hosted many large mammals such as those of the Pleistocene after they went extinct.

What lived on islands during the time of the dinosaurs is an interesting question. I'm not sure the idea of large island mammals is reasonable due in large part to the fact that modern mammals tend to ward dwarfism on islands.

 

Reptiles tend to do well in islands due to their ability to do with out food and water for long periods of time. I'd like to see what the fossil record, if there is any, shows about island animals during the age of dinosaurs.

moontanman is right; fossil records show that the only mammals present during the Mesozoic were rodent- or shrew-sized creatures. What we think of as "large" mammals did not evolve (and did not exist on either continental or arc-island terrains) until after the K/T boundary, due to the "monopoly" the dinosaurs had on the ecological food supply.