Later in the Mesozoic period...
As we get later in the Mesozoic, into the Cretaceous, the next influence of the dance of the continents begins to be felt. The continents of Pangea begin to separate in the Jurassic, and as they do, wet and moist environments become more prevalent
. A wider range of pteridophytes can grow in addition to seed plants. Additionally, in the mid Cretaceous (about 120 million years ago) we see the appearance of the new seed plant on the block, the angiosperm.
Here is a reconstruction from the Smithsonian. In contrast to gymnosperms which can only reproduce sexually, angiosperms have excellent abilities for both sexual reproduction and often vegetative reproduction. They can reproduce asexually by root sprouting, by breaking off pieces and rerooting. They are very damage tolerant in this way. By contrast, every living gymnosperm that we know of, with one exception and it is not a very important exception, cannot reproduce vegetatively in an effective manner, they can only reproduce sexually. So getting angiosperms into the play means that there is an organism that could both spread very rapidly and tolerate herbivore damage. Another feature of angiosperms is that in many cases they are not rich in digestion-inhibiting chemicals. That is why you consume angiosperms when you go out to have your salad or eat your vegetables. You have to pick and choose carefully if you insist on eating really toxic angiosperms. From an herbivore's perspective, the angiosperm is a much more,in a sense, fern-like kind of product. What do we see in response to the appearance and spread of angiosperms? A transition in ecosystems from the very large, high-feeding, herbivores of the Triassic, Jurassic and Early Cretaceous, to the radiation of a whole new group of dinosaurian herbivores who are both smaller and who feed lower to the ground. This is the great ornithischian radiation, duckbills, ceratopsians and their friends. It is this later Cretaceous radiation that accounts for over 50 percent of the total specific diversity of dinosaurs.