But in this world there would still be large herbivores and carnivores- in fact larger by several orders of magnitude than modern ones. Rodent-like mammals would be present and still abundant. Seed eating birds would be around- but would be different in many ways from modern seed eating birds. Social insects would be starting to flex their muscles in ecosystems around the world.
But the pivotal role grass plays in modern ecosystems would be played by other characters in these ecosystems.
And the world you are imagining is the Mesozoic.