Jurassic
Ginkgoales, of which the only living species is Ginkgo biloba, were more diverse during the Jurassic. These plants were not easy to digest, compared to modern flowering plants (Angiosperms). They must have spent longer in the gut than the food of modern herbivores. That would make increased size an advantage for ankylosaurs and sauropods. Sauropods did indeed become much larger in the Jurassic than any animal land life had before.