The dominant land plants were the gymnosperms (conifers). Ferns, large horsetails,'monkey puzzle' trees, ginkgos and cycads were common. These trees were not easy to digest, compared to modern flowering plant trees (Angiosperms). They must have spent longer in the gut than the food of modern herbivores. That would make increased size an advantage for sauropods, which did indeed become much larger in the Jurassic than any any land life had before.