Many modern plants have evolved defenses against herbivores, which include toxins that can swiftly impair any animal that hasn’t adapted to them.
Consequently, a time-traveling Triceratops would be taking a big risk with every visit to its local salad bar. Paleobotanists could try to solve this problem by cataloging fossil plants that lived at the same time as plant-eating dinosaurs, before picking out descendants of those plants that are still around today. Still, plant lists will never be good enough to say whether a Triceratops, Stegosaurus or Brachiosaurus ate those plants or if they could eat their descendants.
The same might hold true for carnivorous dinosaurs, which—for all we know—may have been picky eaters. For instance, although some Triceratops bones hold tooth traces of Tyrannosaurus, there’s no way to be sure a genetically engineered Tyrannosaurus would eat an equally inauthentic Triceratops (even if it were organic and free-range).
So despite a century of dinosaur flicks portraying Tyrannosaurs and other predatory dinosaurs gratuitously munching humans, one bite of our species—or other sizable mammals—might make them sick. In other words, there’s no accounting for taste.