A scientist from North Carolina State University in the US grew plants like those found in dinosaur times in conditions like those millions of years ago.
She found they grew three times quicker than those around now, which is why there would have been so many plant-eating or herbivore dinosaurs.
Back in the Cretaceous period herbivores that weighed as much as a tonne each lived in giant herds.
And other herbivores and meat-eaters - carnivores - wandered around weighing around 10 tonnes each.
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