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I'm reading a book and came across this sentence

トリケラトプスは白亜紀の終わり頃にいた恐竜、草や木などの葉を食べていた。

My translation

The triceratops a dinosaur, which existed towards the end of the cretaceous period, ate things like grass and tree leaves.

What is the purpose / function of the bolded で here? I don't understand why it's needed

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