Final answer:
The simple form of the missing verb is 'roamed.' Dinosaurs existed on Earth for approximately 185 million years before facing extinction.
Explanation:
The simple form of the missing verb in the sentence is roamed.
In the sentence 'Once upon a time, dinosaurs roamed the earth,' the simple past tense form 'roamed' is used to indicate an action that happened in the past.
Dinosaurs existed on Earth for approximately 185 million years before facing extinction.
Learn more about Simple verb forms in English sentences here:
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