Why Aren't Modern Animals As Large As Dinosaurs?

Since the prehistoric age, we haven't seen any animals as big as dinosaurs. So just how did dinosaurs get so big?

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The Titanosaur (AMNH)
"In January 2016, the Museum added another must-see exhibit to its world-famous fossil halls: a cast of a 122-foot-long dinosaur. "

How Did Sauropods Get So Big? (Smithsonian Magazine)
"Without a doubt, the sauropod dinosaurs were the largest animals to have ever walked the earth."

Dinosaur Era Had 5 Times Today's CO2 (Live Science)
"Dinosaurs that roamed the Earth 250 million years ago knew a world with five times more carbon dioxide than is present on Earth today, researchers say, and new techniques for estimating the amount of carbon dioxide on prehistoric Earth may help scientists predict how Earth's climate may change in the future."