Why did some dinosaurs grow so big?

Paleontologists don't know for certain, but perhaps a large body size protected them from most predators, helped to regulate internal body temperature, or let them reach new sources of food (some probably browsed treetops, as giraffes do today). No modern animals except whales are even close in size to the largest dinosaurs; therefore, paleontologists think that the dinosaurs' world was much different from the world today and that climate and food supplies must have been favorable for reaching great size.

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Image: Mammoth Fossils

Mammoth Fossils

Excavation of the initial Columbian mammoth remains found at the Ziegler Reservoir site.

Attribution: Land Resources
Image: Mastodon Fossil

Mastodon Fossil

Denver Museum of Nature and Science personnel excavate a large mastodon tusk at the Ziegler Reservoir site.

Attribution: Land Resources