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What were the biggest and smallest dinosaurs?

There's little doubt that the largest animals ever to live on land were sauropods, which includes Apatosaurus (formerly "Brontosaurus"), Diplodocus, and Brachiosaurus. But deciding which was the largest depends on what one means—longest or heaviest? Based on fairly complete skeletons, the sauropods Argentinosaurus and Seismosaurus, which were up to 50 meters long, are the longest dinosaurs yet discovered.
Among dinosaurs that aren't birds, or non-avian dinosaurs, the Compsognathus longus, an Early Jurassic, chicken-sized meat-eater from Western Europe is the smallest yet discovered. It measures only 70 centimeters long, including the tail, and probably weighed about three kilograms.
Created by American Museum of Natural History.

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Video transcript

The biggest dinosaurs were, of course, these long neck creatures known technically as the sauropods, the famous apatosaurus, used to use the term brontosaurus but that's no longer an official name in the scientific lineature, although it's a very famous name, thunder lizard it means. As far as the traditional extinct dinosaurs, the smallest animals are close bird relatives, probably in the group troodontidae which were pigeon sized that we found remains of in northeastern China. One thing about small animals is that they preserve much more poorly in the fossil record than large animals do so that our record is highly skewed toward larger sorts of animals as fossils and the small ones just quickly become decomposed or abraded or eaten and things like that, and don't make very good fossils so we know nothing about them.