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About Apatosaurus (Brontosaurus):
If the name Apatosaurus sounds unfamiliar, that's because this sauropod was known, until recently, as Brontosaurus. Why the change? When the paleontologist Othniel C. Marsh first discovered the bones of this giant herbivore in 1877, he named it Apatosaurus--and when he unearthed larger, but similar, bones a few years later, he called the supposedly new dinosaur Brontosaurus. It turned out that what Marsh thought were Brontosaurus bones were actually grown-up Apatosaurus bones, and scientists were compelled to revert to the original name. (See 10 Facts About Apatosaurus and a gallery of Apatosaurus pictures.)
With its massive body, long neck, and equally long tail, Apatosaurus was the prototypical sauropod. Because its back legs were slightly longer than its front legs, some paleontologists think Apatosaurus reared up to nibble on the tops of trees--though this would have placed enormous demands on its heart, not to mention the rest of its anatomy.
By the way, the name Apatosaurus ("deceptive lizard") has nothing to do with that whole Brontosaurus affair; this dinosaur was so dubbed because paleontologists once confused it with yet another beast, Mosasaurus, the signature genus of the breed of vicious aquatic reptiles known as mosasaurs. Interested in knowing more? Here's an article that goes into detail about the fossil history of Apatosaurus.