According to a scientific paper published in the Journal of Zoology and summarized in The Daily Mail, math mistakes have caused scientists to overstate how much many of the famous large dinosaurs actually weighed. The apatosaurus, which was called the brontosaurus when I was learning about this stuff several decades ago, has had his estimated weight shrink from about 38 metric tons to a svelte, some might say anorexic, 18 metric tons.
This guy pictured here is not an apatosaurus (I still hate typing that after all these years), but a diplodocus. When I was a kid, the diplodocus was thought to be the longest dinosaur and the brachiosaurus the heaviest, though now there are fossils of a creature bigger than either called the supersaurus.
The diplodocus was about 90 feet long, with half of his length in his tail. No human being was alive when the dinosaurs roamed the earth (or, if a creationist should stumble on this blog, Jesus never talked about the dinosaurs he rode in any of his parables), but it is widely assumed this plant eating dinosaur used his very long tail as a defensive weapon against predators. The estimated weight of the diplodocus has been downgraded from 5.5 metric tons to 4 metric tons. To give that some perspective using living land mammals, the African elephant is about 25 feet long and 4 metric tons is on the low end of the weights of adult males.
Speaking of stuff that used to be true but isn't now, did you know that elephants aren't pachyderms anymore? The word "pachyderm" means thick skinned, and was used to classify the elephants, rhinos and hippos in the same basic category, but better evidence now shows they aren't closely related at all.
Darn those scientists and their pesky obsession with better understanding the world!