What’s bigger than Elasmosaurus, has more teeth than Smilodon, and is fiercer than T-rex? It must be Megalodon, the giant ancestor of the great white shark that roamed the ancient seas 50 million years ago eating whales. O’Brien, author of Gigantic: How Big Were the Dinosaurs? (1999) is back with another toothy monster to delight young dinosaur fans. Beginning simply and dramatically, showing one giant creature after another, O’Brien builds to his subject, which comes crashing out of the water and onto a two-paged spread featuring his bloody mouth. Then continuing in this engaging style, he gives Megalodon plenty of room to show off as he looms and threatens and shows plenty of his enormous teeth. In fact, the author notes that the only evidence of Megalodon scientists have discovered to date is a few vertebrae and some large fossilized teeth. Using the size of the teeth of the great white shark as a comparison, some scientists predict the ancient shark was 50 feet long. O’Brien’s double-paged spreads give ample room to compare this monster to more familiar large beasts: a great white shark or Tyrannosaurus rex. In one telling shot, the jaw of the Megalodon surrounds the standing figure of a man, dressed in a snorkel and pink inner tube. It is this simple approach, laden with enormous kid appeal that will make this sail off the shelves. The author may inspire a whole new generation of treasure hunters as he notes in an afterword that giant fossil teeth have been found all over the world, but “the best place to find them is the eastern United States.” For younger readers than Caroline Arnold’s Giant Shark: Megalodon, Prehistoric Super Predator (2000). (timeline, tooth facts) (Picture book/nonfiction. 5-8)