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Cretaceous-era dinosaurs called Iguanodons could weigh several tons and grow to be over 30 feet long. In this essential volume presented by Scientific American, readers find out why scientists think this animal switched between walking on four feet to two feet and what its thumb spike was used for. Fact boxes provide extra excellent tidbits, and full-color images help readers visualize the information in the text. The subject matter helps readers build knowledge, a key part of the science of reading.