Hypsilophodon (Greek for "Hypsilophus-toothed"); pronounced HIP-sih-LOAF-oh-don
Forests of western Europe
Middle Cretaceous (125-120 million years ago)
Size and Weight:
About five feet long and 50 pounds
Small size; bipedal posture; numerous teeth lining cheeks
The initial fossil specimens of Hypsilophodon were discovered in England in 1849, but it wasn't until 20 years later that they were recognized as belonging to an entirely new genus of dinosaur, and not to a juvenile Iguanodon (as paleontologists first believed). That wasn't the only misconception about Hypsilophodon: nineteenth-century scientists once speculated that this dinosaur lived high up in the branches of trees (since they couldn't imagine such a puny beast holding its own against contemporary giants like Megalosaurus) and/or walked on all fours, and some naturalists even thought it had armor plating on its skin!
Here's what we do know about Hypsilophodon: this roughly human-sized dinosaur appears to have been built for speed, with long legs and a long, straight, stiff tail, which it held parallel to the ground for balance. Since we know from the shape and arrangement of its teeth that Hypsilophodon was a herbivore (technically a type of small, slender dinosaur known as an ornithopod), we can surmise that it evolved its sprinting ability as a way of escaping the large theropods (i.e., meat-eating dinosaurs) of its middle Cretaceous habitat, such as (possibly) Baryonyx and Eotyrannus. We also know that Hypsilophodon was closely related to Valdosaurus, another small ornithopod discovered on England's Isle of Wight.
Because it was discovered so early in the history of paleontology, Hypsilophodon is a case study in confusion. (Even this dinosaur's name is widely misunderstood: it technically means "Hypsilophus-toothed," after a genus of modern lizard, in the same way that Iguanodon means "Iguana-toothed," back when naturalists thought it actually resembled an iguana.) The fact is that it took decades for early paleontologists to reconstruct the ornithopod family tree, to which Hypsilophodon belongs, and even today ornithopods as a whole are virtually ignored by the general public, which prefers terrrifying meat-eating dinosaurs like Tyrannosaurus Rex or gigantic sauropods like Diplodocus.