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The article How do we know what dinosaurs looked like? discusses the evolution of how dinosaurs were pictured through the years. From that article:
When ancient people were faced with strange bones, they did exactly what we do today, and used the best knowledge available to reconstruct the creatures that left them behind. Sometimes this resulted in poor conclusions. The first name assigned in print to any dinosaur remains was the ignominious title of Scrotum humanum – a label given by British physician Richard Brookes to the broken end of a femur in 1763, believing it to be the fossilised testicles of a Biblical giant.
We now know that the leg bone belonged to a Megalosaurus – correctly described as an extinct reptile by William Buckland in 1824. You can’t entirely blame Brookes for his conclusions, as dinosaurs would not be described as a group until 1842. That was when Richard Owen, head of what is now the Natural History Museum, revealed to the world a new class of strange, extinct creatures he called dinosaurs, meaning ‘fearfully great reptiles’. He imagined Iguanodon, Megalosaurus and Hylaeosaurus to be reptiles with legs sprawled out to the sides, with scaly grey or green skin: something like modern lizards or crocodiles.
In 1854 artist Benjamin Waterhouse Hawkins created life-sized sculptures of these animals as directed by Owen, and you can still see these on display in Crystal Palace Park in south London. Visit them and you will see they look very different to how we depict dinosaurs today.
Over time, we have come to completely revise our understanding of the appearance of dinosaurs, and much of this began with the description of another American dromaeosaur called Deinonychus in the 1960s. John Ostrom at Yale University made the revolutionary suggestion that this species was a bird-like, fast, warm-blooded pack hunter, and so began the ‘dinosaur renaissance’ of the 1960s and 70s. Ostrom championed the idea that birds were dinosaurs, and was spectacularly vindicated when Sinosauropteryx, the first known feathered dinosaur, was found in China in 1996.