Iguanodon's thumb spike was originally placed on its nose

In the 1820s-1830s, early Iguanodon reconstructions by Gideon Mantell and others placed the thumb spike (a defensive weapon) on the nose due to incomplete skeletal understanding. This iconic error was perpetuated in Crystal Palace sculptures (1854) showing Iguanodon with a nasal horn. By the late 19th century, as more complete Iguanodon skeletons emerged from Belgium, paleontologists correctly identified the spike's true position on the thumb. This correction demonstrates how paleontological understanding improved with better fossil evidence and represents a seminal error that shaped early dinosaur science.