A Mysterious Thumb
The peculiar false thumb of Iguanodon was originally thought to set into the dinosaur’s nose. When Gideon Mantell first described the animal in 1825, the various bits and pieces of the dinosaur were thought to represent the remains of an enormous, iguana-like reptile. As a result, it seemed reasonable that a conical, bony spike corresponded to the same structure on the snouts of rhinoceros iguanas. This placement made sense within the prevailing view that creatures like Iguanodon were lizards writ large, but the idea was tossed when a series of more complete Iguanodon were found in a Belgian coal mine in 1878. The “horn” actually belonged on a mitten-like hand, opposite a prehensile finger.
But why should Iguanodon have a hand spike? The most popular idea is that the dinosaur used the appendage for defense—an illustration by John Sibbick in The Book of Dinosaurs shows and Iguanodon stabbing its spike into the neck of an attacking allosaurid. The restoration looks more than a little ridiculous. In order to get within poking range, the defending Iguanodon would have to place itself right in front of its assailant, perfectly within the range of the slicing dental cutlery of the carnivore. Such maneuvers would require the attacker to hold still while being prodded. One popular-audience book suggested that the spike might house a venom gland, but there is no evidence for this and, furthermore, the Iguanodon would still have to get within biting range of the attacking theropod to use the weapon.
There are a few other speculative hypotheses. Maybe Iguanodon used the spikes in combat with one another. Or perhaps, as David Norman briefly suggested in his section on basal iguanodontia in the second edition of The Dinosauria, the spike was used for “breaking into seeds and fruits.” These are not unreasonable notions, but there is also no positive evidence to suggest that they are correct, either. The Iguanodon thumb spike is a strange specialization that must have originated for a reason. The question is whether we can test any of these ideas.