A Mysterious Thumb

There is much we still don’t know about dinosaurs. In fact, some aspects of dinosaurs have puzzled paleontologists for well over a century. Among the most frustrating is why the great herbivore Iguanodon had prominent thumb spikes. Despite all the possibly explanations provided for this appendage, none are especially satisfying.

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.

Though my own suggestion is not any better than those I have been disappointed by, I wonder if the Iguanodon spike is a Mesozoic equivalent of another false thumb seen among animals today—the enlarged wrist bones of red and giant pandas. Perhaps the Iguanodon thumb spike was an adaptation for stripping foliage from tree branches. The dinosaur could have grasped the branch with the prehensile finger, or flexed the main fingers of the palm around a bough, and run the spike down the branch to remove the greens without having to chew through the less-nutritious twigs. But this hypothesis has problems, too. The false thumbs of pandas flex so that they help the mammals grip bamboo, whereas the Iguanodon spike was rigid. And why would an Iguanodon preferentially select greener browse, especially when supplied with a formidable battery or self-replacing teeth? Furthermore, this idea is difficult to test—a preserved thumb spike wouldn’t show wear from use the same way a fossil tooth would. The Iguanodon spike was surrounded by a tough, keratinous sheath, so the actual wear wouldn’t be seen on the bone itself. A functional model of an Iguanodon hand could help investigate this idea, but even then, direct evidence would be lacking.

Perhaps there isn’t a good modern analog for the Iguanodon spikes. The bones look like they could be used for any number of things, from defense to feeding, but frustratingly, there isn’t any unambiguous indication of what they were used for or why they evolved. Perhaps, to solve this mystery, we need to go beyond the obvious and try to think like a dinosaur.

Riley Black | | READ MORE

Riley Black is the author of The Last Days of the Dinosaurs and many other books. She is a science correspondent for Smithsonian magazine covering fossils and natural history, and she writes about the prehistoric past for a variety of publications.