Iguanodon is a genus of large, herbivorous dinosaur whose fossilized remains have helped define our understanding of prehistoric life. The name, meaning “iguana-tooth,” stems from the initial discovery of its distinctive teeth, which greatly resembled those of a modern iguana, only massively enlarged. This bulky plant-eater became one of the first dinosaurs ever formally named, securing a significant place in the history of paleontology.
Taxonomy, Geography, and Epoch
Iguanodon belongs to the group of dinosaurs known as Ornithopoda, or “bird-footed” dinosaurs, which includes the later duck-billed hadrosaurs. It is classified as a key member of the Iguanodontia, representing a successful lineage of herbivores that diversified across the globe. The genus lived primarily during the Early Cretaceous period, roughly 126 to 122 million years ago.
The geographic distribution of Iguanodon bernissartensis, the most completely understood species, was centered in Western Europe. Fossil evidence has been unearthed across modern-day England, Germany, Spain, and notably, Belgium, where a large bonebed provided exceptional insight into the animal. This concentration of finds indicates that Iguanodon was a prominent figure in the continent’s ecosystems during this time.
Defining Anatomy and Physical Traits
The Iguanodon was a substantial animal; adult I. bernissartensis are estimated to reach lengths of 9 to 11 meters and a body mass of up to 4.5 metric tons. Its powerful hind limbs suggest an ability to move on two legs, but the robust structure of its forelimbs indicates it likely spent significant time walking on all fours, making it a facultative biped. This dual-mode locomotion allowed it to browse at different heights, switching between quadrupedal grazing and bipedal browsing.
The skull was long and narrow, ending in a toothless, keratin-covered beak used for clipping tough vegetation. Behind the beak, the dinosaur possessed unique dental batteries where individual teeth were designed to be self-sharpening as they ground against plant material. Unlike its hadrosaur relatives, Iguanodon featured only a single replacement tooth at each position. This specialized chewing surface, combined with evidence suggesting the presence of cheeks, allowed for efficient processing of its fibrous diet.
Its hands were highly specialized, featuring three central fingers that were sturdy and hoof-like, adapted to bear the animal’s weight during quadrupedal movement. The outermost, or fifth, finger was long and flexible, potentially acting as a prehensile digit used for grasping food. The hind feet were strong and ended in three functional toes, providing a stable foundation for its considerable weight.
The Function of the Thumb Spike
The most distinctive feature of the Iguanodon’s anatomy is the large conical bone that replaced the thumb, often referred to as the pollex spike. This specialized structure has been a topic of debate among paleontologists regarding its function. One theory suggests the spike served as a defensive weapon against large predatory dinosaurs, such as Baryonyx. A well-placed thrust could potentially deter an attacker.
The spike may also have been a specialized foraging tool, used to break open tough plant matter or to strip bark and foliage from branches. The combination of the spike with the flexible fifth finger suggests a sophisticated hand structure for manipulating its food source. Some researchers propose that the spike played a role in intraspecies conflict, used in non-lethal fights between rivals for territory or mating rights, similar to the horns of modern ungulates.
The exact purpose remains difficult to determine, as wear on the bone is often obscured by the keratinous sheath that would have covered the spike in life. It is possible the spike was a versatile adaptation, serving multiple roles in defense, feeding, and social interaction.
The Iguanodon’s Place in Paleontology History
The Iguanodon holds a foundational position in the history of paleontology. The first scientifically recognized remains were fossilized teeth found in England in the 1820s by Gideon Mantell, with credit often extended to his wife, Mary Ann Mantell, for the initial discovery. Mantell formally named the creature Iguanodon in 1825, making it the second dinosaur ever described, following Megalosaurus.
This discovery was pivotal, as the Iguanodon, along with Megalosaurus and Hylaeosaurus, would later be used by Sir Richard Owen to define the new group of reptiles he named Dinosauria in 1842. Early reconstructions of the Iguanodon were famously inaccurate due to the fragmentary nature of the initial finds, often depicting the animal as a giant, lizard-like creature.
A significant error in the 1850s Crystal Palace models placed the thumb spike mistakenly on the creature’s nose, a reconstruction based on the horn of a rhinoceros iguana. This misconception persisted until the late 1870s, when a massive find of complete Iguanodon skeletons in a Belgian coal mine revealed the true location and structure of the thumb spike on the hand.