Description and Classification

Iguanodon was a large, heavily built herbivorous dinosaur. Adults typically reached lengths of about 9 to 11 meters (30 to 36 feet) and stood about 3 to 4 meters (10 to 13 feet) tall at the hips, weighing an estimated 4 to 5 metric tons. It had a long, horse-like skull ending in a toothless, keratinous beak used for cropping vegetation. Further back in its jaws were rows of leaf-shaped cheek teeth, ideal for grinding tough plant material.

Iguanodon walked primarily on its two hind legs (bipedal locomotion) but was also capable of walking on all fours (quadrupedal locomotion), making it a facultative quadruped. Its strong hind limbs were much larger than its forelimbs. The hands were unique, featuring a large, conical thumb spike and a flexible fifth digit, or “pinky,” which could have been used for grasping food. The three middle fingers were stout and ended in hoof-like structures, likely supporting weight when walking on four limbs.

Iguanodon belongs to the order Ornithischia, the “bird-hipped” dinosaurs. It is a key member of the group Iguanodontia, which also includes other well-known dinosaurs like Hadrosaurus and the later “duck-billed” dinosaurs (hadrosaurids). Iguanodon bernissartensis is the best-known species, largely due to the discovery of numerous complete skeletons in a coal mine in Bernissart, Belgium, in the late 19th century.

Distinguishing Features

Iguanodon possessed several unique characteristics that set it apart from other dinosaurs:

• Thumb Spike: A large, sharp, conical spike on the first digit (thumb) of each hand. Its exact purpose is debated, but it may have been used for defense against predators or for breaking open seeds and fruits.
• Prehensile Fifth Finger: The fifth digit (pinky finger) on its hands was elongated and flexible, likely used to help gather or manipulate food.
• Weight-Bearing Middle Fingers: The three central fingers (digits II, III, and IV) of the hand were robust, bound together, and ended in hoof-like structures, adapted for bearing weight when the animal moved on all fours.
• Specialized Teeth: Its teeth were similar to those of an iguana but larger and more packed, forming efficient grinding surfaces for processing tough plant matter. They showed a pattern of wear that suggested a complex chewing motion.
• Toothless Beak: The front of its snout was equipped with a horny beak, ideal for stripping leaves and twigs from plants.

Paleoenvironment and Diet

Iguanodon lived during the Early Cretaceous period, a time when flowering plants were beginning to diversify, although conifers, cycads, and ferns still dominated the landscape. It inhabited environments such as floodplains, river valleys, and open woodlands. The famous Bernissart fossils were found in what was likely a swampy, marsh-like area.

As a herbivore, Iguanodon would have fed on a variety of plants available in its habitat. Its beak was suited for cropping vegetation, and its powerful jaws and teeth could process tough plant material. It likely browsed on low-growing plants, but its bipedal stance might have allowed it to reach vegetation at higher levels as well. Evidence from bonebeds, like the one at Bernissart, suggests that Iguanodon may have lived in herds or social groups, which could have offered protection from predators such as Baryonyx or Neovenator.

Significance and Ongoing Research

Iguanodon holds a significant place in the history of paleontology. It was the second dinosaur to be formally named (after Megalosaurus) and played a vital role in the initial recognition of dinosaurs as a distinct group of reptiles. The interpretation of Iguanodon has changed dramatically over time, reflecting advancements in paleontological understanding. Early reconstructions incorrectly depicted it as a lumbering, quadrupedal creature with its thumb spike placed on its nose, similar to a rhinoceros horn. The Bernissart skeletons dramatically improved this understanding, showing its more agile, bipedal capabilities and the true nature of its thumb spike.

Ongoing research continues to refine our knowledge of Iguanodon‘s biology and behavior. Studies focus on its locomotion, particularly the transition between bipedal and quadrupedal stances, the exact function of its unique hand structures, and its feeding mechanics. The classification within the genus Iguanodon has also been revised, with several species once assigned to it now recognized as distinct genera, such as Mantellisaurus and Dollodon. Iguanodon remains a crucial taxon for understanding the evolution of ornithopod dinosaurs and their eventual radiation into the diverse hadrosaurids of the Late Cretaceous.