Iguanodon

About Iguanodon

Features: Swiss Army Knife-like Forelimbs and Skeleton

Iguanodon was a large herbivorous dinosaur with a massive, barrel-shaped body that weighed up to 4 to 4.5 tons. One of its most striking features is the structure of its forelimbs (hands), often compared to a "Swiss Army knife", where each of the five fingers served a different function.

• First finger (thumb): Formed into a massive, sharp, and robust conical spike, it is thought to have been used as a defensive weapon against carnivorous dinosaurs or to crack open hard plants and fruits.
• Second, third, and fourth fingers (index, middle, and ring fingers): These fingers ended in hoof-like tips, serving as weight-bearing pillars when touching the ground during quadrupedal walking, dispersing its massive weight. Footprint fossils indicate that while walking, the palms did not face backward completely but were angled slightly inward.
• Fifth finger (pinky): Highly flexible and capable of moving in a different direction from the other fingers. It was useful for grasping objects, such as pulling in branches of plants.

Iguanodon basically walked on all fours, but could also stand up on two legs when fleeing in a hurry or browsing high vegetation (facultative bipedalism). To support its immense weight, the bones forming the pelvis (sacral vertebrae) were fused into a robust structure consisting of eight vertebrae.

Furthermore, along the spine from its back to its tail, there was a lattice of bone-like stiffened tendons called "ossified tendons". This structure firmly fixed the back and tail, playing a role in maintaining a posture horizontal to the ground and supporting the muscles that pulled the hindlimbs.

Diet and Chewing Mechanism of Iguanodon

Iguanodon lacked teeth at the front of its mouth. Instead, it had a hard beak covered in keratin, which efficiently cropped fibrous plants like ferns.

At the back of its mouth, it possessed a "dental battery" where densely packed teeth, resembling enlarged versions of those of modern iguanas, grew. Furthermore, they had a highly advanced mechanism for chewing (grinding up food).

Discovery of Mass Fossils at the Bernissart Coal Mine

A major discovery of a fossil assemblage at the Bernissart coal mine in Belgium in 1878 revolutionized the entirety of dinosaur paleontology. From a stratum about 322 meters underground, more than 14 fully articulated Iguanodon skeletons were found clustered together.

This location is speculated to have been a sinkhole or a swampy environment at the time, and it is believed that multiple individuals died and were buried simultaneously due to some natural disaster. This discovery conclusively proved that what was previously thought to be a horn on the nose was actually the "thumb spike" of the forelimb.

During this excavation, to prevent the fossils from collapsing deep underground, a field excavation technique of protecting the skeletons with plaster before transporting them was established, a method still relevant in modern paleontology.

Ecology Revealed by Habitat and Footprint Fossils

Numerous trace fossils (footprints) attributed to iguanodontians have been discovered in the UK and Germany. The presence of footprint sizes ranging widely from small juveniles to large adults suggests that they were not solitary, but formed multigenerational herds. Furthermore, trackways show quadrupedal impressions when moving slowly and the disappearance of forelimb impressions when running faster, corroborating that they skillfully switched their walking style depending on speed.

Skin impressions have also been found from some exceptionally well-preserved footprint fossils. They had no feathers and were covered in polygonal scales resembling those on the feet of modern crocodilians and birds.

Among iguanodontian skeletons, there are two types: a robust, thick-boned "robust morph" and a slightly slenderer "gracile morph". Whether these represent sexual dimorphism (differences between males and females) or merely individual variation has been debated for years, and research utilizing statistical simulations continues today.

Historical Transition of Discovery and Restoration

Iguanodon fossils were discovered in 1822, before the word "Dinosauria" even existed. It is one of the most important episodes representing the dawn of paleontology as a discipline.

Gideon Mantell, an amateur paleontologist practicing medicine in rural England, discovered unknown giant fossilized teeth. For many years, a romantic anecdote was widely circulated that "Mantell's wife, Mary Ann, discovered the first tooth among crushed stones by the roadside while he was on a house call," but modern historical verification suggests this anecdote is likely untrue. However, there is no doubt that his wife Mary Ann made an academically vital contribution by creating precise sketches of the fossils.

Even prominent scholars at the time initially misidentified them as "rhino or fish teeth," but Mantell conducted comparative studies with modern iguana skeletons and became convinced that this was a giant extinct reptile. In 1825, he named it "Iguanodon," meaning "iguana tooth."

Later, when the contemporary anatomist Richard Owen coined the taxon "Dinosauria" in 1841, it became one of the three foundational genera defining the group (along with Iguanodon, Megalosaurus, and Hylaeosaurus).

Breaking Away from a "Wastebasket Taxon" and Refining Classification

Because Iguanodon was historically so famous, many Early Cretaceous ornithopod fossils discovered in places like Europe and Asia were once easily assigned to the "genus Iguanodon" without rigorous comparison. It had become a "wastebasket taxon" into which unidentified fossils were simply tossed.

However, through detailed re-examinations and advancements in taxonomy in recent years, many former "Iguanodons" have been revealed to actually be different dinosaurs, each with distinct features. For instance, new genus names such as Mantellisaurus and Barilium have now been given.

As a result, currently, only Iguanodon bernissartensis, based on the fossils discovered at the Bernissart coal mine in Belgium, is considered a true "Iguanodon." Regarding fossils found in places like Spain, there is still ongoing debate over whether they represent a new species of the genus Iguanodon or a completely new and separate genus (such as Paulodon).