An Iguanodon tooth embedded in rock

An Iguanodon tooth, discovered and described by the Mantells in the early 1800s

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Iguanodon: the teeth that led to a dinosaur discovery

Our perception of Iguanodon has been changing for almost 200 years, since the discovery of sparkling rocks by the side of the road.

But even now, this prehistoric reptile remains shrouded in mystery.

Discovered in Sussex in 1822, a collection of teeth was the first evidence of a gigantic herbivorous reptile named Iguanodon. This dinosaur and its closest relatives lived during the Early Cretaceous, between 140 and 110 million years ago. The crucial discovery helped to kick-start our fascination with dinosaurs.

Who discovered Iguanodon?

Mary Ann Mantell (1795-1869) and her husband Dr Gideon Algernon Mantell (1790-1852), are recorded as having found the first evidence of Iguanodon

(Left to right) Side by side black and white portraits of Gideon Mantell and Mary Ann Mantell

Husband and wife, Gideon Algernon and Mary Ann Mantell

Most accounts hold that Mary was accompanying her husband on a trip to visit a patient in Sussex, when she noticed something glinting by the side of the road. When Mary went to investigate she discovered a collection of fairly large teeth embedded in the rocks.

Her husband Gideon, also an amateur palaeontologist, studied the teeth. To try and determine what they belonged to, he reached out to experts for advice, including Georges Cuvier, who is sometimes referred to as the father of palaeontology. 

Iguanodon teeth embedded in two rocks

Two of the Iguanodon teeth found by Mary Ann Mantell, still embedded in rock. See the teeth on display in Treasures in the Cadogan gallery.

Cuvier initially thought that the teeth could be from a fish comparable to pufferfish (Tetraodon) or porcupinefish (Diodon), based on their external appearance. However, he noted that the fossils' internal structure was different, and that they may instead be from a new animal - possibly an herbivorous reptile.

Until that time, no evidence of giant prehistoric reptilian herbivores had been discovered. Remains of the large carnivore Megalosaurus had been in the collections of the Oxford University Museum of Natural History since the late 1600s, but they were only described by William Buckland in 1824 - two years after the discovery of these Iguanodon teeth.

Encouraged by Cuvier's suggestion, Gideon visited the Hunterian Museum (at the Royal College of Surgeons in London) to search for jaws and teeth of living reptiles with curator William Clift. They had little success until they were shown an iguana skeleton that had been recently prepared by assistant curator Samuel Stutchbury. 

A rhinoceros iguana

Gideon Mantell noticed some similarities between living iguanas and his discovery Iguanodon. The rhinoceros iguana may also have been the inspiration for Iguanodon's thumb spike originally being depicted on its nose. © H. Zell via Wikimedia Commons (CC BY-SA 3.0)

The fossilised teeth resembled those of the iguana, only many times larger. Iguanas are relatively large lizards, but scaled up the prehistoric owner of the fossilised teeth could have been up to 18 metres or longer. We now know this dinosaur reached a length of about 10 metres. Gideon based the name Iguanodon on this link to iguanas (the name means iguana tooth).

The Maidstone slab

For several years Gideon searched for more evidence of Iguanodon. Although finds were plentiful, they were usually isolated bones and teeth.

But his streak of smaller discoveries would come to an end when in 1834 some workmen accidentally blew up a slab of rock in a limestone quarry near Maidstone in Kent. 

A collection of iguanodon bones embedded in a slab of rock

The Maidstone Slab, also known as the Mantell-piece, was discovered after being blown apart by explosives in a limestone quarry

The quarry owner, William Harding Bensted, noticed a 'remarkable substance' resembling petrified wood in a large fragment of rock. He then searched for and reassembled the many pieces blown apart by gunpowder.

With a hammer and chisel Bensted followed the outline of the bones until he had 'brought into view portions of the skeleton of an extraordinary animal, which had been buried in the bowels of the earth probably in the earliest ages of existence'.

Among the bones embedded in the rock were rib fragments, vertebrae, limb bones, parts of the pelvis as well as, crucially, part of a tooth and a clear impression of another.

A line drawing of iguanodon with a thumb spike on its nose

From the Maidstone Slab and living lizard skeletons, Gideon Mantell conceptualised how Iguanodon might have been structured

Bensted contacted Gideon Mantell, who then travelled to Maidstone to investigate and attributed the bones to Iguanodon

Rhino horn to thumb spike

With the Iguanodon fossils Mantell had available to him, he determined that Iguanodon had a horn. This concept may have been based on the living iguana species Cyclura cornuta - the rhinoceros iguana - which features a small horn near its snout.

A drawing of the Crystal Palace Iguanodon and Hylaeosaurus models

 Two Iguanodons (L) and Hylaeosaurus (R) drawn by Benjamin Waterhouse Hawkins

Sir Richard Owen used Iguanodon (along with Megalosaurus and Hylaeosaurusto coin the word 'dinosaur' in 1842. Iguanodon's horn was still a prevailing theory when Owen consulted on the construction of the Crystal Palace dinosaur sculptures in the 1850s.

A drawing of a banquet being held inside a Crystal Palace Iguanodon model

Prior to the completion of the Crystal Palace dinosaurs in 1853, several notable scientists were invited to a dinner inside the Iguanodon sculpture. 

It wasn't until a significant Iguanodon discovery was made in the 1870s that it was proved that the spikes were thumbs rather than horns. But there is still speculation about what they were used for.

Iguanodon's thumbs may have been used as defensive weapons, either against its meat-eating predators or others of its species. But other scientists believe that the spiked thumbs were specialised tools, used for stripping foliage from branches or breaking into seeds.

One of the completed Iguanadon models in Crystal Palace Park

The Crystal Palace Iguanodon sculpture was based on the belief that these dinosaurs had a spike on their nose, rather than two thumb spikes as they would have had in life

Although scientists can interpret from fossils that the dinosaur's spikes were large, the bones are not representative of their full size in life. The spikes would have been covered in keratin, whereas dinosaur fossils only show the bony core.

An iguanodon hand

Iguanodon's hands were multifunctional. It had thumb spikes for food preparation or defence, three middle fingers fused into a 'hoof' for walking on and a fifth finger that was possibly used for grasping. 

The Bernissart Iguanodons

By this point no complete Iguanodon skeleton had been found - until a chance discovery in a Belgian coal mine in March 1878.

Miners digging at 322 metres below the surface near the town of Bernissart began to dig through a pocket of clay when they discovered numerous bones encrusted with pyrite (also known as fool's gold).

Informed by telegram, the head fossil preparator at the Musée Royal d'Histoire Naturelle de Belgique (now the Royal Belgian Institute of Natural Sciences), Louis De Pauw, travelled to the site to investigate. Over the following three years around 30 Iguanodon skeletons were uncovered, many of which were relatively complete.

The construction of a Bernissart Iguanodon in an upright pose

One of the Bernissart Iguanodons is being assembled in Brussels in 1882. This chapel was the only location with a high enough ceiling for the job. Image via Wikimedia Commons

Covered in plaster for protection and broken into 600 blocks, the fossils were transported to Brussels to be reassembled and fitted with iron frames. For the first time, scientists were able to see what a complete dinosaur might have looked like.

The giant dinosaur puzzles were assembled based on knowledge of the time, that Iguanodon was bipedal. We now know the dinosaur predominantly walked on all fours. The Belgian Iguanodons are now too fragile to be repositioned into life-like postures, however, so they remain a vestige of nineteenth-century thinking.

The Bernissart mineshafts have long since been filled and sealed, making it impossible to search for other Iguanodons that may be buried there. But samples taken from the mines in the early 2000s contained bone fragments, suggesting that there may still be dinosaurs far below the surface.