Iguanodon

Kingdom: Animalia

Phylum: Chordata

Class: Reptilia

Order: Ornithischia

Family: Iguanodontidae

Genus: Iguanodon

Species: Iguanodon bernissartensis

Introduction

Discovered in 1822, Iguanodon was only the second dinosaur to be formally named in 1825 (after Megalosaurus). The initial discovery was a tooth found in England. The amateur geologist who discovered it recognized it to be a much larger version of the modern iguana's teeth—thus the name “iguana tooth” seemed appropriate.

Scientific understanding of Iguanodon has experienced significant shifts since its discovery. It was originally thought to be a quadruped (i.e., walking on four legs) with a horned nose. Further excavations and discoveries led scientists to believe that Iguanodon was actually bipedal (i.e., walking upright on two legs), and the “horns” were actually conical thumb spikes.

During the Early Cretaceous, the supercontinent Pangaea was breaking apart, but there were still opportunities for animals to spread over vast areas of land. Iguanodon did exactly that, and excavations have turned up Iguanodon fossils on more than one continent. Because it was one of the first dinosaurs to be discovered and studied, it seems to have found a place in the public imagination. It routinely appears in popular culture and is highly recognizable.

Classification

Paleontologists use two major systems for classifying organisms. Linnaean taxonomy, also known as systematics, uses overall physical similarity to place organisms into different groups. By contrast, cladistics seeks to create groups of organisms based on evolutionary relationships; these groups are called “clades.” Cladistic analysis focuses on important key traits and attempts to trace the inheritance of a trait among descendants of a common ancestor.

Under the Linnaean classification system, the quadrupedal (four-legged), herbivorousIguanodon falls into the Iguanodontidae family. The creature lived during the Early Cretaceous period (specifically between 130 and 125 million years ago). Iguanodon possessed traits common to dinosaurs of the order Ornithischia and infraorder Ornithopoda, including bird-like feet, beaks, and an herbivorous diet. Fossil evidence for many dinosaurs in this larger group date back to the Jurassic and Cretaceous periods. Iguanodon also shared many traits with other members of the Iguanodontidae family. Related animals include Ouranosaurus and Probactrosaurus, and their shared traits include massive size, long toes, and thumb spikes.

Following the first discovery, many different species were assigned to the genus Iguanodon. Considerable controversy over classification within this genus ensued, however. Many scientists contend that most of the species assigned to the genus are misclassified and will eventually be reassigned. Today, the species I. bernissartensis is considered the neotype of the genus; it is also the most well known and most widely studied. Many of the other species that had previously been assigned to Iguanodon have since been reassigned.

The cladistics method of classification seeks to draw relationships between species. Considered a somewhat fluid system, cladistics helps scientists to understand the similarities and differences between and among species, as well as to develop theories about how different species evolved. In a cladistic analysis, Iguanodon falls under the larger Dryomorpha group of dinosaurs, and then within the subgroup Ankylopollexia, as it featured stiff, cone-shaped thumbs.

As with any dynamic study in the field of science, cladistic analyses serve as hypotheses and can change over time as new specimens are discovered and as the science advances. In 2017 Matthew Baron and his colleagues posited a common ancestor for the ornithischians, such as Iguanodon, and theropods, such as Allosaurus, and proposed they be reclassified into a new clade, Ornithoscelida.

Anatomy

Iguanodon had a large, narrow skull, a toothless beak, and tightly packed cheek teeth (similar to those of an iguana). Its forelegs consisted of relatively inflexible hands of four fingers, plus a cone-shaped thumb spike. These spikes were from 5 to 15 centimeters (2 to 6 inches) long and were perpendicular to the other fingers. They would have been sheathed in keratin, a hornlike material. They may have been used for defense or feeding, but their exact purpose is still under debate.

The front legs were significantly shorter than the hind legs (roughly 75 percent of their length). There were three toes on the hind feet, along with hoof-like claws. Iguanodon had a flat, stiff tail that did not drag along the ground. Because its legs were much larger and more powerful than its arms, Iguanodon was able to alternate between two-legged and four-legged locomotion. It was not a fast dinosaur but could run on two legs or walk on four, reaching a supposed top speed of 15 to 20 kilometers (9 to 12 miles) per hour.

Intelligence

Iguanodon was an ornithopod, whose intelligence (as measured by its relative brain to body weight, or EQ) was fairly average among dinosaurs. Ornithopod EQs have been charted from 0.9 to 1.5. Relative to other dinosaurs, Iguanodon cannot be classified as unusually intelligent or unintelligent. It is likely that the creature's sensory abilities were well developed, helping it find edible vegetation and avoid predators. It is also likely that it fell prey to other, smarter dinosaurs.

Reproduction and Population

Fossil evidence offers little information regarding population numbers, gender differences, and mating and reproduction for Iguanodon. The 1878 discovery of more than thirty adult Iguanodon remains in Bernissart, Belgium, was significant because the discovery of so many remains in one place suggests that Iguanodon may have lived and travelled in herds. This theory is still debated among scientists, however. Discoveries of Iguanodon remains have been found on nearly every continent, indicating that it was a very successful species and had a relatively large population.

Other evidence indicates that most dinosaurs, including Iguanodon, were oviparous, meaning that they laid eggs (normally several eggs at a time). Some dinosaurs laid their eggs in nests that were eventually abandoned, leaving the eggs vulnerable to predators. Some species’ nests and eggs were safely hidden or buried, while fossil evidence proves that other species’ eggs were covered and kept warm by a parent or parents.

Diet

Iguanodon was an herbivore with teeth like those of the modern iguana. It had as many as 29 teeth on each side of its upper jaw and 25 larger teeth in the lower jaw; some of its teeth measured up to 2 inches long. Teeth were continually replaced, so Iguanodon presumably had a full set of teeth for its entire lifetime.

Scientists believe that this dinosaur grabbed prehistoric plants with its toothless beak and then used the strong cheek teeth to chew and grind up the food. Based on its skull and jaw structure, scientists think that Iguanodon was able to grind its food in a manner that is similar to mammalian chewing. It may have also gathered food with comparatively flexible fingers on its forelimbs.

Though the exact plants that Iguanodon ate are unknown, it would have required more or less continual eating, based on its size and weight. Based on its height, this herbivorous dinosaur would have eaten plants available no more than 5 meters (15 ft) above ground.

Behavior

Because of its size, Iguanodon moved relatively slowly. However, it had the unique advantage of being able to run on two legs or walk on four. Based on the evidence from paleontological discoveries in Bernissart, Belgium, many scientists believe that Iguanodon was a herding animal (although this has been questioned). Herding dinosaurs were docile and nonaggressive compared to the solitary carnivores that lived in the same environment. Travelling together protected the group as a whole, especially the young.

Dinosaurs that are supposed to be herding species, such as hadrosaurs, are typically sexually dimorphic. In other words, there is a significant difference in physical characteristics, such as body size, between the male and female of the species. This is not the case with Iguanodon, however, as the skeletons discovered show no significant difference between sexes despite significant individual variation. Many scientists point to this apparent lack of sexual dimorphism as evidence that Iguanodon was not a herding dinosaur.

Habitat and Other Life Forms

The Early Cretaceous period was characterized by a mild cooling trend. Cool temperate forests covered the polar regions. This trend soon reversed, however, due to increased volcanic activity. The Cretaceous period as a whole was characterized by an increasingly warm climate.

Flowering plants (angiosperms) became abundant and widespread during the Cretaceous. Gymnosperms continued to thrive, and many leafy trees also began to appear. A variety of low-lying plants would have constituted the massive diet that Iguanodon required on a daily basis.

Modern mammals began to appear during the Early Cretaceous, though an incredible variety of dinosaurs ruled the land. Pterosaurs (flying reptiles) were common animals in the sky, but they faced growing competition from various bird species, and they were all but extinct by the end of the Cretaceous. In the sea, rays, sharks, and echinoderms (spiny-skinned marine animals such as the modern-day starfish) were common.

Iguanodon had many predators, including megalosaurid theropods. Iguanodon's sharp thumb spike was its best defense, and the ability to alternate between bipedal and quadrupedal locomotion gave it a distinct advantage over other dinosaurs.

Research

Iguanodon was one of the first species of dinosaur to have been discovered and named. Over time, especially during the nineteenth and early twentieth centuries, when dinosaur discoveries were relatively new, many different species were assigned to the Iguanodon genus. Remains (of various iguanodontids) have been found across Europe, and in Central Asia, North Africa, and North America.

The dinosaur was first named by English medical doctor Gideon Mantell after he and his wife discovered teeth and bones in Sussex, England, in 1822. It was only the second dinosaur to be named (after Megalosaurus). In 1834, a much better (and more complete) specimen was discovered in Maidstone, Kent, England. It was from this excavation that the first descriptions and illustrations of Iguanodon came. Over the years, many incorrect assumptions were made about this dinosaur—that it had a nose horn (which later turned out to be the spiked thumbs), that it was permanently quadrupedal, and that its tail dragged along the ground.

The largest discovery was made in 1878 in Bernissart, Belgium, by Louis Dollo and Louis de Pauw. They uncovered, excavated, and constructed more than thirty separate individuals—some of the first complete dinosaur skeletons ever found. This species was later named Iguanodon bernissartensis. In 2000, the International Commission on Zoological Nomenclature (ICZN) defined the genus Iguanodon by this species.

The first-ever fossilized dinosaur brain found was that of an iguanodon in 2004 in Sussex, England. It was identified as a brain in part by the presence of branching blood vessel hollows in the fossil. Soft organic tissues like brains usually decay too quickly to become fossilized; researchers hypothesized, however, that this brain was plunged into water, limiting its exposure to oxygen, and pushed against the skull, allowing its organic matter to be replaced by phosphate and carbonate minerals. The findings, published in the Geological Society of London in 2016, may enable paleontologists to identify additional fossilized soft tissues and to further refine their understanding of dinosaur intelligence relative to that of modern birds, whose brains take up most of their skulls.

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