The Iguanodontia

The significance of the iguanodonts to the history of dinosaur science is a matter of record. In the year 1822—19 years before the term dinosaur became a part of scientific literature—a physician and amateur geologist named Gideon Mantell (1790-1852) was visiting a patient near Lewes, in the English countryside. Accompanying him was his wife, Mary Ann. As the story goes, while the doctor tended to his patient, Mary Ann took a stroll along a country road, where she noticed a jumbled pile of fossils and rocks that had been dug up because of a road repair. Recognizing that some of the fossils resembled teeth, she brought them to the attention of her husband.

Both Mantells were quite familiar with fossil finds from the area, particularly of invertebrates. Mary Ann often provided scientific illustrations for papers and books that Gideon wrote from time to time describing such specimens. Greatly excited by his wife's discovery, Mantell continued the search for more pieces of the fossil creature and wrote a memoir describing it in the same year. Most importantly, Mantell and his wife recognized that the remains were from a large animal with reptilian affinities, and his paper became the first to correctly describe such fossils as being those of a large, unique reptilian creature that had become extinct.

The Mantells continued their search for such fossils, and after having procured some additional evidence for their ancient saurian, Gideon wrote another paper in 1825 and gave the creature the name Iguanodon ("iguana tooth"). The first dinosaur ever described was an ornithopod.

Iguanodon has since become one of the best-known ornithopods, partly because of the remarkable discovery in the 1880s of more than 26 nearly complete and associated skulls and skeletons in a mine in Belgium. Currently, there are more than two dozen recognized taxa of nonhadrosaurid iguanodonts, including basal forms, iguanodontids such as Iguanodon itself, and animals on the line to hadrosaurids.

Nonhadrosaurid iguanodonts are noted for having a toothless premaxilla with a deeply rounded snout and large, anterior nasal opening. They ranged in size from small, basal members measuring 6.6 to 10 feet (2 to 3 m) long to the largest members that were up to 37 feet (11 m) long. The fossil record of iguanodonts is well documented and shows remarkable stages of transformation in the size, posture, and dental adaptations of the clade.

Some of the primitive iguanodonts, such as Dryosaurus (Late Jurassic, western North America and Tanzania); Tenontosaurus (Early Cretaceous, western North America); and Camptosaurus (Late Jurassic, western North America and England) illustrate adaptive stages leading to the larger iguanodontids and hadrosaurs. Many of these early members had long arms and were partly quadrupedal. The feet of Tenontosaurus and Camptosaurus were four- toed, while those of Dryosaurus had three weight-bearing toes, as was the case with more derived iguanodonts. The tails of iguanodonts also modified over time into a deep, stiff counter-balance that probably aided the animals in walking bipedally.

The evolution of iguanodonts from basal forms into taxa such as Iguanodon also involved increasingly robust adaptations to their plant-grinding jaws. The front of the jaws had become a tough, toothless beak for snipping off stems and twigs. The skull was long and narrow with an impressive row of ridged grinding teeth in each cheek. The upper and lower jaws each had about fifty closely packed, chisel-shaped teeth that formed a long grinding surface. Most animals that can grind food with their teeth, including cows and people, can move the lower jaw up and down as well as side to side. Iguanodon could not do this. Its lower jaw could only move up and down. Instead, it evolved a way to spread the upper jaw sideways to rub the inner surface of its upper teeth against the outer surface of the lower teeth. This movement was propelled by powerful jaw muscles, allowing Iguanodon to thoroughly grind the fibrous plants that it ate.

Large iguanodonts such as Iguanodon were probably slow-moving, making them subject to attack by predatory dinosaurs. Like many other ornithopods, iguanodonts probably survived by

Iguanodontia

Iguanodon could protect itself by using its spike thumbs to stab predators; this early example of a lively depiction of Iguanodon was rendered in the 1960s and is based on the best-known facts at that time.

breeding in large numbers and living in herds. When faced with defending itself, Iguanodon had stout spikes for thumbs that could have been used to stab its attackers.

Iguanodon and its close relatives are mostly known from the Northern Hemisphere and were quite populous in what is now Europe and Asia. A few such taxa have been found south of the Equator, including Lurdusaurus (Early Cretaceous, Niger) and Ouranosaurus (Early Cretaceous, Niger). Ouranosaurus is noted for having extraordinarily long spines on its back that may have formed a sail. If such a sail was indeed present, its function is not entirely understood, but it has been interpreted variously as a visual signaling mechanism (e.g., for recognizing individuals or gender differences) or for regulation of body temperature. The presence of another tall-spined but unrelated dinosaur— Spinosaurus, a predator best known from Egypt and Morocco—from the same tropical region lends some credence to the need for these large animals to have adapted an anatomical means for effectively venting heat, or even absorbing it from the Sun during the cooler hours of the day.

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