Ornithopoda

Ornithopoda was one of the most successful dinosaur lineages, achieving significant ecological dominance during the Mesozoic. The group includes small bipedal species like Hypsilophodon and Thescelosaurus, intermediate iguanodontians such as Iguanodon and Ouranosaurus, and the diverse duck-billed hadrosaurids. By the Late Cretaceous, hadrosaurs like Edmontosaurus and Shantungosaurus had become the most common large herbivores on the planet.

A key factor in ornithopod success was the evolution of a complex chewing mechanism. Unlike most reptiles, derived hadrosaurs possessed dental batteries containing hundreds of teeth arranged in columns. These teeth were replaced continuously and provided a robust surface for grinding plants. Studies by Norman & Weishampel (1985) suggest these dinosaurs used pleurokinetic skull movements, which allowed the upper jaws to flex outward against the lower ones. This sophisticated chewing system rivalled that of modern cattle and enabled hadrosaurs to process large volumes of tough, fibrous vegetation.

Body designs varied across the clade. Early members were light, bipedal runners. As the group evolved, many species grew larger and became capable of four-legged movement. This transition included specialized hand adaptations, such as the conical thumb spike of Iguanodon. Hadrosaurs were primarily quadrupedal but could still move on two legs when necessary. Many were highly social and possessed elaborate head crests.

The social lives of ornithopods are best understood through the fossil record of hadrosaurs. Sites like Egg Mountain in Montana provide evidence of colonial nesting and parental care for Maiasaura peeblesorum hatchlings. Massive bonebeds of Edmontosaurus also indicate that these animals travelled in herds numbering in the thousands. Crested species like Parasaurolophus and Lambeosaurus likely used their hollow cranial crests for vocalisation. Models by Weishampel and other researchers suggest these structures acted as resonating chambers for low-frequency sounds. Preserved skin impressions reveal that ornithopods had small polygonal scales and lacked bony armour, though some, like Edmontosaurus regalis, had soft-tissue structures like a fleshy crest.

• Iguanodon bernissartensis Bernissart specimens (RBINS) — ~38 articulated skeletons recovered from a Belgian coal mine, 1878.
• Maiasaura peeblesorum Egg Mountain — colonial nesting site, Montana.
• Edmontosaurus regalis "Mummies" — multiple specimens with skin impressions; AMNH 5060 the famous "Trachodon mummy."
• Parasaurolophus walkeri holotype (ROM 768) — Royal Ontario Museum.

The hadrosaur crest function — display, species recognition, sound resonance, thermoregulation, or all of the above — has been argued for nearly a century; current consensus accepts species recognition + acoustic resonance as primary, possibly with display. The pleurokinetic skull model has been challenged by Cuthbertson et al. (2012); the response (Erickson 2013) substantially supports the original. Locomotion — whether large hadrosaurs were primarily bipedal or quadrupedal — current consensus: facultatively both.

Hadrosaurs are the herd-animal default of dinosaur media. The Land Before Time's Ducky is a hadrosaur (Saurolophus-grade); Walking with Dinosaurs (1999) has a Parasaurolophus song; Disney's Dinosaur (2000) opens with hadrosaur stampede.

• iguanodon
• hadrosaurus
• parasaurolophus
• edmontosaurus
• lambeosaurus
• ornithischia

• Norman, D. B. (2004). Basal Iguanodontia. In The Dinosauria (2nd ed.).
• Horner, J. R., Weishampel, D. B., & Forster, C. A. (2004). Hadrosauridae. In The Dinosauria (2nd ed.).
• Norman, D. B., & Weishampel, D. B. (1985). Ornithopod feeding mechanisms: their bearing on the evolution of herbivory. American Naturalist, 126, 151–164.
• Bell, P. R., et al. (2014). A mummified duck-billed dinosaur with a soft-tissue cock's comb. Current Biology, 24, 70–75.
• Weishampel, D. B. (1981). Acoustic analyses of potential vocalization in lambeosaurine dinosaurs. Paleobiology, 7, 252–261.