In their own way, ornithopods--the small, mostly two-legged herbivorous dinosaurs of the Mesozoic Era--have had a disproportionate impact on the history of paleontology. By a geographical fluke, many of the dinosaurs dug up in Europe in the early 19th century happened to be ornithopods (the most noteworthy being Iguanodon), and today more ornithopods are named after famous paleontologists than any other kinds of dinosaur. (See a gallery of ornithopod piotures.)
Ornithopods (the name is Greek for "bird-footed") are one of the classes of ornithischian ("bird-hipped") dinosaurs, the others being pachycephalosaurs, stegosaurs, ankylosaurs and ceratopsians. The most well-known subgroup of ornithopods are the hadrosaurs, or duck-billed dinosaurs, which are discussed in a separate article; this piece focuses on the smaller, non-hadrosaur ornithopods.
Technically speaking, ornithopods (including hadrosaurs) were plant-eating dinosaurs with bird-shaped hips, three- or four-toed feet, powerful teeth and jaws, and a lack of the anatomical "extras" (armor plating, thickened skulls, clubbed tails, etc.) found on other ornithischian dinosaurs. The earliest ornithopods were exclusively bipedal, but the larger species of the Cretaceous period spent most of their time on all fours (though it's conjectured that they could run on two feet if they had to get away in a hurry).
Ornithopod Behavior and Habitats
Paleontologists often find it helpful to infer the behavior of long-extinct dinosaurs from the modern creatures they most resemble. In that respect, the modern analogs of ancient ornithopods seem to be herbivorous mammals like deer, bison, and wildebeests. Since they were relatively low on the food chain, it's believed that most genera of ornithopods roamed the plains and woodlands in herds of hundreds or thousands, to better protect themselves from raptors and tyrannosaurs, and it's also likely that they took care of their hatchlings until they were able to fend for themselves.
Ornithopods were widespread geographically; fossils have been dug up on every continent except Antarctica. Paleontologists have noted some regional differences between genera: for example, Leaellynasaura and Qantassaurus, which both lived in near-Antarctic Australia, had unusually large eyes, presumably to make the most of the limited sunlight, while the north African Ouranosaurus may have sported a camel-like hump to help it through the parched summer months.
As with many types of dinosaurs, our state of knowledge about ornithopods is constantly changing. For example, recent years have seen the discovery of two enormous genera, Lanzhousaurus and Lurdusaurus, which lived in mid-Cretaceous Asia and Africa, respectively. These dinosaurs weighed about 5 or 6 tons each, making them the heaviest ornithopods until the evolution of plus-sized hadrosaurs in the later Cretaceous--an unexpected development that has caused scientists to revise their views of ornithopod evolution.
As noted above, ornithopods featured prominently in the early development of paleontology, thanks to the fact that an unusual number of Iguanodons (or herbivores that closely resembled Iguanodon) wound up fossilized in the British Isles. In fact, Iguanodon was only the second dinosaur ever to be officially named (the first was Megalosaurus), one unintended consequence being that subsequent Iguanodon-like remains were assigned to that genus, whether they belonged there or not.
To this day, paleontologists are still undoing the damage. An entire book could be written about the slow, laborious untangling of the various "species" of Iguanodon, but suffice it to say that new genera are still being coined to make room for the reshuffling. For example, the genus Mantellisaurus was created as recently as 2006, based on its obvious differences from Iguanodon (to which it's still closely related, of course).
Mantellisaurus evokes another long-standing fracas in the hallowed halls of paleontology. This ornithopod was named after Gideon Mantell, whose original discovery of Iguanodon in 1822 was appropriated by the egotistical Richard Owen. Today, Owen has no dinosaurs bearing his name, but Mantell's eponymous ornithopod goes a long way toward correcting a historical injustice.
The naming of small ornithopods also figures in another famous paleontological feud. During their lifetimes, Edward Drinker Cope and Othniel C. Marsh were mortal enemies, the result of an Elasmosaurus head being placed on its tail rather than its neck (don't ask). Today, both of these paleontologists have been immortalized in ornithopod form--Drinker and Othnielia--but there's some suspicion that these dinosaurs may actually have been two species of the same genus!
Here's a list of the most notable ornithopods; just click on the links for more information.
Agilisaurus This "agile lizard" was one of the earliest ornithopods.
Albertadromeus This petite ornithopod was recently discovered in Canada.
Anabisetia The best-attested South American ornithopod.
Barilium Yet another iguanodontid ornithopod of the British Isles.
Camptosaurus A close relative of Iguanodon.
Cumnoria It was once mistakenly classified as a species of Iguanodon.
Darwinsaurus "Darwin's lizard" may or may not be a valid dinosaur genus.
Delapparentia This ornithopod was initially classified as a species of Iguanodon.
Dollodon Named after the Belgian paleontologist Louis Dollo.
Draconyx This "dragon claw" lived in late Jurassic Portugal.
Drinker Named after the famous paleontologist Edward Drinker Cope.
Dryosaurus A typical ornithopod of the late Jurassic.
Dysalotosaurus We know a lot about this dinosaur's growth stages.
Echinodon One of the few ornithopods to sport a set of canines.
Elrhazosaurus Once classified as a species of Valdosaurus.
Fabrosaurus This early ornithopod may have been a species of Lesothosaurus.
Fukuisaurus This ornithopod was discovered in Japan.
Fulgurotherium Very little is known about this "lightning beast."
Gasparinisaura One of the few ornithopods known to have lived in South America.
Gideonmantellia Guess what naturalist this dinosaur was named after?
Haya This dinosaur was named after a horse-headed Mongolian god.
Heterodontosaurus This "different-toothed" dinosaur was a dentist's nightmare.
Hexinlusaurus Named after the Chinese professor He Xin-Lu.
Hippodraco This "horse dragon" was recently discovered in Utah.
Huxleysaurus Named after the famous biologist Thomas Henry Huxley.
Hypselospinus It was once classified as a species of Iguanodon.
Hypsilophodon This man-sized herbivore liked to eat and run.
Iguanacolossus A brand-new ornithopod from North America.
Iguanodon The second dinosaur in history ever to receive a name.
Jeholosaurus This ornithopod may have had an omnivorous diet.
Koreanosaurus Guess what country this ornithopod was discovered in?
Kukufeldia Yet another ornithopod that was once lumped in with Iguanodon.
Lanzhousaurus This herbivore's teeth were half a foot long.
Laosaurus This dubious ornithopod was named by Othniel C. Marsh.
Leaellynosaura One of the few dinosaurs to be named after a little girl.
Lesothosaurus One of the earliest ornithischian dinosaurs.
Lurdusaurus This ornithopod resembled a giant sloth.
Macrogryphosaurus Otherwise known as the Big Enigmatic Lizard.
Manidens A strangely toothed relative of Heterodontosaurus.
Mantellisaurus Named after the famous fossil hunter Gideon Mantell.
Mantellodon This Iguanodon refugee may or may not deserve its own genus.
Mochlodon One of the few dinosaurs ever to be discovered in Austria.
Muttaburrasaurus The most complete dinosaur fossil ever found in Australia.
Nanosaurus This "tiny lizard" was named by Othniel C. Marsh.
Nanyangosaurus An iguanodontid ornithopod of middle Cretaceous Asia.
Notohypsilophodon A rare South American ornithopod.
Orodromeus This tiny herbivore was on Troodon's dinner menu.
Oryctodromeus The only ornithopod known to have lived in burrows.
Othnielia Named after the famous paleontologist Othniel C. Marsh.
Othnielosaurus Also named after the famous paleontologist Othniel C. Marsh.
Ouranosaurus Scientists can't decide if this herbivore had a sail or a hump.
Parksosaurus It was once classified as a species of Thescelosaurus.
Planicoxa A medium-sized iguanodont of early Cretaceous North America.
Proa This ornithopod was named after its prow-shaped jaw.
Protohadros Despite its name, it wasn't really the "first hadrosaur."
Qantassaurus Named after the national airline of Australia.
Rhabdodon A possible "missing link" between Iguanodon and Hypsilophodon.
Talenkauen A rare ornithopod from South America.
Tenontosaurus This long-tailed herbivore was hunted by Deinonychus.
Theiophytalia Its name means "garden of the gods."
Thescelosaurus Did paleontologists find this dinosaur's mummified heart?
Trinisaura The first ornithopod ever to be discovered in Antarctica.
Valdosaurus This ornithopod was discovered on the Isle of Wight.
Xuwulong This iguanodontid ornithopod was recently discovered in China.
Yandusaurus A small ornithopod of middle Jurassic China.
Zalmoxes A strange-looking ornithopod from Romania.