01 of 74 Meet the Small, Plant-Eating Dinosaurs of the Mesozoic Era Wikimedia Commons Ornithopods—small- to medium-sized, bipedal, plant-eating dinosaurs—were some of the most common vertebrate animals of the later Mesozoic Era. On the following slides, you'll find pictures and detailed profiles of over 70 ornithopod dinosaurs, ranging from A (Abrictosaurus) to Z (Zalmoxes).

02 of 74 Abrictosaurus Wikimedia Commons Name: Abrictosaurus (Greek for "waking lizard"); pronounced AH-brick-toe-SORE-us Habitat: Woodlands of southern Africa Historical Period: Early Jurassic (200 million years ago) Size and Weight: About four feet long and 100 pounds Diet: Plants Distinguishing Characteristics: Small size; a combination of beak and teeth As with many dinosaurs, Abrictosaurus is known from limited remains, the incomplete fossils of two individuals. This dinosaur's distinctive teeth mark it as a close relative of Heterodontosaurus, and like many reptiles of the early Jurassic period, it was fairly small, adults reaching sizes of only 100 pounds or so--and it may have existed at the time of the ancient split between ornithischian and saurischian dinosaurs. Based on the presence of primitive tusks in one specimen of Abrictosaurus, it's believed this species may have been sexually dimorphic, with males differing from females.

03 of 74 Agilisaurus Joao Boto Name: Agilisaurus (Greek for "agile lizard"); pronounced AH-jih-lih-SORE-us Habitat: Woodlands of eastern Asia Historical Period: Middle Jurassic (170-160 million years ago) Size and Weight: About four feet long and 75-100 pounds Diet: Plants Distinguishing Characteristics: Small size; lightweight build; stiff tail Ironically enough, the near-complete skeleton of Agilisaurus was discovered during the construction of a dinosaur museum adjacent to China's famous Dashanpu fossil beds. Judging by its slender build, long hind legs and stiff tail, Agilisaurus was one of the earliest ornithopod dinosaurs, though its exact place on the ornithopod family tree remains a matter of dispute: it may have been more closely related to either Heteredontosaurus or Fabrosaurus, or it may even have occupied an intermediate position between true ornithopods and the earliest marginocephalians (a family of herbivorous dinosaurs that comprises both pachycephalosaurs and ceratopsians).

04 of 74 Albertadromeus Julius Csotonyi Name: Albertadromeus (Greek for "Alberta runner"); pronounced al-BERT-ah-DRO-may-us Habitat: Plains of North America Historical Period: Late Cretaceous (80-75 million years ago) Size and Weight: About five feet long and 25-30 pounds Diet: Plants Distinguishing Characteristics: Small size; long hind legs The smallest ornithopod yet to be discovered in Canada's Alberta province, Albertadromeus only measured about five feet from its head to its slender tail and weighed as much as a good-sized turkey--which made it a true runt of its late Cretaceous ecosystem. In fact, to hear its discoverers describe it, Albertadromeus basically played the role of tasty hors d'oeuvre for much larger North American predators like the similarly named Albertosaurus. Presumably, this speedy, bipedal plant-eater was able to at least give its pursuers a good workout before being swallowed whole like a Cretaceous dumpling.

05 of 74 Altirhinus Wikimedia Commons Name: Altirhinus (Greek for "high nose"); pronounced AL-tih-RYE-nuss Habitat: Woodlands of Central Asia Historical Period: Middle Cretaceous (125-100 million years ago) Size and Weight: About 26 feet long and 2-3 tons Diet: Plants Distinguishing Characteristics: Long, stiff tail; strange crest on the snout At some point during the middle Cretaceous period, the later ornithopods evolved into the early hadrosaurs, or duck-billed dinosaurs (technically, hadrosaurs are classified under the ornithopod umbrella). Altirhinus is often pointed to as a transitional form between these two closely related dinosaur families, mostly because of the very hadrosaur-like bump on its nose, which resembles an early version of the elaborate crests of later duck-billed dinosaurs like Parasaurolophus. If you ignore this growth, though, Altirhinus also looked a lot like Iguanodon, which is why most experts classify it as an iguanodont ornithopod rather than a true hadrosaur.

06 of 74 Anabisetia Anabisetia. Eduardo Camarga Name: Anabisetia (after the archaeologist Ana Biset); pronounced AH-an-biss-ET-ee-ah Habitat: Woodlands of South America Historical Period: Late Cretaceous (95 million years ago) Size and Weight: About 6-7 feet long and 40-50 pounds Diet: Plants Distinguishing Characteristics: Small size; bipedal posture For reasons that remain mysterious, very few ornithopods—the family of small, bipedal, plant-eating dinosaurs—have been discovered in South America. Anabisetia (named after the archaeologist Ana Biset) is the best-attested of this select group, with a complete skeleton, lacking only the head, reconstructed from four separate fossil specimens. Anabisetia was closely related to its fellow South American ornithopod, Gasparinisaura, and probably to the more obscure Notohypsilophodon as well. Judging by the profusion of large, carnivorous theropods that prowled late Cretaceous South America, Anabisetia must have been a very fast (and very nervous) dinosaur.

07 of 74 Atlascopcosaurus Jura Park Name: Atlascopcosaurus (Greek for "Atlas Copco lizard"); pronounced AT-lass-COP-coe-SORE-us Habitat: Woodlands of Australia Historical Period: Early-Middle Cretaceous (120-100 million years ago) Size and Weight: About 10 feet long and 300 pounds Diet: Plants Distinguishing Characteristics: Small size; long, stiff tail One of the few dinosaurs to be named after a corporation (Atlas Copco, a Swedish manufacturer of mining equipment, which paleontologists find very useful in their field work), Atlascopcosaurus was a small ornithopod of the Cretaceous period that bore a marked resemblance to Hypsilophodon. This Australian dinosaur was discovered and described by the husband-and-wife team of Tim and Patricia Vickers-Rich, who diagnosed Atlascopcosaurus on the basis of widely scattered fossil remains, almost 100 separate bone fragments consisting mostly of jaws and teeth.

08 of 74 Camptosaurus Julio Lacerda Name: Camptosaurus (Greek for "bent lizard"); pronounced CAMP-toe-SORE-us Habitat: Woodlands of North America Historical Period: Late Jurassic (155-145 million years ago) Size and Weight: About 20 feet long and 1-2 tons Diet: Plants Distinguishing Characteristics: Four toes on back feet; long, narrow snout with hundreds of teeth The golden age of dinosaur discovery, which spanned the mid-to-late nineteenth century, was also the golden age of dinosaur confusion. Because Camptosaurus was one of the earliest ornithopods ever to be discovered, it suffered the fate of having more species pushed under its umbrella than it could comfortably handle. For this reason, it's now believed that only one identified fossil specimen was a true Camptosaurus; the others may well have been species of Iguanodon (which lived much later, during the Cretaceous period).

09 of 74 Cumnoria Wikimedia Commons Name: Cumnoria (after Cumnor Hirst, a hill in England); pronounced kum-NOOR-ee-ah Habitat: Woodlands of western Europe Historical Period: Late Jurassic (155 million years ago) Size and Weight: About 20 feet long and one ton Diet: Plants Distinguishing Characteristics: Stiff tail; bulky torso; quadrupedal posture An entire book can be written about the dinosaurs that were mistakenly classified as species of Iguanodon in the late 19th century. Cumnoria is a good example: when this ornithopod's "type fossil" was unearthed from England's Kimmeridge Clay Formation, it was assigned as an Iguanodon species by an Oxford paleontologist, in 1879 (at a time when the full extent of ornithopod diversity was not yet known). A few years later, Harry Seeley erected the new genus Cumnoria (after the hill where the bones were discovered), but he was overturned shortly thereafter by yet another paleontologist, who lumped Cumnoria in with Camptosaurus. The matter was finally settled over a century later, in 1998, when Cumnoria was once again granted its own genus after a reexamination of its remains.

10 of 74 Darwinsaurus Nobu Tamura Name: Darwinsaurus (Greek for "Darwin's lizard"); pronounced DAR-win-SORE-us Habitat: Woodlands of western Europe Historical Period: Early Cretaceous (140 million years ago) Size and Weight: About 20 feet long and 2-3 tons Diet: Plants Distinguishing Characteristics: Small head; bulky torso; occasional bipedal posture Darwinsaurus has come a long way since its fossil was described by the famous naturalist Richard Owen in 1842, following its discovery on the English coast. In 1889, this plant-eating dinosaur was assigned as a species of Iguanodon (not an uncommon fate for the newly discovered ornithopods of that time), and over a century later, in 2010, it was reassigned to the even more obscure genus Hypselospinus. Finally, in 2012, the paleontologist and illustrator Gregory Paul decided that this dinosaur's type fossil was distinctive enough to merit its own genus and species, Darwinsaurus evolutionis, though not all of his fellow experts are convinced.

11 of 74 Delapparentia Nobu Tamura Name: Delapparentia ("de Lapparent's lizard"); pronounced DAY-lap-ah-REN-tee-ah Habitat: Woodlands of western Europe Historical Period: Early Cretaceous (130-125 million years ago) Size and Weight: About 27 feet long and 4-5 tons Diet: Plants Distinguishing Characteristics: Large size; heavy trunk A close relative of Iguanodon—in fact, when this dinosaur's remains were discovered in Spain in 1958, they were initially assigned to Iguanodon bernissartensis—Delapparentia was even bigger than its more famous relative, about 27 feet from head to tail and weighing upwards of four or five tons. Delapparentia was only assigned its own genus in 2011, its name, oddly enough, honoring the paleontologist who misidentified the type fossil, Albert-Felix de Lapparent. Its twisted taxonomy aside, Delapparentia was a typical ornithopod of the early Cretaceous period, an ungainly looking plant-eater that may have been capable of running on its hind legs when startled by predators.

12 of 74 Dollodon Wikimedia Commons Name: Dollodon (Greek for "Dollo's tooth"); pronounced DOLL-oh-don Habitat: Woodlands of western Europe Historical Period: Early Cretaceous (130-125 million years ago) Size and Weight: About 20 feet long and one ton Diet: Plants Distinguishing Characteristics: Long, thick body; small head The euphonious-sounding Dollodon—named after the Belgian paleontologist Louis Dollo, and not because it looked like a child's doll—is another of those dinosaurs that had the misfortune to be lumped in as a species of Iguanodon in the late 19th century. Further examination of this ornithopod's remains resulted in its being assigned to its own genus; with its long, thick body and small, narrow head, there's no mistaking Dollodon's kinship to Iguanodon, but its relatively long arms and distinctively rounded beak peg it as its own dinosaur.

13 of 74 Drinker Wikimedia Commons Name: Drinker (after the American paleontologist Edward Drinker Cope) Habitat: Swamps of North Africa Historical Period: Late Jurassic (155 to 145 million years ago) Size and Weight: About six feet long and 25-50 pounds Diet: Plants Distinguishing Characteristics: Small size; flexible tail; complex tooth structure In the late 19th century, the American fossil hunters Edward Drinker Cope and Othniel C. Marsh were mortal enemies, constantly trying to one-up (and even sabotage) one another on their numerous paleontological digs. That's why it's ironic that the small, two-legged ornithopod Drinker (named after Cope) may be exactly the same animal as the small, two-legged ornithopod Othnielia (named after Marsh); the differences between these dinosaurs are so minimal that they may one day be collapsed into the same genus.

14 of 74 Dryosaurus Jura Park Name: Dryosaurus (Greek for "oak lizard"); pronounced DRY-oh-SORE-us Habitat: Woodlands of Africa and North America Historical Period: Late Jurassic (155-145 million years ago) Size and Weight: About 10 feet long and 200 pounds Diet: Plants Distinguishing Characteristics: Long neck; five-fingered hands; stiff tail In most ways, Dryosaurus (its name, "oak lizard," refers to the oak-leaf-like shape of some of its teeth) was a plain-vanilla ornithopod, typical in its small size, bipedal posture, stiff tail, and five-fingered hands. Like most ornithopods, Dryosaurus probably lived in herds, and this dinosaur may have raised its young at least halfway (that is, at least for a year or two after they hatched). Dryosaurus also had especially large eyes, which raises the possibility that it was a smidgen more intelligent than other herbivores of the late Jurassic period.

15 of 74 Dysalotosaurus Wikimedia Commons Name: Dysalotosaurus (Greek for "uncatchable lizard"); pronounced DISS-ah-LOW-toe-SORE-us Habitat: Woodlands of Africa Historical Period: Late Jurassic (150 million years ago) Size and Weight: About 15 feet long and 1,000-2,000 pounds Diet: Plants Distinguishing Characteristics: Long tail; bipedal stance; low-slung posture Considering how obscure it is, Dysalotosaurus has a lot to teach us about dinosaur growth stages. Various specimens of this medium-sized herbivore have been discovered in Africa, enough for paleontologists to conclude that a) Dysalotosaurus reached maturity in a relatively quick 10 years, b) this dinosaur was subject to viral infections of its skeleton, similar to Padget's disease, and c) the brain of Dysalotosaurus went through major structural changes between early childhood and maturity, though its auditory centers were well-developed early on. Otherwise, though, Dysalotosaurus was a plain-vanilla plant eater, indistinguishable from the other ornithopods of its time and place.

16 of 74 Echinodon Nobu Tamura Name: Echinodon (Greek for "hedgehog tooth"); pronounced eh-KIN-oh-don Habitat: Woodlands of western Europe Historical Period: Early Cretaceous (140 million years ago) Size and Weight: About two feet long and 5-10 pounds Diet: Plants Distinguishing Characteristics: Small size; paired canine teeth Ornithopods—the family of mostly small, mostly bipedal, and completely unfeathered herbivorous dinosaurs—are the last creatures you would expect to sport mammal-like canines in their jaws, the strange feature that makes Echinodon such an unusual fossil find. Like other ornithopods, Echinodon was a confirmed plant-eater, so this dental equipment is a bit of a mystery—but perhaps a bit less so once you realize this tiny dinosaur was related to the equally strangely toothed Heterodontosaurus (the "different toothed lizard"), and possibly to Fabrosaurus as well.

17 of 74 Elrhazosaurus Nobu Tamura Name: Elrhazosaurus (Greek for "Elrhaz lizard"); pronounced ell-RAZZ-oh-SORE-us Habitat: Woodlands of Africa Historical Period: Early Cretaceous (130-125 million years ago) Size and Weight: About four feet long and 20-25 pounds Diet: Plants Distinguishing Characteristics: Small size; bipedal posture Dinosaur fossils not only have a lot to tell us about local ecosystems but also about the distribution of the world's continents tens of millions of years ago, during the Mesozoic Era. Until recently, the early Cretaceous Elrhazosaurus—the bones of which were discovered in central Africa—was considered to be a species of a similar dinosaur, Valdosaurus, hinting at a land connection between these two continents. The assignment of Elrhazosaurus to its own genus has muddied the waters somewhat, though there's no disputing the kinship between these two bipedal, plant-eating, toddler-sized ornithopods.

18 of 74 Fabrosaurus Wikimedia Commons Name: Fabrosaurus (Greek for "Fabre's lizard"); pronounced FAB-roe-SORE-us Habitat: Woodlands of Africa Historical Period: Early Jurassic (200-190 million years ago) Size and Weight: About three feet long and 10-20 pounds Diet: Plants Distinguishing Characteristics: Small size; bipedal posture Fabrosaurus—named after the French geologist Jean Fabre—occupies a murky place in the annals of dinosaur history. This tiny, two-legged, plant-eating ornithopod was "diagnosed" based on a single incomplete skull, and many paleontologists believe that it was actually a species (or specimen) of another herbivorous dinosaur from early Jurassic Africa, Lesothosaurus. Fabrosaurus (if it really existed as such) may also have been ancestral to a slightly later ornithopod of eastern Asia, Xiaosaurus. Any more conclusive determination of its status will have to await future fossil discoveries.

19 of 74 Fukuisaurus Name: Fukuisaurus (Greek for "Fukui lizard"); pronounced FOO-kwee-SORE-us Habitat: Woodlands of Asia Historical Period: Early Cretaceous (110 million years ago) Size and Weight: About 15 feet long and 750-1,000 pounds Diet: Plants Distinguishing Characteristics: Long, thick body; narrow head Not to be confused with Fukuiraptor—a moderately sized theropod discovered in the same region of Japan—Fukuisaurus was a moderately sized ornithopod that probably resembled (and was closely related to) the much better-known Iguanodon from Eurasia and North America. Since they lived at roughly the same time, the early to middle Cretaceous period, it's possible that Fukuisaurus figured on Fukuiraptor's lunch menu, but as yet there's no direct evidence for this--and because ornithopods are so rare on the ground in Japan, it's difficult to establish Fukuisaurus' exact evolutionary provenance.

20 of 74 Gasparinisaura Wikimedia Commons Name: Gasparinisaura (Greek for "Gasparini’s lizard"); pronounced GAS-par-EE-knee-SORE-ah Habitat: Woodlands of South America Historical Period: Late Cretaceous (90-85 million years ago) Size and Weight: About three feet long and 50 pounds Diet: Plants Distinguishing Characteristics: Small size; short, blunt head About the size and weight of a typical second-grader, Gasparinisaura is important because it's one of the few ornithopod dinosaurs known to have lived in South America during the late Cretaceous period. Judging by the discovery of numerous fossil remains in the same area, this small plant-eater probably lived in herds, which helped protect it from the larger predators in its ecosystem (as did its ability to run away very quickly when threatened). As you may have noticed, Gasparinisaura is one of the few dinosaurs to be named after the female, rather than the male, of the species, an honor it shares with Maiasaura and Leaellynasaura.

21 of 74 Gideonmantellia Nobu Tamura Name: Gideonmantellia (after naturalist Gideon Mantell); pronounced GIH-dee-on-man-TELL-ee-ah Habitat: Woodlands of western Europe Historical Period: Early Cretaceous (130-125 million years ago) Size and Weight: Unknown Diet: Plants Distinguishing Characteristics: Slender build; bipedal posture When the name Gideonmantellia was coined in 2006, the 19th-century naturalist Gideon Mantell became one of the few people to have not one, not two, but three dinosaurs named after him, the others being Mantellisaurus and the somewhat more dubious Mantellodon. Confusingly, Gideonmantellia and Mantellisaurus lived at about the same time (the early Cretaceous period) and in the same ecosystem (the woodlands of western Europe), and they're both classified as ornithopods closely related to Iguanodon. Why does Gideon Mantell deserve this double honor? Well, in his own lifetime, he was overshadowed by more powerful and self-centered paleontologists like Richard Owen, and modern researchers feel that he's been unjustly overlooked by history.

22 of 74 Haya Nobu Tamura Name: Haya (after a Mongolian deity); pronounced HI-yah Habitat: Woodlands of central Asia Historical Period: Late Cretaceous (85 million years ago) Size and Weight: About five feet long and 50 pounds Diet: Plants Distinguishing Characteristics: Small size; bipedal posture Compared to other parts of the world, very few "basal" ornithopods—small, bipedal, plant-eating dinosaurs—have been identified in Asia (one notable exception is the early Cretaceous Jeholosaurus, which weighed about 100 pounds soaking wet). That's why the discovery of Haya made such big news: this lightweight ornithopod lived during the late Cretaceous period, about 85 million years ago, in an area of central Asia corresponding to modern-day Mongolia. (Still, we can't tell whether the paucity of basal ornithopods is because they were indeed rare animals, or just didn't fossilize all that well). Haya is also one of the few ornithopods known to have swallowed gastroliths, stones that helped grind down vegetable matter in this dinosaur's stomach.

23 of 74 Heterodontosaurus Wikimedia Commons Name: Heterodontosaurus (Greek for "different-toothed lizard"); pronounced HET-er-oh-DON-toe-SORE-us Habitat: Scrublands of South Africa Historical Period: Early Jurassic (200-190 million years ago) Size and Weight: About three feet long and 5-10 pounds Diet: Probably omnivorous Distinguishing Characteristics: Small size; three different types of teeth in the jaw The name Heterodontosaurus is a mouthful, in more ways than one. This tiny ornithopod earned its moniker, which means "different-toothed lizard," thanks to its three distinct kinds of teeth: incisors (for slicing through vegetation) on the upper jaw, chisel-shaped teeth (for grinding said vegetation) further back, and two pairs of tusks jutting out from the upper and lower lip. From an evolutionary point of view, Heterodontosaurus' incisors and molars are easy to explain. The tusks pose more of a problem: some experts think these were only found on males and were thus a sexually selected characteristic (meaning female Heterodontosaurus were more inclined to mate with big-tusked males). However, it's also possible that both males and females had these tusks, and used them to intimidate predators. The recent discovery of a juvenile Heterodontosaurus bearing a full set of canines has shed more light on this issue. It's now believed that this tiny dinosaur may have been omnivorous, supplementing its largely vegetarian diet with the occasional small mammal or lizard.

24 of 74 Hexinlusaurus Joao Boto Name: Hexinlusaurus ("He Xin-Lu's lizard"); pronounced HAY-zhin-loo-SORE-us Habitat: Woodlands of Asia Historical Period: Middle Jurassic (175 million years ago) Size and Weight: About five feet long and 25 pounds Diet: Plants Distinguishing Characteristics: Small size; bipedal posture It has proven difficult to classify the early, or "basal," ornithopods of middle Jurassic China, most of which looked alike. Hexinlusaurus (named after a Chinese professor) was until recently classified as a species of the equally obscure Yandusaurus, and both of these plant-eaters had traits in common with Agilisaurus (in fact, some paleontologists believe that the diagnostic specimen of Hexinlusaurus was really a juvenile of this better-known genus). Wherever you choose to place it on the dinosaur family tree, Hexinlusaurus was a small, skittery reptile that ran on two legs to avoid being eaten by larger theropods.

25 of 74 Hippodraco Lukas Panzarin Name: Hippodraco (Greek for "horse dragon"); pronounced HIP-oh-DRAKE-oh Habitat: Woodlands of North America Historical Period: Early Cretaceous (125 million years ago) Size and Weight: About 15 feet long and half a ton Diet: Plants Distinguishing Characteristics: Bulky body; small head; occasional bipedal posture One of a pair of ornithopod dinosaurs recently unearthed in Utah—the other being the impressively named Iguanacolossus—Hippodraco, the "horse dragon," was on the small side for an Iguanodon relative, only about 15 feet long and half a ton (which may be a clue that the sole, incomplete specimen is of a juvenile rather than a full-grown adult). Dating to the early Cretaceous period, about 125 million years ago, Hippodraco appears to have been a comparatively "basal" iguanodont whose closest relative was the slightly later (and still extremely obscure) Theiophytalia.

26 of 74 Huxleysaurus Nobu Tamura Name: Huxleysaurus (after the biologist Thomas Henry Huxley); pronounced HUCKS-lee-SORE-us Habitat: Woodlands of western Europe Historical Period: Early Cretaceous (140 million years ago) Size and Weight: Unknown Diet: Plants Distinguishing Characteristics: Narrow snout; stiff tail; bipedal posture During the 19th century, a huge number of ornithopods were classified as species of Iguanodon, and then promptly consigned to the fringes of paleontology. In 2012, Gregory S. Paul rescued one of these forgotten species, Iguanodon hollingtoniensis, and elevated it to genus status under the name Huxleysaurus (honoring Thomas Henry Huxley, one of the first devoted defenders of Charles Darwin's theory of evolution). A couple of years earlier, in 2010, another scientist had "synonymized" I. hollingtoniensis with Hypselospinus, so as you can imagine, the ultimate fate of Huxleysaurus is still up in the air.

27 of 74 Hypselospinus Nobu Tamura Name: Hypselospinus (Greek for "high spine"); pronounced HIP-sell-oh-SPY-nuss Habitat: Woodlands of western Europe Historical Period: Early Cretaceous (140 million years ago) Size and Weight: About 20 feet long and 2-3 tons Diet: Plants Distinguishing Characteristics: Long, stiff tail; bulky torso Hypselospinus is just one of many dinosaurs that started its taxonomic life as a species of Iguanodon (since Iguanodon was discovered so early in the history of modern paleontology, it became a "wastebasket genus" to which many poorly understood dinosaurs were assigned). Classified as Iguanodon fittoni in 1889, by Richard Lydekker, this ornithopod lumbered in obscurity for well over 100 years, until a re-examination of its remains in 2010 prompted the creation of a new genus. Otherwise very similar to Iguanodon, the early Cretaceous Hypselospinus was distinguished by the short vertebral spines along its upper back, which likely supported a flexible flap of skin.

28 of 74 Hypsilophodon Wikimedia Commons The type fossil of Hypsilophodon was discovered in England in 1849, but it wasn't until 20 years later that the bones were recognized as belonging to an entirely new genus of ornithopod dinosaur, and not to a juvenile Iguanodon.

29 of 74 Iguanacolossus Lukas Panzarin Name: Iguanacolossus (Greek for "colossal iguana"); pronounced ih-GWA-no-coe-LAH-suss Habitat: Woodlands of North America Historical Period: Early Cretaceous (130-125 million years ago) Size and Weight: About 30 feet long and 2-3 tons Diet: Plants Distinguishing Characteristics: Large size; long, thick trunk and tail One of the more imaginatively named ornithopod dinosaurs of the early Cretaceous period, Iguanacolossus was recently discovered in Utah alongside the slightly later, and much smaller, Hippodraco. (As you might have guessed, the "iguana" in this dinosaur's name refers to its more famous, and comparatively more advanced, relative Iguanodon, and not to modern iguanas.) The most impressive thing about Iguanacolossus was its sheer bulk; at 30 feet long and 2 to 3 tons, this dinosaur would have been one of the biggest non-titanosaur plant-eaters of its North American ecosystem.

30 of 74 Iguanodon Jura Park The fossils of the ornithopod dinosaur Iguanodon have been discovered as far afield as Asia, Europe, and North America, but it's unclear exactly how many individual species there were—and how closely related they are to other ornithopod genera.

31 of 74 Jeholosaurus Wikimedia Commons Name: Jeholosaurus (Greek for "Jehol lizard"); pronounced jeh-HOE-lo-SORE-us Habitat: Woodlands of Asia Historical Period: Early Cretaceous (130-125 million years ago) Size and Weight: About six feet long and 100 pounds Diet: Possibly omnivorous Distinguishing Characteristics: Small size; sharp front teeth There's something about prehistoric reptiles named after the Jehol region of northern China that occasions controversy. Jeholopterus, a genus of pterosaur, has been reconstructed by one scientist as having fangs, and possibly sucking the blood of bigger dinosaurs (granted, very few people in the scientific community subscribe to this hypothesis). Jeholosaurus, a small, ornithopod dinosaur, also possessed some odd dentition—sharp, carnivore-like teeth in the front of its mouth and blunt, herbivore-like grinders in the back. In fact, some paleontologists speculate that this presumed close relative of Hypsilophodon may have pursued an omnivorous diet, a startling adaptation (if true) since the vast majority of ornithischian dinosaurs were strict vegetarians.

32 of 74 Jeyawati Lukas Panzarin Name: Jeyawati (Zuni Indian for "grinding mouth"); pronounced HEY-ah-WATT-ee Habitat: Woodlands of western North America Historical Period: Middle-Late Cretaceous (95-90 million years ago) Size and Weight: About 20 feet long and 1,000-2,000 pounds Diet: Plants Distinguishing Characteristics: Wrinkly growths around eyes; sophisticated teeth and jaws The hadrosaurs (duck-billed dinosaurs), the most abundant herbivores by the end of the Cretaceous period, were part of the larger dinosaur breed known as ornithopods--and the line between the most advanced ornithopods and the earliest hadrosaurs is very fuzzy indeed. If you only examined its head, you might mistake Jeyawati for a true hadrosaur, but subtle details of its anatomy have placed it in the ornithopod camp—more specifically, paleontologists believe Jeyawati was an iguanodont dinosaur, and thus closely related to Iguanodon. However you choose to classify it, Jeyawati was a medium-sized, mostly bipedal plant-eater distinguished by its sophisticated dental apparatus (which was well-suited to grinding down the tough vegetable matter of the middle Cretaceous) and the strange, wrinkled ridges around its eye sockets. As so often happens, the partial fossil of this dinosaur was unearthed in 1996, in New Mexico, but it wasn't until 2010 that paleontologists finally got around to "diagnosing" this new genus.

33 of 74 Koreanosaurus Nobu Tamura Name: Koreanosaurus (Greek for "Korean lizard"); pronounced core-REE-ah-no-SORE-us Habitat: Woodlands of southeast Asia Historical Period: Late Cretaceous (85-65 million years ago) Size and Weight: Unknown Diet: Plants Distinguishing Characteristics: Long tail; bipedal posture; longer hind than front legs One doesn't normally associate South Korea with major dinosaur discoveries, so you may be surprised to learn that Koreanosaurus is represented by no less than three separate (but incomplete) fossil specimens, discovered in this country's Seonso Conglomerate in 2003. To date, not a lot has been published about Koreanosaurus, which seems to have been a classic, small-bodied ornithopod of the late Cretaceous period, perhaps closely related to Jeholosaurus and perhaps (though this is far from proven) a burrowing dinosaur along the lines of the better-known Oryctodromeus.

34 of 74 Kukufeldia Wikimedia Commons Name: Kukufeldia (Old English for "cuckoo's field"); pronounced COO-coo-FELL-dee-ah Habitat: Woodlands of western Europe Historical Period: Early Cretaceous (135-125 million years ago) Size and Weight: About 30 feet long and 2-3 tons Diet: Plants Distinguishing Characteristics: Narrow snout; longer hind than front legs You could write an entire book about all the dinosaurs that were once mistaken for Iguanodon (or, rather, assigned to this genus by the puzzled paleontologists of the 19th century, such as Gideon Mantell). For well over a hundred years, Kukufeldia was classified as a species of Iguanodon, on the evidence of single fossilized jaw housed at the London Natural History Museum. That all changed in 2010 when a student inspecting the jaw noticed some subtle anatomical peculiarities and convinced the scientific community to erect the new ornithopod genus Kukufeldia ("cuckoo's field," after the Old English name for the locality where the jaw was discovered).

35 of 74 Kulindadromeus Andrey Atuchin Name: Kulindadromeus (Greek for "Kulinda runner"); pronounced coo-LIN-dah-DROE-mee-us Habitat: Plains of northern Asia Historical Period: Late Jurassic (160 million years ago) Size and Weight: About 4-5 feet long and 20-30 pounds Diet: Plants Distinguishing Characteristics: Small size; bipedal posture; feathers Despite what you may have read in the popular media, Kulindadromeus isn't the first identified ornithopod dinosaur to possess feathers: that honor belongs to Tianyulong, which was discovered in China a few years ago. But whereas the fossilized feather-like imprints of Tianyulong were open to at least some interpretation, there's no doubting the existence of feathers in the late Jurassic Kulindadromeus, the existence of which implies that feathers were much more widespread in the dinosaur kingdom than had been previously believed (the vast majority of feathered dinosaurs were theropods, from which birds are thought to have evolved).

36 of 74 Lanzhousaurus Wikimedia Commons Name: Lanzhousaurus (Greek for "Lanzhou lizard"); pronounced LAN-zhoo-SORE-us Habitat: Woodlands of Asia Historical Period: Early Cretaceous (120-110 million years ago) Size and Weight: About 30 feet long and five tons Diet: Plants Distinguishing Characteristics: Large size; enormous teeth When its partial remains were discovered in China in 2005, Lanzhousaurus caused a stir for two reasons. First, this dinosaur measured an enormous 30 feet in length, making it one of the biggest ornithopods before the rise of the hadrosaurs in the late Cretaceous period. And second, at least some of this dinosaur's teeth were equally enormous: with choppers up to 14 centimeters long (in a meter-long lower jaw), Lanzhousaurus may be the longest-toothed herbivorous dinosaur that ever lived. Lanzhousaurus appears to have been closely related to Lurdusaurus, another giant ornithopod from central Africa—a strong hint that dinosaurs migrated from Africa to Eurasia (and vice-versa) during the early Cretaceous.

37 of 74 Laosaurus Wikimedia Commons Name: Laosaurus (Greek for "fossil lizard"); pronounced LAY-oh-SORE-us Habitat: Woodlands of North America Historical Period: Late Jurassic (160-150 million years ago) Size and Weight: Unknown Diet: Plants Distinguishing Characteristics: Slender build; bipedal posture At the height of the Bone Wars, in the late 19th century, new dinosaurs were being named faster than convincing fossil evidence could be gathered to support them. A good example is Laosaurus, which was erected by the famous paleontologist Othniel C. Marsh on the basis of a handful of vertebrae discovered in Wyoming. (Soon after, Marsh created two new Laosaurus species, but then reconsidered and assigned one specimen to the genus Dryosaurus.) After decades of further confusion—in which species of Laosaurus were transferred to, or were considered for inclusion under, Orodromeus and Othnielia—this late Jurassic ornithopod lapsed into obscurity and is today considered a nomen dubium.

38 of 74 Laquintasaura Mark Witton Name: Laquintasaura ("La Quinta lizard"); pronounced la-KWIN-tah-SORE-ah Habitat: Woodlands of South America Historical Period: Early Jurassic (200 million years ago) Size and Weight: About three feet long and 10 pounds Diet: Plants; possibly insects as well Distinguishing Characteristics: Small size; bipedal posture; distinctively serrated teeth The first plant-eating dinosaur ever to be discovered in Venezuela--and only the second dinosaur, period, since it was announced at the same time as the meat-eating Tachiraptor—Laquintasaura was a tiny ornithischian that prospered shortly after the Triassic/Jurassic boundary, 200 million years ago. What this means is that Laquintasaura was only recently evolved from its carnivorous ancestors (the first dinosaurs that sprang up in South America 30 million years before)—which may explain the odd shape of this dinosaur's teeth, which seem to have been equally suited to scarfing down small insects and animals as well as the usual diet of ferns and leaves.

39 of 74 Leaellynasaura Australia National Dinosaur Museum If the name Leaellynasaura seems odd, that's because this is one of the few dinosaurs to be named after a living person: the daughter of Australian paleontologists Thomas Rich and Patricia Vickers-Rich, who discovered this ornithopod in 1989.

40 of 74 Lesothosaurus Getty Images Lesothosaurus may or may not have been the same dinosaur as Fabrosaurus (the remains of which were discovered much earlier), and it may also have been ancestral to the equally obscure Xiaosaurus, yet another tiny ornithopod native to Asia.

41 of 74 Lurdusaurus Nobu Tamura Name: Lurdusaurus (Greek for "heavy lizard"); pronounced LORE-duh-SORE-us Habitat: Woodlands of Africa Historical Period: Early Cretaceous (120-110 million years ago) Size and Weight: About 30 feet long and six tons Diet: Plants Distinguishing Characteristics: Long neck; low-lying trunk with a short tail Lurdusaurus is one of those dinosaurs that shakes paleontologists out of their complacency. When its remains were discovered in central Africa in 1999, this herbivore's huge size upset longstanding notions about ornithopod evolution (that is, that the "small" ornithopods of the Jurassic and early Cretaceous periods gradually gave way to the "big" ornithopods, i.e. hadrosaurs, of the late Cretaceous). At 30 feet long and 6 tons, Lurdusaurus (and its equally gigantic sister genus, Lanzhousaurus, which was discovered in China in 2005) approached the bulk of the largest known hadrosaur, Shantungosaurus, which lived 40 million years later.

42 of 74 Lycorhinus Getty Images Name: Lycorhinus (Greek for "wolf snout"); pronounced LIE-coe-RYE-nuss Habitat: Woodlands of southern Africa Historical Period: Early Jurassic (200 million years ago) Size and Weight: About four feet long and 50 pounds Diet: Plants Distinguishing Characteristics: Small size; occasional bipedal posture; large canine teeth As you may have guessed from its name—Greek for "wolf snout"—Lycorhinus wasn't identified as a dinosaur when its remains were first discovered way back in 1924, but as a therapsid, or "mammal-like reptile" (this was the branch of non-dinosaur reptiles that eventually evolved into true mammals during the course of the Triassic period). It took nearly 40 years for paleontologists to recognize Lycorhinus as an early ornithopod dinosaur closely related to Heterodontosaurus, with which it shared some strangely shaped teeth (notably the two pairs of oversized canines in front of its jaws).

43 of 74 Macrogryphosaurus BBC Name: Macrogryphosaurus (Greek for "big enigmatic lizard"); pronounced MACK-roe-GRIFF-oh-SORE-us Habitat: Woodlands of South America Historical Period: Late Cretaceous (90 million years ago) Size and Weight: About 20 feet long and 1-2 tons Diet: Plants Distinguishing Characteristics: Narrow skull; squat trunk; longer hind than front legs You've got to admire any dinosaur whose name translates as "big enigmatic lizard"—a view apparently shared by the producers of the BBC series "Walking with Dinosaurs," who once gave Macrogryphosaurus a small cameo. One of the rare ornithopods to be discovered in South America, Macrogryphosaurus seems to have been closely related to the equally obscure Talenkauen and is classified as a "basal" iguanodont. Since the type fossil is of a juvenile, no one is quite sure how big Macrogryphosaurus adults were, although three or four tons is not out of the question.

44 of 74 Manidens Nobu Tamura Name: Manidens (Greek for "hand tooth"); pronounced MAN-ih-denz Habitat: Woodlands of South America Historical Period: Middle Jurassic (170-165 million years ago) Size and Weight: About 2-3 feet long and 5-10 pounds Diet: Plants; possibly omnivorous Distinguishing Characteristics: Small size; prominent teeth; bipedal posture The heterodontosaurids—the family of ornithopod dinosaurs epitomized by, you guessed it, Heterodontosaurus—were some of the strangest and most poorly understood dinosaurs of the early to middle Jurassic period. The recently discovered Manidens ("hand tooth") lived a few million years after Heterodontosaurus, but (judging by its strange dentition) it seems to have pursued roughly the same lifestyle, possibly including an omnivorous diet. As a rule, heterodontosaurids were fairly small (the largest example of the genus, Lycorhinus, didn't exceed 50 pounds soaking wet), and it's likely that they had to adapt their diets to their close-to-the-ground position in the dinosaur food chain.

45 of 74 Mantellisaurus Wikimedia Commons Name: Mantellisaurus (Greek for "Mantell's lizard"); pronounced man-TELL-ih-SORE-us Habitat: Woodlands of western Europe Historical Period: Early Cretaceous (135-125 million years ago) Size and Weight: About 30 feet long and 3 tons Diet: Plants Distinguishing Characteristics: Long, flat head; streamlined body Well into the twenty-first century, paleontologists are still clearing up the confusion created by their well-meaning predecessors of the 1800s. A good example is Mantellisaurus, which until 2006 was classified as a species of Iguanodon—primarily because Iguanodon was discovered so early in the history of paleontology (way back in 1822) that every dinosaur that looked remotely like it was assigned to its genus.

46 of 74 Mantellodon Wikimedia Commons Name: Mantellodon (Greek for "Mantell's tooth"); pronounced man-TELL-oh-don Habitat: Woodlands of western Europe Historical Period: Early Cretaceous (135-125 million years ago) Size and Weight: About 30 feet long and three tons Diet: Plants Distinguishing Characteristics: Spiked thumbs; bipedal posture Gideon Mantell was often ignored in his time (notably by the famous paleontologist Richard Owen), but today he has no less than three dinosaurs named after him: Gideonmantellia, Mantellisaurus, and (the most dubious of the bunch) Mantellodon. In 2012, Gregory Paul "rescued" Mantellodon from Iguanodon, where it had previously been assigned as a separate species, and raised it to genus status. The trouble is, there's significant disagreement about whether Mantellodon merits this distinction; at least one scientist insists it should properly be assigned as a species of the Iguanodon-like ornithopod Mantellisaurus.

47 of 74 Mochlodon Magyar Dinosaurs Name: Mochlodon (Greek for "bar tooth"); pronounced MOCK-low-don Habitat: Woodlands of western Europe Historical Period: Late Cretaceous (75-70 million years ago) Size and Weight: About 10 feet long and 500 pounds Diet: Plants Distinguishing Characteristics: Moderate size; bipedal posture As a general rule, any dinosaur that was ever classified as a species of Iguanodon has had a complicated taxonomic history. One of the few dinosaurs to be discovered in modern-day Austria, Mochlodon was designated as Iguanodon suessii in 1871, but it soon became clear that this was a much more petite ornithopod that deserved its own genus, created by Harry Seeley in 1881. A few years later, one Mochlodon species was referred to the better-known Rhabdodon, and in 2003, another was split off into the new genus Zalmoxes. Today, so little is left of the original Mochlodon that it's widely considered a nomen dubium, though some paleontologists continue to use the name.

48 of 74 Muttaburrasaurus Wikimedia Commons Thanks to the discovery of an almost-complete skeleton in Australia, paleontologists know more about the skull of Muttaburrasaurus than they do about the noggin of almost any other ornithopod dinosaur.

49 of 74 Nanyangosaurus Mariana Ruiz Name: Nanyangosaurus (Greek for "Nanyang lizard"); pronounced nan-YANG-oh-SORE-us Habitat: Woodlands of eastern Asia Historical Period: Middle Cretaceous (110-100 million years ago) Size and Weight: About 12 feet long and 1,000 pounds Diet: Plants Distinguishing Characteristics: Moderate size; long arms and hands In the course of the early Cretaceous period, the largest and most advanced ornithopods (typified by Iguanodon) began to evolve into the very first hadrosaurs, or duck-billed dinosaurs. Dating to about 100 million years ago, Nanyangosaurus has been classified as an iguanodontid ornithopod laying near (or at) the base of the hadrosaur family tree. Specifically, this plant-eater was significantly smaller than later duckbills (only about 12 feet long and half a ton), and may already have lost the prominent thumb spikes that characterized other iguanodont dinosaurs.

50 of 74 Orodromeus Wikimedia Commons Name: Orodromeus (Greek for "mountain runner"); pronounced ORE-oh-DROME-ee-us Habitat: Woodlands of North America Historical Period: Late Cretaceous (75 million years ago) Size and Weight: About eight feet long and 50 pounds Diet: Plants Distinguishing Characteristics: Small size; bipedal posture One of the smallest ornithopods of the late Cretaceous period, Orodromeus was the subject of an understandable goof by paleontologists. When this plant-eater's remains were first discovered, in a fossilized nesting ground in Montana known as "Egg Mountain," their proximity to a clutch of eggs prompted the conclusion that those eggs belonged to Orodromeus. We now know that the eggs were really laid by a female Troodon, which also lived on Egg Mountain—the inescapable conclusion being that Orodromeus was hunted by these slightly larger, but much smarter, theropod dinosaurs.

51 of 74 Oryctodromeus Joao Boto Name: Oryctodromeus (Greek for "burrowing runner"); pronounced or-RICK-toe-DROE-mee-us Habitat: Woodlands of North America Historical Period: Middle Cretaceous (95 million years ago) Size and Weight: About six feet long and 50-100 pounds Diet: Plants Distinguishing Characteristics: Small size; burrowing behavior A small, swift dinosaur closely related to Hypsilophodon, Oryctodromeus is the only ornithopod proven to have lived in burrows—that is, the adults of this genus dug deep holes in the forest floor, where they hid from predators and (probably) laid their eggs. Oddly enough, though, Oryctodromeus didn't have the type of elongated, specialized hands and arms one would expect in a digging animal; paleontologists speculate that it may have used its pointed snout as a supplementary tool. Another clue to the specialized lifestyle of Oryctodromeus is that this dinosaur's tail was relatively flexible compared to those of other ornithopods, so it could more easily have curled up in its underground burrows.

52 of 74 Othnielia Wikimedia Commons Name: Othnielia (after the 19th-century paleontologist Othniel C. Marsh); pronounced OTH-nee-ELL-ee-ah Habitat: Plains of western North America Historical Period: Late Jurassic (155-145 million years ago) Size and Weight: About four feet long and 50 pounds Diet: Plants Distinguishing Characteristics: Small size; thin legs; long, stiff tail The slim, fast, two-legged Othnielia was named after the famous paleontologist Othniel C. Marsh—not by Marsh himself (who lived in the 19th century), but by a tribute-paying paleontologist in 1977. (Oddly, Othnielia is very similar to Drinker, another small, Jurassic plant-eater named after Marsh's arch-nemesis Edward Drinker Cope.) In many ways, Othnielia was a typical ornithopod of the late Jurassic period. This dinosaur may have lived in herds, and it certainly figured on the dinner menu of the larger, carnivorous theropods of its day—which goes a long way toward explaining its presumed speed and agility.

53 of 74 Othnielosaurus Wikimedia Commons Name: Othnielosaurus ("Othniel's lizard"); pronounced OTH-nee-ELL-oh-SORE-us Habitat: Woodlands of North America Historical Period: Late Jurassic (155-150 million years ago) Size and Weight: About six feet long and 20-25 pounds Diet: Plants Distinguishing Characteristics: Slender build; bipedal posture Considering how famous and talented they were, Othniel C. Marsh and Edward Drinker Cope left a lot of damage in his wake, which has taken over a century to clean up. Othnielosaurus was erected in the 20th century to house the homeless remnants of a series of plant-eating dinosaurs named by Marsh and Cope during the late 19th-century Bone Wars, often on the basis of insufficient evidence, including Othnielia, Laosaurus, and Nanosaurus. As definitive as a genus can get, given the vast reams of confusion that preceded it, Othnielosaurus was a small, bipedal, herbivorous dinosaur closely related to Hypsilophodon, and was certainly hunted and eaten by the larger theropods of its North American ecosystem.

54 of 74 Parksosaurus Wikimedia Commons Name: Parksosaurus (after paleontologist William Parks); pronounced PARK-so-SORE-us Habitat: Woodlands of North America Historical Period: Late Cretaceous (70 million years ago) Size and Weight: About five feet long and 75 pounds Diet: Plants Distinguishing Characteristics: Small size; bipedal posture Since hadrosaurs (duck-billed dinosaurs) evolved from smaller ornithopods, you might be forgiven for thinking that most of the ornithopods of the late Cretaceous period were duckbills. Parksosaurus counts as evidence to the contrary: this five-foot-long, 75-pound plant muncher was way too small to count as a hadrosaur, and is one of the latest identified ornithopods from the time shortly before the dinosaurs went extinct. For over half a century, Parksosaurus was identified as a species of Thescelosaurus (T. warreni), until a re-examination of its remains cemented its kinship with smaller ornithopod dinosaurs like Hypsilophodon.

55 of 74 Pegomastax Tyler Keillor The stubby, spiny Pegomastax was an odd-looking dinosaur, even by the standards of the early Mesozoic Era, and (depending on the artist that illustrates it) it may have been one of the ugliest ornithopods that ever lived.

56 of 74 Pisanosaurus Wikimedia Commons Name: Pisanosaurus (Greek for "Pisano's lizard"): pronounced pih-SAHN-oh-SORE-us Habitat: Woodlands of South America Historical Period: Late Triassic (220 million years ago) Size and Weight: About three feet long and 15 pounds Diet: Plants Distinguishing Characteristics: Small size; probably long tail Few issues in paleontology are more complicated than when, exactly, the first dinosaurs split off into the two major dinosaur families: ornithischian ("bird-hipped") and saurischian ("lizard-hipped") dinosaurs. What makes Pisanosaurus such an unusual discovery is that it was apparently an ornithischian dinosaur that lived 220 million years ago in South America, at the same time as early theropods like Eoraptor and Herrerasaurus (which would push the ornithischian line to millions of years earlier than had been previously believed). Further complicating matters, Pisanosaurus possessed an ornithischian-style head perched atop a saurischian-style body. Its closest relative seems to have been the southern African Eocursor, which may have pursued an omnivorous diet.

57 of 74 Planicoxa Wikimedia Commons Name: Planicoxa (Greek for "flat ilium"); pronounced PLAN-ih-COK-sah Habitat: Woodlands of North America Historical Period: Early Cretaceous (125 million years ago) Size and Weight: About 18 feet long and 1-2 tons Diet: Plants Distinguishing Characteristics: Squat torso; occasional bipedal posture The large theropods of early Cretaceous North America, about 125 million years ago, needed a reliable source of prey, and no prey was more reliable than squat, bulky, ungainly ornithopods like Planicoxa. This "iguanodontid" ornithopod (so named because it was closely related to Iguanodon) wasn't entirely defenseless, especially when full grown, but it must have been quite a sight when it bolted away from predators on two feet after quietly grazing in its usual quadrupedal posture. One species of a related ornithopod, Camptosaurus, has been assigned to Planicoxa, while one Planicoxa species has since been stripped away to erect the genus Osmakasaurus.

58 of 74 Proa Nobu Tamura Name: Proa (Greek for "prow"); pronounced PRO-ah Habitat: Woodlands of western Europe Historical Period: Early Cretaceous (110 million years ago) Size and Weight: About 20 feet long and one ton Diet: Plants Distinguishing Characteristics: Squat torso; small head; occasional bipedal posture Not a week goes by, it seems, without somebody, somewhere, discovering yet another iguanodont ornithopod of the middle Cretaceous period. The fragmented fossils of Proa were unearthed in Spain's Teruel Province a few years ago; the oddly shaped "predentary" bone in this dinosaur's lower jaw inspired its name, which is Greek for "prow." All we know for sure about Proa is that it was a classic ornithopod, similar in appearance to Iguanodon and literally dozens of other genera, whose main function was to serve as a reliable food source for hungry raptors and tyrannosaurs.

59 of 74 Protohadros Karen Carr Name: Protohadros (Greek for "first hadrosaur"); pronounced PRO-to-HAY-dross Habitat: Woodlands of North America Historical Period: Late Cretaceous (95 million years ago) Size and Weight: About 25 feet long and 1-2 tons Diet: Plants Distinguishing Characteristics: Small head; bulky torso; occasional bipedal posture As with so many evolutionary transitions, there wasn't a single "aha!" moment when the most advanced ornithopods evolved into the first hadrosaurs, or duck-billed dinosaurs. In the late 1990's, Protohadros was touted by its discoverer as the first-ever hadrosaur, and its name reflects his confidence in this assessment. Other paleontologists, however, are less certain, and have since concluded that Protohadros was an iguanodontid ornithopod, almost, but not quite, on the cusp of being a true duckbill. Not only is this a more sober assessment of the evidence, but it leaves intact the current theory that the first true hadrosaurs evolved in Asia rather than North America (the type specimen of Protohadros was unearthed in Texas.)

60 of 74 Qantassaurus Wikimedia Commons The tiny, big-eyed ornithopod Qantassaurus lived in Australia when that continent was much farther south than it is today, meaning it thrived in cold, wintry conditions that would have killed most dinosaurs.

61 of 74 Rhabdodon Alain Beneteau Name: Rhabdodon (Greek for "rod tooth"); pronounced RAB-doe-don Habitat: Woodlands of western Europe Historical Period: Late Cretaceous (75 million years ago) Size and Weight: About 12 feet long and 250-500 pounds Diet: Plants Distinguishing Characteristics: Blunt head; large, rod-shaped teeth Ornithopods were some of the most common dinosaurs unearthed in the 19th century, mainly because so many of them lived in Europe (where paleontology was pretty much invented back in the 18th and 19th centuries). Discovered in 1869, Rhabdodon has yet to be properly classified, since (not to get too technical) it shares some of the characteristics of two types of ornithopods: iguanodonts (herbivorous dinosaurs similar in size and build to Iguanodon) and hypsilophodonts (dinosaurs similar to, you guessed it, Hypsilophodon). Rhabdodon was a fairly small ornithopod for its time and place; its most notable features were its roundish teeth and its unusually blunt head.

62 of 74 Siamodon Wikimedia Commons Name: Siamodon (Greek for "Siamese tooth"); pronounced sie-AM-oh-don Habitat: Woodlands of Asia Historical Period: Early Cretaceous (110-100 million years ago) Size and Weight: About 20 feet long and 1-2 tons Diet: Plants Distinguishing Characteristics: Small head; thick tail; occasional bipedal posture Ornithopods, like titanosaurs, had a worldwide distribution during the middle to late Cretaceous period. The importance of Siamodon is that it's one of the few dinosaurs to be discovered in modern-day Thailand (a country that used to be known as Siam)--and, like its close cousin Probactrosaurus, it lay close to the evolutionary juncture when the first true hadrosaurs branched off from their ornithopod forebears. To date, Siamodon is known from only a single tooth and a fossilized braincase; further discoveries should shed additional light on its appearance and lifestyle.

63 of 74 Talenkauen Nobu Tamura Name: Talenkauen (indigenous for "small skull"); pronounced TA-len-cow-en Habitat: Woodlands of South America Historical Period: Late Cretaceous (70-65 million years ago) Size and Weight: About 15 feet long and 500-750 pounds Diet: Plants Distinguishing Characteristics: Moderate size; small head Ornithopods—small, herbivorous, bipedal dinosaurs—were sparse on the ground in late Cretaceous South America, with only a handful of genera discovered so far. Talenkauen stands apart from other South American ornithopods like Anabisetia and Gasparinisaura in that it bore a distinct resemblance to the much better known Iguanodon, with a long, thick body and an almost comically small head. The fossils of this dinosaur include an intriguing set of oval-shaped plates lining the rib cage; it's unclear if all ornithopods shared this feature (which has been rarely preserved in the fossil record) or if it was limited to just a few species.

64 of 74 Tenontosaurus Wikimedia Commons Some dinosaurs are more famous for how they got eaten than for how they actually lived. That’s the case with Tenontosaurus, a medium-sized ornithopod that's notorious for having been on the lunch menu of the voracious raptor Deinonychus.

65 of 74 Theiophytalia Wikimedia Commons Name: Theiophytalia (Greek for "garden of the gods"); pronounced THAY-oh-fie-TAL-ya Habitat: Woodlands of North America Historical Period: Early Cretaceous (110 million years ago) Size and Weight: About 16 feet long and 1,000 pounds Diet: Plants Distinguishing Characteristics: Long, thick body; small head When the intact skull of Theiophytalia was discovered in the late 19th century—near a park called "Garden of the Gods," hence this dinosaur's name—the famous paleontologist Othniel C. Marsh assumed it was a species of Camptosaurus. Later, it was realized that this ornithopod dated from the early Cretaceous rather than the late Jurassic period, prompting another expert to assign it to its own genus. Today, paleontologists believe that Theiophytalia was intermediate in appearance between Camptosaurus and Iguanodon; like these other ornithopods, this half-ton herbivore probably ran on two legs when chased by predators.

66 of 74 Thescelosaurus Wikimedia Commons In 1993, paleontologists discovered an almost-intact specimen of Thescelosaurus containing the fossilized remains of what seemed to be a four-chambered heart. Was this a genuine artifact, or some byproduct of the fossilization process?

67 of 74 Tianyulong Nobu Tamura Name: Tianyulong (Greek for "Tianyu dragon"); pronounced tee-ANN-you-LONG Habitat: Woodlands of Asia Historical Period: Late Jurassic (155 million years ago) Size and Weight: About three feet long and 10 pounds Diet: Plants Distinguishing Characteristics: Small size; bipedal posture; primitive feathers Tianyulong has thrown the dinosaur equivalent of a monkey wrench into the carefully wrought classification schemes of paleontologists. Previously, the only dinosaurs known to have sported feathers were small theropods (two-legged carnivores), mostly raptors and associated dino-birds (but possibly juvenile tyrannosaurs as well). Tianyulong was a different creature entirely: an ornithopod (small, herbivorous dinosaur) whose fossil bears the unmistakable imprint of long, hairy proto-feathers, thus possibly hinting at a warm-blooded metabolism. Long story short: if Tianyulong sported feathers, so could any dinosaur, no matter what its diet or lifestyle.

68 of 74 Trinisaura Nobu Tamura Name: Trinisaurus (after paleontologist Trinidad Diaz); pronounced TREE-nee-SORE-ah Habitat: Plains of Antarctica Historical Period: Late Cretaceous (75-70 million years ago) Size and Weight: About four feet long and 30-40 pounds Diet: Plants Distinguishing Characteristics: Small size; large eyes; bipedal posture Discovered in Antarctica in 2008, Trinisaura is the first identified ornithopod from this massive continent, and one of the few to be named after the female of the species (another is the very similar Leaellynasaura, from Australia). What makes Trinisaura important is that it inhabited an unusually harsh landscape by Mesozoic standards; 70 million years ago, Antarctica wasn't nearly as frigid as it is today, but it was still plunged in darkness for much of the year. Like other dinosaurs from Australia and Antarctica, Trinisaura adapted to its environment by evolving unusually large eyes, which helped it to gather in sparse sunlight and spot voracious theropods from a healthy distance away.

69 of 74 Uteodon Wikimedia Commons Name: Uteodon (Greek for "Utah tooth"); pronounced YOU-toe-don Habitat: Woodlands of North America Historical Period: Late Jurassic (150 million years ago) Size and Weight: About 20 feet long and one ton Diet: Plants Distinguishing Characteristics: Bipedal posture; long, narrow snout There seems to be a rule in paleontology that the number of genera remains constant: while some dinosaurs are demoted from their genus status (that is, reclassified as individuals of already-named genera), others are promoted in the opposite direction. Such is the case with Uteodon, which for over a century was considered a specimen, and then a separate species, of the well-known North American ornithopod Camptosaurus. Even though it was technically distinct from Camptosaurus (specifically as concerns the morphology of its braincase and shoulders), Uteodon probably led the same kind of lifestyle, browsing vegetation and running away at top speed from hungry predators.

70 of 74 Valdosaurus London Natural History Museum Name: Valdosaurus (Greek for "weald lizard"); pronounced VAL-doe-SORE-us Habitat: Woodlands of western Europe Historical Period: Early Cretaceous (130-125 million years ago) Size and Weight: About four feet long and 20-25 pounds Diet: Plants Distinguishing Characteristics: Small size; bipedal posture Valdosaurus was a typical ornithopod of early Cretaceous Europe: a small, two-legged, nimble plant-eater that was probably capable of impressive bursts of speed when it was being chased by the larger theropods of its habitat. Until recently, this dinosaur was classified as a species of the better-known Dryosaurus, but upon reexamination of the fossil remains, it was awarded its own genus. An "iguanodont" ornithopod, Valdosaurus was closely related to, you guessed it, Iguanodon. (Recently, a central African species of Valdosaurus was reassigned to its own genus, Elrhazosaurus.)

71 of 74 Xiaosaurus Getty Images Name: Xiaosaurus (Chinese/Greek for "little lizard"); pronounced show-SORE-us Habitat: Woodlands of Asia Historical Period: Late Jurassic (170-160 million years ago) Size and Weight: About five feet long and 75-100 pounds Diet: Plants Distinguishing Characteristics: Small size; bipedal posture; leaf-shaped teeth Yet another notch in the belt of the famous Chinese paleontologist Dong Zhiming, who discovered its scattered fossils in 1983, Xiaosaurus was a small, inoffensive, plant-eating ornithopod of the late Jurassic period that may have been ancestral to Hypsilophodon (and may itself have been descended from Fabrosaurus). Other than those bare facts, though, not much is known about this dinosaur, and Xiaosaurus may yet turn out to be a species of an already-named genus of ornithopod (a situation that can only be resolved pending further fossil discoveries).

72 of 74 Xuwulong Nobu Tamura Name: Xuwulong (Chinese for "Xuwu dragon"); pronounced zhoo-woo-LONG Habitat: Woodlands of eastern Asia Historical Period: Early Cretaceous (130 million years ago) Size and Weight: Unknown Diet: Plants Distinguishing Characteristics: Thick, stiff tail; short front legs There hasn't been a lot published about Xuwulong, an early Cretaceous ornithopod from China that lay near the split between the "iguanodontid" ornithopods (that is, those with a marked resemblance to Iguanodon) and the very first hadrosaurs, or duck-billed dinosaurs. Like other iguandontids, the ungainly-looking Xuwolong possessed a thick tail, a narrow beak, and long hind legs on which it could run away when threatened by predators. Perhaps the most unusual thing about this dinosaur is the "long," meaning "dragon," at the end of its name; usually, this Chinese root is reserved for more fearsome meat-eaters like Guanlong or Dilong.

73 of 74 Yandusaurus Wikimedia Commons Name: Yandusaurus (Greek for "Yandu lizard"); pronounced YAN-doo-SORE-us Habitat: Woodlands of Asia Historical Period: Middle Jurassic (170-160 million years ago) Size and Weight: About 3-5 feet long and 15-25 pounds Diet: Plants Distinguishing Characteristics: Small size; bipedal posture Once a fairly secure dinosaur genus comprising two named species, Yandusaurus has since been whittled down by paleontologists to the point that this small ornithopod is no longer even included in some dinosaur bestiaries. The most prominent Yandusaurus species was reassigned a few years ago to the better-known Agilisaurus and was subsequently re-re-reassigned to a completely new genus, Hexinlusaurus. Classified as "hypsilophodonts," all of these small, herbivorous, bipedal dinosaurs were closely related to, you guessed it, Hypsilophodon, and had a worldwide distribution during most of the Mesozoic Era.