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One of the hugest dinosaurs known.
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Among non-hadrosaur/non-Iguanodon ornithopods, the ones you've more chances to see in media are: Hypsilophodon (the prototype of the "hypsilophodonts" aka small-slender members of the Group); Camptosaurus & Dryosaurus (the two iconic ornithopods from the Late Jurassic; the former was big and Iguanodon-like, the latter small and Hypsilophodon-like): Tenontosaurus (looking like it's in the middle between an Iguanodon and a Hypsilophodon, but with a distinctively long tail); and Ouranosaurus (with an evident crest on its back, it's traditionally considered an "iguanodont" but was actually closer to hadrosaurs). Among the other examples, the "iguanodont" Muttaburrasaurus and the "hypsilophodont" Leaellynasaura (both Australian) — the latter, according to recent research, could not be a proper ornithopod however — were portrayed in 1999 by Walking with Dinosaurs, while Orodromeus and Thescelosaurus (both "hypsilophodonts") have had notable Science Marches On stories.

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Hypsilophodon (the animal of the image, no bigger than an adult human) is classically countered against Iguanodon, which is the traditional prototype of the the “iguanodonts” (large/heavy non-hadrosaurian ornithopods). Now scientists have found “hypsilophodonts” is an artificial assemblage including the most basal ornithopods, while “iguanodonts” now indicates a natural group including not only the most Iguanodon-like animals but also duckbills, pre-duckbills and also some traditional “hypsilophodontians” (see below).

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    "Iguanodonts" 


Iguanodons Everywhere?: Mantellisaurus & Altirhinus

  • Since its first discovery made in the first decades of the XIX Century, Iguanodon remains have been found everywhere from Africa to Mongolia, Europe and North America; Chased By Dinosaurs added some sorta iguanodons even in South American settings - even though some iguanodontians are known from South America, they were much smaller. Science Marches On however, and now many of these Iguanodon species have been reclassified in other genera, while a formerly distinct iguanodontid genus, "Vectisaurus" from Early Cretaceous England, is now classified within Iguanodon. Several genera from Europe have been created as an homage to some of the greatest XIX century paleontologists - at the time, dinosaur remains were mainly from Iguanodons. So we have Mantellisaurus atherfieldensis from Gideon Mantell (Iguanodon’s Trope Namer); formerly "Iguanodon atherfieldensis", it was found in the same coal mine near Bernissart in Belgium in which the 30 or so skeletons of the more robust Iguanodon bernissartensis have been dug out.note  Then, Owenodon from Richard Owen (dinosaurs’ Trope Namer), and Dollodon from Louis Dollo, the guy who described the famous iguanodonts found in the “Dinosaur Mine” in Belgium as erect bipedal beasts. However, Dollodon is very likely the same as Mantellisaurus. Another former Iguanodon species has been renamed "Huxleysaurus" (after Thomas Henry Huxley, supporter of Darwin's theory of evolution), but this name is unofficial at present. Other Europeans were named for their unique physical characteristics, such as Hypselospinus ("tall spines") and Barilium ("heavy hips"). The North American Iguanodon lakotaensis was renamed Dakotadon after the state it was found in. A similar fate befell another South Dakotan iguanodont: Osmakasaurus, from the same rock unit as Dakotadon, was originally named as a species of Camptosaurus. True camptosaurids include Bihariosaurus from Romania, Draconyx from Portugal, and maybe North American Theiophytalia. Finally, the Mongolian specimen "Iguanodon orientalis" has been renamed Altirhinus kurzanovi. As with most former Iguanodon specimens, Altirhinus was actually closer to hadrosaurs than to Iguanodon. Indeed its name, “high nose”, was given from its humped nose similar to the duckbill Gryposaurus or the more primitive ornithopod Muttaburrasaurus. Interesting that Kron (the villainous Iguanodon in Dinosaur) has a hump-nose that could mean he’s actually an Altirhinus.


The First Hadrosaur?: Protohadros

  • Ouranosaurus is the most well-known among those middle-ways between Iguanodon and hadrosaurs called “basal hadrosauroids”, and was actually closer to duckbills than to Iguanodon despite sharing thumbspikes with the latter (though less-developed than the Iguanodon's ones). Unlike true hadrosaurs whose teeth were in number of hundreds and crammed in "batteries", Ouranosaurus and the other pre-hadrosaurs had less-numerous teeth placed in one single line on each half-jaw — the primitive condition of almost all the non-hadrosaur dinosaurs. Among the other possible basal hadrosauroids, other than Altirhinus and Probactrosaurus, are also worth of mention Protohadros and Eolambia, both discovered in 1998 in the USA. The former was initially considered the earliest hadrosaur (its name just means “the first hadrosaur”); the latter received a similar treatment, initially described as the first ancestor of crested hadrosaurs (Eolambia means "dawn lambeosaurine").


Aquatic Dinosaur?: Lurdusaurus

  • On the other hand, the heavily-built Lurdusaurus (informally named "Gravisaurus" before its official description: both names mean "heavy lizard") seems very closely-related to Iguanodon, even though shared its habitat with Ouranosaurus in North Africa. Interestingly, Lurdusaurus seems showing adaptations for a semi-aquatic lifestyle, which is unusual for ornithischian dinosaurs. This hyp was also made for another ornithischian, the basal ceratopsian Koreaceratops from Korea: found only in 2012, its describer believed it was actually a marine dinosaur. Today, thanks to the most recent findings, the saurischian Spinosaurus is known to have been a really sea-dwelling dinosaur.


Dwarf Iguanodont: Rhabdodon

  • As a whole, non-hadrosaurian ornithopods have been found everywhere, even in Antarctica (as we'll see later). Almost all the main dinosaurian faunas had at least one known ornithopod: even the famous Late Cretaceous islets which were where today is Central Europe. Rhabdodon priscus was a sort of “dwarf iguanodont”, a primitive spike-less iguanodontian analogue to the earlier Dryosaurus and Tenontosaurus; it was a late-surviving form which managed to reach the K/T extinction event just thanks to its insulation and absence of competition from the much more evolved hadrosaurs. But wait: some hadrosaurs are actually known as well from that habitat, such as Telmatosaurus; only, they too were small and primitive. From the same fauna are the close Rhabdodon relatives Mochlodon and Zalmoxes, making together the Rhabdodontids.


The Most Ancient?: Callovosaurus

  • On the other hand, the 3/4 m long Callovosaurus found in England comes from a far more ancient period: the Callovian stage of the Middle Jurassic (hence the name).It was one of the most ancient iguanodontians known: once considered a camptosaurid, it's actually more closely related to Dryosaurus. Valdosaurus ("Weald lizard") was another dryosaurid, also English but Early Cretaceous (like Hypsilophodon); some alleged "Valdosaurus" remains were found in Africa, too. Kangnasaurus was bigger, and lived in Early Cretaceous South Africa: known from scanty remains, it owes its name from a local Ranch. Phyllodon, Alocodon, Taveirosaurus, and Trimucrodon from Portugal are known only from teeth, and are thus hard to classify.


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    "Hypsilophodonts" 


Pietrified Nests?: Orodromeus

  • There were other “hypsilophodontians” from Late Cretaceous North America that were smaller and lived slightly earlier than Thescelosaurus. Orodromeus makelai, meaning "Makela's runner of the (Egg) Mountain", was discovered in Montana in year 1988 by Jack Horner and his assistant Robert Makela in the same site in which they had found Maiasaura peeblesorum eight years before. They noted some small unusually spiral-shaped nests full of eggs containing fossilized embryos, next to the bigger & more famous Maiasaura ones, which they attributed to Orodromeus: as the bones inside those eggs were already well-formed, they said that the orodromeus' hatchlings were more independent after birth than the maiasaura's ones. Science Marches On however, and later it was found that those eggs/embryos were from the theropod Stenonychosaurus instead. The ironical thing is, fossils of stenonychosaurs (aka the "troodons") were discovered as well around those putative Orodromeus nests, but it was thought that they were actually preying on Orodromeus nestlings: an astonishingly similar story to the “Oviraptor robbing Protoceratops' eggs”. Found in 2007, its relative Oryctodromeus ("digging runner") lived quite a bit earlier than the similar-named Orodromeus in Middle Cretaceous, but has also shown the first proof of digging behavior among non-avian dinosaurs: its skeleton has been found inside a fossilized burrow. Another relative found in Alberta is known since the start of the XX century: Parksosaurus ("William Parks' lizard", sometimes misspelled "Parkosaurus"). Similar in size and shape to Orodromeus but living few million years later, it could be actually closer to Thescelosaurus (which lived even later), and has recently become the namesake of its own family of ornithopods, the Parksosaurids. Zephyrosaurus ("lizard of the western wind"), lived earlier than all these, in the Early Cretaceous Montana: it could have met Deinonychus in real life, and possibly was one of its preys.


Named after a Girl: Leaellynasaura

  • If you’ve seen the fifth episode of Walking With Dinosaurs, you’ll already have the idea what we’re talking about. Dinosaur names are often thought bizarre-sounding, and this animal certainly does match the commonplace very well, like its bigger compatriot Muttaburrasaurus. This one is called from the village of Muttaburra in Queensland, Australia, where its only skeleton was found in 1981; Leaellynasaura amicagraphica was named after the daughter of its discoverers, Leaellyn. Another little-known australian dinosaur, Timimus, was named after Leaellyn's brother, Tim. It has mimus at the end because was originally thought an ornithomimosaur. Leaellynasaura (sometimes misspelled "Leaellynosaura") owes well its feminine suffix saura, just like the hadrosaur Maiasaura which means “good-mother lizard”. Together with the shorter-named but equally bizarre-sounding ankylosaur Minmi, these are the most well-known Aussie Dinos. First-found in 1989, Leaellynasaura was a small (1 m long) bipedal animal similar to Hypsilophodon. Once considered an “hypsilophodontian”, even its ornithopod status is disputed today, and is now generally regarded as a more basal ornithischian. The fossil material attributed to Leaellynasaura has been recently revised. Fossils discovered in Australia in the 2000s indicate presence of a small Early Cretaceous ornithischian with tail 3 times longer that its own body (even more than the Tenontosaurus tail); whether this is the same taxon as Leaellynasaura or not remains to be seen. The discover of Muttaburrasaurus, Minmi, and Leaellynasaura in the 1980s made sensation in Australia, because very few dinosaurs were known before in the Land Down Under, all fragmentary. Muttaburrasaurus, like Minmi, still is one of the most complete dinosaurs found there; Leaellynasaura ‘s skeletons are more incomplete, but the latter's importance was due to having contributed to enforce the “warm-blooded dinosaurs” hypothesis even more. In Early Cretaceous, Australia was not the temperate/tropical/desertic country we know today, but a colder world with warm summers but cold winters - because was much closer to the South Pole. How could such a small, clearly non-migratory animal like this manage to survive that icy winter? The only explanation was: Leaellynasaura was warm-blooded. Furthermore, its unusually big eyes could have been used to see throughout the darkness of the polar winter. All these arguments have been discussed in Walking with Dinosaurs, in which a family of Leaellynasaura makes the main characters. The show also portrayed Muttaburrasaurus, as a migrating animal that flees the winter in herd like caribous; it also added to it speculative nasal sacs to make loud sounds (like what's been hypothized for some hadrosaurs), but we don't have direct evidence for this.


Chinese Gazelle: Yandusaurus

  • An even more primitive ornithopod from the same period of Callovosaurus was the Chinese “hypsilophodont” Yandusaurus, once considered the most basal true ornithopod known to science. It was originally named "Yubasaurus", but since that name was not made official, Yandusaurus got the precedence. Other “hypsilophodonts" from the same fauna were also too primitive to be real ornithopods. The most scientifically-known is Agilisaurus ("agile lizard"); others are Xiaosaurus, Gongbusaurus, and former Yandusaurus species Hexinlusaurus. From Early Cretaceous China comes the enigmatic true-ornithopod Jeholosaurus (named after the geological formation it was dug out), whose pointed frontal teeth seem indicating an omnivorous diet. It appears as a prey of the "gliding raptor" Sinornithosaurus in Planet Dinosaur.


The Never-Ending Story: Nanosaurus

  • Travelling in Late Jurassic USA, other than Camptosaurus the Wildebeest and Dryosaurus the Gazelle we'd encounter also Nanosaurus the Dik-Dik. Meaning "dwarf lizard", this was indeed a very small animal, 1.5 m long (smaller than an Hypsilophodon), with a very convoluted Science Marches On story. Discovered during Cope’s and Marsh’s “war”, Nanosaurus agilis ("agile dwarf lizard") was very commonly-portrayed in old textbooks for having detained the record of “the smallest North-American dinosaur” for almost a century. In 1977 it was described from the same sites a similar animal, Othnielia rex, which was renamed more recently Othnielosaurus because its type material was not diagnostic. Both curious names derive from Othniel Charles Marsh, one of the two scientists who “fought” the Bone-Wars in the XIX century. As it seems, its notorious rivalry with Edward Drinker Cope has lasted until today, with another similar animal from the same habitat named Drinker nisti in 1990 out of spite! Today, all these three genus names are usually considered as junior synonyms of Nanosaurus agilis. Today, the record of "the smallest North-American dinosaur" pertains to a tiny heterodontosaurid found only in 2009, Fruitadens. The very fragmentary Laosaurus celer ("speedy fossil lizard", described in USA in the same period of Nanosaurus) has been involved in this taxonomic tangle as well, with its remains often found mixed with those of Dryosaurus.


Icy Amnesia: the "Polar Hypsilophodon"

  • Let’s not forget the “Mysterious Antarctic Dino”. In year 1987, just one year after the ankylosaur named Antarctopelta in 2006, the second Antarctic dinosaur was found, described as a “polar hypsilophodont”; the thing is, it has had an even worse fate than the ankylosaur itself. At least, after 20 years of waiting, the latter has received a name; the polar "hypsy" has yet to wait a formal naming and description, and sadly seems to be almost forgotten today. In classic textbooks the "polar Hypsilophodon" is usually depicted with a shape very similar to the real Hypsilophodon, but being it still-undescribed, we cannot say much about its life apart from the fact that was Late Cretaceous like its neighbor Antarctopelta.


Plane-Traveler: Qantassaurus

  • Partially compensating, several small bipedal ornithischians have been then discovered in other southern continents, the best-known being Leaellynasaura amicagraphica. It was found in 1989 in Australia along with the single lower jaw of the larger Atlascopcosaurus loadsi, so-called from the Atlas-Copco Corporation that funded its excavation. Today, the most-complete Australian "hypsilophodont" is Qantassaurus intrepidus ("courageous Qantas Lizard": its genus name is a homage to Qantas, the Land Down Under airlines). However, there's another relative discovered in Australia at the start of the XX century, but is known only from a femur: Fulgurotherium australe (the "lightening beast of the South"). Its name is odd because recalls more that of a prehistoric mammal ("Megatherium", "Uintatherium", "Hyracotherium", and so on) than that of a dinosaur.


Feminine Dinosaurs: Gasparinisaura

  • About South American discoveries, apart from the enigmatic Loncosaurus (found as well in the early XX century but described from a single femur mixed with a theropod tooth), there are few non-hadrosaur ornithopods found in the 1990s or later, the largest one being the 6 m long Macrogryphosaurus. Another is meaningfully named Notohypsilophodon, the "Southern Hypsilophodon". Curiously, one small South American ornithopod described in 1996 has got a feminine name reminescent of Leaellynasaura: Gasparinisaura ("Gasparini's lizard"). Since that other small ornithischians around the world have received the suffix "-saura", for example "Bugenasaura" (now regarded as a synonym of Thescelosaurus) and, last example, Trinisaura, found in Antarctica in the earliest part of 2013 (this one has immediately received a name). Well, Unfair Sex -related names do fit better for these graceful “gazelle dinos” rather than the badass-looking (and very masculine) ankylosaurs.


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