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Useful Notes: Prehistoric Life Hadrosaur Predecessors
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): Tenontosaurus (apparently in the middle between an Iguanodon and a Hypsilophodon); and Ouranosaurus (traditionally considered an "iguanodont" but actually closer to hadrosaurs). Among the other examples, the Australian Muttaburrasaurus and Leaellynasaura note  were portrayed in 1999 by Walking with Dinosaurs, while Orodromeus and Thescelosaurus have had notable Science Marches On stories.


When dinosaurs went down trees: Hypsilophodon

When we think about small bipedal dinosaurs, our mind authomatically comes to guys like the “raptors”, the “compies”, or the ornithomimids. But there were also several ornithopods which shared an analogue body-shape with the latter; even though they are usually ignored by film-makers. The most iconic of them has always been Hypsilophodon.

One of the first dinosaurs discovered (middle XIX century), lived in Europe 130-125 million years ago together with its gigantic relative Iguanodon, and was originally considered the latter’s juvenile specimen. Unusually for such a small animal, dozens complete individuals have been found, and this also explains its historical role as the stock small ornithopod. Even though most remains come from England (especially the Isle of Wight), some uncertain remains come from North America. A very small dinosaur, 2 m long or less, the size of a dog, Hypsilophodon is easily distinguishable from theropods by its horny beak on the lower jaw, small mouth-opening typical of ornithischians, large grinding teeth at the bottom of the mouth hidden by cheeks in the living animal, hands with five digits (coelurosaurs never have more than three fingers), and a more round belly to contain the typical large gut of a herbivore.

Hypsilophodon is nicknamed “the gazelle dinosaur”. The comparison works very well: it was a graceful, harmless, wide-eyed biped that escaped predators thanks to its agile legs well adapted for high-speed runs: it was certainly one of the fastest-running dinosaurs. If alive today, it would probably appear one of the cutest-looking dinos, maybe even suitable as a good household pet. But before the 1970s, Hypsilophodon used to be depicted as a tree-climbing animal, vaguely similar to a large, long-legged, spike-less iguana, and long depicted in this way in books, 3D models, and perhaps even fiction. Few other dinosaurs have had such a great Science Marches On change during their story.

The traditional prototype of the eponymous "hypsilophodonts" (small/slender non-hadrosaurian ornithopods), Hypsilophodon is classically countered against Iguanodon, which in turn is the classic 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).


The Jurassic antelopes: Camptosaurus & Dryosaurus

Let’s face it: it’s Rule of Cool that undisputedly dominates when coping with dinosaurs. Camptosaurus is the perfect example. In spite of being one of the most abundant dinosaurs in fossil record, and also one of the most common dinosaurs in museums around the world… when was the last time you’ve you watched it in recent documentaries? The problem is, Camptosaurus lived just alongside dino-stars like these: Apatosaurus, Diplodocus, Brachiosaurus, Stegosaurus... and Allosaurus. Thus, it could have shown up in Walking with Dinosaurs, Ballad of Big Al or even When Dinosaurs Roamed America. But, as it seems, its very generic appearance was judiced too incospicuous to capture the watchers’ interest.

On the other hand, the other well-known Late Jurassic ornithopod, Dryosaurus, despite being even less-conspicuous than Camptosaurus, has received a better treatment showing up in all the three documentaries above, though with very minor roles (in one case, it serves only to give a prey to Allosaurus).

Camptosaurus was similar to Iguanodon but smaller (5-7 m long) and with mere hints of thumbspikes. note  This because was one of the most primitive large-sized ornithopods, and a possible ancestor of Iguanodon and, indirectly, hadrosaurs. In the Jurassic world still dominated by sauropods, camptosaurids and stegosaurs were the only big ornithischians which were successful, anticipating the great diversity bird-hipped dinosaurs reached later in the Cretaceous. Dryosaurus was much smaller than Camptosaurus and more similar to a Hypsilophodon in shape, being totally bipedal. Compared with Hypsilophodon, the dryosaur was larger (3-4 m long), slightly more robust, and with a toothless beak. Nonetheless, Dryosaurus was a basal iguanodont, closer to Iguanodon than to Hypsilophodon.

Both discovered during the Bone Wars, Camptosaurus and Dryosaurus have been found in the USA. Some close relatives known from Europe include the camptosaurid Draconyx and the dryosaurid Valdosaurus. Specimens once referred to Dryosaurus have also been discovered in Africa - more precisely in Tendaguru (together with Giraffatitan and Kentrosaurus); they have been recently re-classified as Dysalotosaurus.

Camptosaurus and Dryosaurus are very frequently portrayed in dinosaur books, especially Camptosaurus; here is typically shown as one of the favorite prey of Allosaurus, a concept that is almost certainly Truth In Books. Indeed, in North American placements, camptosaurids and dryosaurids were respectively the “wildebeest” and the “Thompson’s gazelles” of their fauna, that escaped their reptilian “lions” and “hyenas” (carnosaurs and ceratosaurs) by running fast on two legs. Dryosaurids, being smaller and more manouvreable, were arguably less-easy prey to catch than camptosaurids, at least for an adult Allosaurus - a fully-grown, 2-ton Camptosaurus was surely a difficult task for young carnosaurs and maybe adult ceratosaurs too.


Hearts of stone and spiral nests: Thescelosaurus & Orodromeus

Hypsilophodon-like animals existed also in Late Cretaceous, but tend to be overshadowed by the spectacular ornithischians of their period (hadrosaurs, ceratopsians etc.). In North-America, while duckbills ruled the “wildebeest” role, the “gazelle” one was mainly played by Thescelosaurus. 3-4 m long and rather similar to Dryosaurus, it was not a basal iguanodont however unlike the latter. One peculiar trait are the small bony plates on its back, maybe placed under the skin and not-visible in the living animal.

Thescelosaurus is the animal from which the controversial “fossilized heart” comes from, which is almost certainly a fossilization artifact: that is, a piece of stone found in one specimen, which casually resembles a heart. Discovered in year 2000, this stony concretion was celebrated as the proof of “warm-bloodedness” among dinosaurs, because it seemingly showed a four-chambered heart just like bird and mammals and unlike most modern reptiles — crocodilians have four-chambered hearts, but their ancestors could have been warm-blooded as hypothized in the late 2000s.

Other “hypsilophodontians” from Late Cretaceous North America were Parksosaurus and Orodromeus. Both were smaller than Thescelosaurus, and lived slightly earlier. Orodromeus was once thought to be the source of some small nests full of eggs containing fossilized embryos, discovered in 1988 just next to the more famous Maiasaura ones. We now know that those eggs were from the theropod Troodon instead. The ironical thing is, fossils of troodonts 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 lived quite a bit earlier than the similar-named Orodromeus, 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, Zephyrosaurus, lived even earlier in the Early Cretaceous alongside Deinonychus and was possibly its prey.


Duck-billed spinosaur: Ouranosaurus

Here’s one of the best dinosaurian Mix-and-Match Critter examples: Ouranosaurus. This medium sized (7 m long) ornithopod looked like a cross between other more familiar dinosaurs. Flat-billed head like Anatosaurus; thumbspikes like Iguanodon; and, above all, a wide spinal crest on its back, similar to Spinosaurus but less tall and extending from the shoulders down to the tip of the tail.

Named in the 1970s, Ouranosaurus lived in Cretaceous Sahara like Spinosaurus, and the latter has often been shown as the predator of the former - this is actually a mistake, because Ouranosaurus lived 15 million years before "Spino". Nevertheless, its one documentary appearance in "Planet Dinosaur" showed the two living at the same time. Today, some scientists argue that Ouranosaurus had a fleshy hump instead of a “sail”, because its vertebrae are similar to those of modern bison. But others say that comparing dinosaurs with modern big mammals is not correct, since these are two completely distinct zoological groups.

Ouranosaurus is the most well-known among those middle-ways between Iguanodon and hadrosaurs called “basal hadrosauroids”, and was closer to duckbills than to Iguanodon. Unlike true hadrosaurs (whose teeth were crammed in "batteries") the ouranosaur had teeth placed in one single line on each half-jaw — the primitive condition of the non-hadrosaur iguanodonts. Among the other possible pre-hadrosaurs, other than Altirhinus (see below) and Probactrosaurus, is 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"). On the other hand, the heavily-built Lurdusaurus 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. note 


Stock Fodder: Tenontosaurus

One of the most iconic scenes in those paleo-artistic works made in full Dino-Renaissance was a fight between a whole pack of Deinonychus and a much heavier ornithopod; even though Iguanodon was often chosen in this role, the most classic choice has been another relative, Tenontosaurus.

This was one of the most basal known iguanodontians, an Early Cretaceous animal similar in size to the Jurassic Camptosaurus but totally devoid of thumbspikes (it was once considered an overgrown "hypsilophodont") and with a much longer tail, twice the length of the rest of its body. First found in the 1970s in Montana, its first skeleton was surrounded by several Deinonychus skeletons; it was just this detail that made John Ostrom to think about these predators as wolf-like pack-hunters capable to bring down giant preys with their agility and their sickle-claws. Even though in an indirect way, Tenontosaurus has thus given a strong contribute to the public image of dinosaurs.

In these struggles, Tenontosaurus is usually shown swinging its enormous tail and hitting some “raptors” to death, before being killed and eaten by the remaining Deinonychus. The Tenontosaur-Deinonychus battle is more justified than the Iguanodon-Deinonychus one, both because the former has at least one possible proof, and because an adult Iguanodon would have weighed eighty times more than Deinonychus (see Raptor Attack). Some scientists, however, have recently suggested that the carnivores simply ate the carcass of the Tenontosaurus they found already dead. The presence of their skeletons around the herbivore could be explained if some “raptors” fought each other to the point that some ended killed by their own companions.


Land Down Under dinosaurs: Leaellynasaura & Muttaburrasaurus

If you’ve seen the fifth episode of Walking with Dinosaurs, you’ll already have the idea what we’re talking about. If dinosaur names are often thought bizarre-sounding, these ones certainly do match the commonplace very well. Muttaburrasaurus is called from the small town of Muttaburra in Queensland, Australia, where its only skeleton was found in 1980. Leaellynasaura was named after the daughter of its discoverers, Leaellyn: note  hence its feminine suffix saura - just like Maiasaura which means “good-mother lizard”. Together with the much simpler-named but equally bizarre-sounding ankylosaur Minmi, these are the most well-known Aussie Dinos.

Similar but smaller than Iguanodon (about the same size of Camptosaurus, Ouranosaurus or Tenontosaurus), Muttaburrasaurus is easy to tell apart from its relatives thanks to its prominent nose, similar to other ornithopods but more bulbous. We don't know if it had thumbspikes: being more basal than the almost-spikeless Camptosaurus, this is unlikely, but it is traditionally shown with them in drawings. Found in 1989, Leaellynasaura was a tiny (less than 1 m long) bipedal animal similar to Hypsilophodon, but with the possibility of having a tail 3 times longer that its own body. Once considered an “hypsilophodontian”, even its ornithopod status is disputed today, and is now generally regarded as a more basal ornithischian.

The discover of these dinosaurs in the 1980s made sensation in Australia, because very few dinosaurs were known before in the Land Down Under, all fragmentary. Muttaburrasaurus still is one of the most complete dinosaurs found there; on the other hand, Leaellynasaura ‘s only skeleton is very incomplete, but its 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 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 and added to it speculative nasal sacs to make loud sounds, but we don't have direct evidence for this.


Iguanodons everywhere?: Altirhinus & Mantellisaurus

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.

Several 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 from Gideon Mantell (Iguanodon’s Trope Namer), 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. note  Other Europeans were named for their unique physical characteristics, such as Hypselospinus ("tall spines") and Barilium ("heavy hips"). The North American Iguanodon 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.

Finally, the Mongolian specimen "Iguanodon orientalis" has been renamed Altirhinus. 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. Interesting that Kron (the villainous Iguanodon in Dinosaur) has a hump-nose that could mean he’s actually an Altirhinus.


More primitive guys: Rhabdodon & Yandusaurus

As a whole, non-hadrosaurian ornithopods have been found everywhere. 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 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. On the other hand, Callovosaurus (found in England) comes from a far more ancient period; living in the Middle Jurassic, it was one of the most ancient iguanodontians known, and is more closely related to Dryosaurus.

An even more primitive ornithopod from the same period was the Chinese “hypsilophodont” Yandusaurus, perhaps the most basal ornithopod known to science. On the other hand, other “hypsies” from the same fauna were probably too primitive to be real ornithopods: the most scientifically-known is Agilisaurus, others are Xiaosaurus, Gongbusaurus and former Yandusaurus species Hexinlusaurus. From Early Cretaceous China comes the enigmatic true-ornithopod Jeholosaurus, whose pointed frontal teeth seem indicating an omnivorous diet.


An enduring rivarly: Othnielosaurus & Drinker

Returning for a moment in Late Jurassic USA, other than Camptosaurus the Wildebeest and Dryosaurus the Gazelle, there was also Othnielosaurus the Dik-Dik, a very small animal with a very convoluted Science Marches On story. It was described in 1977 as Othnielia (this is the name usually heard in docu-media), and renamed more recently 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 in 1990 out of spite!

To complicate the matter, we also have Nanosaurus and Laosaurus. Discovered during Cope’s and Marsh’s “war”, Nanosaurus was very commonly-portrayed in old textbooks for having detained the record of “the smallest North-American dinosaur” for almost a century (its name simply means “dwarf lizard”). But today it might not even be a valid name. Today, the record pertains to a tiny heterodontosaurid found only in 2009, Fruitadens (see in another page). The very fragmentary Laosaurus (described in USA in the same period of Nanosaurus) has been involved in this taxonomic tangle as well.


Icy amnesia: The "Antarctic hypsilophodont"

Finally, let’s not forget the “Mysterious Polar Dino”. In year 1987, just one year after the ankylosaur Antarctopelta, 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 poor polar "hypsy" has yet to wait a formal naming and description, and sadly seems to be almost forgotten today.

Partially compensating, several small bipedal ornithischians have been then discovered in other southern continents, the best-known being Leaellynasaura. It was found in 1989 in Australia along with the single lower jaw of the larger Atlascopcosaurus (so-called from the Atlas-Copco Corporation that funded its excavation). Today, the most-complete Australian "hypsilophodont" is Qantassaurus (its 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 (the "lightening beast")

About South American discoveries, apart from the enigmatic Loncosaurus (found as well in the early XX century but known from a single femur), there are few non-hadrosaur ornithopods found in the 2000s, the largest one being the 6 m long Macrogryphosaurus. Curiously, one small South American ornithopod described in 1996 has got a feminine name reminescent of Leaellynasaura, Gasparinisaura. 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 Bad Ass -looking (and very masculine) ankylosaurs...
Prehistoric Life - HadrosaursUsefulNotes/Prehistoric Life - DinosaursPrehistoric Life Primitive Ornithischians
Prehistoric Life - HadrosaursUsefulNotes/Prehistoric LifePrehistoric Life Primitive Ornithischians
Prehistoric Life - HadrosaursTropesaurus IndexPrehistoric Life Primitive Ornithischians

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