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Leaellynasaura amicagraphica in the Mid Cretaceous polar forest
from Walking with Dinosaurs © 1999 ABC, BBC

Introduction to the Hypsilophodonts

The hypsilophodonts were a paraphyletic assemblage of mostly small and lightly built, fairly primitive, ornithopods, some members of which quite early on (presumably during the middle Jurassic) evolved into Iguanodonts. These small active bipedal animals were clearly the dinosaurian equivalent of small ungulate mammals like deer and gazelles.

In the current dinosaur cladograms the term Hypsilophodontia, originally referring to a sister taxon to the iguanodontia, is now discarded as these animals are now seen as a paraphyletic sucession (like "Pelycosauria"), rather than a clade in themselves. While I agree that hypsilophodonts are the most plausible ancestors of the iguanodontia, I am not happy with the current phylogeny, as it ignores too much of the evidence of fossil record. In the evolutionary sequence Hypsilophodon - Dryosaurus - Camptosaurus - Iguanodon, the hypsilophondontids gave rise very early to the Dryosaurs, perhaps only a few million years after evolving (both groups appeared in the middle or late Jurassic). Rather than the numerous ghost lineages implied in the currently popular cladogram of early ornithopod relationships, it is more likely there were a few groups, evolving alongside each other. However, details of hypsilophodont evolutionary relationships will have to awauit more and better discoveries, and analysis thereof.

As well as typically small forms there were also unusual and specialised larger types. Tenontosaurus is sometimes considered a basal Iguanodontian; Muttaburrasaurus is variously considered a basal Iguanodontian, a specialised Hypsilophodont, Camptosaurid, or an Iguanodontid. Both were large (around 6 meters) middle Cretaceous animals. Personally I think it is more reasonable to assume that a lot of the supposedly advanced iguanodntian features of these two genera are simply be a result of adaptation to larger size, more efficient food-processing, etc. After all, the original iguanodontians appeared some fifty million years earlier at least, in the form of late or even middle Jurassic dryosaurs and camptosaurs. It seems to me implausible that these larger hypsilophodonts should continue invisible for millions of years, suddenly appearing fully formed in the mid Cretaceous like Athena from the forehead of Zeus. What is more likely is that they were side branches of one or more contemporary hypsilophohodontid lineages already known from fossils. In any case, there is no doubt that a number of ornithopod types were evolving alongside each other and in parallel during the late Jurassic and throughout the Cretaceous. Thus Thescelosaurus, one of the last ornithopods, is also one of the most primitive, a living fossil continuing alongside more advanced forms.

In older books on dinosaurs and vertebrate paleontology, only a single family of these creatures is proposed, the Hypsilophodontidae. But more recently, with further discoveries and phylogenetic analysis, additional subfamilies and even families have been suggested. Although some are the result of a simple inflation of taxa (especially in popular web-based cladograms two sister genera together are often automatically considered a subfamily, or even a family!), others may well turn out to be genuine. Especially if, as seems extremely likley, the hypsilophodonts were evolving in parallel for some 50 to 100 million years, that implies a number of families. But remains are often fragmentary, and the affiliation of most forms is unclear. The following therefore can be taken as a very speculative classification of the Hypsilophodontias. The actual diversity of these small animals was probably a lot higher than the meagre fossil remains indicate.

superfamily "Hypsilophodontoidea"

Note that there is no official taxon "Hypsilophodontoidea". It is here suggested as a provisional paraphyletic grouping of pre-iguanodontoidean euornithopods (including basal Iguanodontia).
Characteristics: Primitively lightly built, bipedal, cursorial; larger forms quadrapedal. The teeth are tall and bear distinctive grooves. The upper and lower tooth rows occluded (met as regular rows) giving a more efficient chewing and grinding surface. Loss of ridges on crowns of cheek teeth. Premaxillary teeth retained, lost only in some advanced forms. Part of pubis bone (the prepubic process) projects forward in rod-like structure, to provide extra attachment for leg muscles, hence greater running power. External nares (nostrils) mostly small. Three to four (rarely five) toes on hind foot

family Thescelosauridae

family Hypsilophodontidae
   subfamily Othnieliidae
   subfamily Zephyrosauridae
   subfamily Hypsilophodontidae

family Tenontosauridae
   subfamily "Muttaburrasaurinae"
   subfamily Tenontosaurinae

Family and Subfamily Uncertain

Agilisaurus louderbacki (Peng,1990)

Comments: originally included under Yandusaurus, it is unrelated to Y. hongheensis. This is either an extremely primitive hypsilophodont (perhaps an Othnieliine) or an advanced fabrosaur. It had large eyes (indicating a possibly nocturnal lifestyle?), a fairly short neck and long tail. This animal might be considered a transitional form between proto-ornithopods and higher members of the taxon. For more info, see the DinoData

family Thescelosauridae

Thescelosaurus neglectus, life reconstruction. Illustration by Steve Kirk - Illustrated Encyclopedia of Dinosaurs and Prehistoric Animals, ed.. Barry Cox

This group of very primitive hyspilophodontians includes only two similar and contemporary latest Cretaceous species, Thescelosaurus neglectus and Bugenasaura infernalis. A third species, B. garbanii, may be the headless skeleton of the pachycephalosaur Stygimoloch spinifer

drawing © Øyvind M. Padron

Thescelosaurus neglectus Gilmore, 1913

Comments: one of the last of the dinosaurs, a contemporary of the great Tyrannosaurus and Triceratops, this animal was apparently very primitive by ornithopod standards. It was more heavily-built and stouter of limb then most hypsilophodonts, and probably, like its bigger cousins Tenontosaurus and Muttaburrasaurus, or even a small iguanodontid, and capable of getting about on all fours. Features distinguishing it from other hypsilophodonts are the presence of premaxillary (front upper jaw) teeth and five toes per foot. It's femur and tibia were of equal length, indicating a not very fast moving animal. There seem to have been bony studs or osteoderms along the back, perhaps to compensate for lack of fleetness of foot.

It is a strange fact that such a primitive and seemingly clumsy animal was able to flourish at the very end of the age of dinosaurs, at a time when many more specialised forms had already been driven into extinction. Perhaps its very primitiveness and generalised adaptations helped it in this regard. Thescelosaurus and its close cousin Bugenasaura must have had immediate ancestors living during the late Jurassic and the Cretaceous, but none have been found as yet.

Family Hypsilophodontidae

Hypsilophodon foxii
image from un monde de dinosaures

The hypsilophodontidae are a paraphyletic assemblage of small and lightly built, forms, which gave rise to several other families, such as the Tenontosaurs and the Dryosaurs. The skull, shown at the left, is very Heterodontosaur-like, only lacking the canines and differing in a few other small details. The teeth are tall and grooved, the upper and lower teeth meeting to form a grinding surface. Like the thescelosaurids, most hypsilophodontids still have premaxillary (front) teeth, but they have only four toes per foot, and the tibia (shin bone) is longer than the femur (thigh bone), indicating a fast moving animal. Interestingly one sees similar adaptations in the evolution of the mamamlian ungulates - e.g. reduction in the number of toes (horses only have a single toe per foot). The hands retain five fingers.

Subfamily Othnieliinae

Othnielia rex, life reconstruction. Illustration by Steve Kirk - Illustrated Encyclopedia of Dinosaurs and Prehistoric Animals, ed.. Barry Cox

A group of small Jurassic hypsilophodonts. As with others of the clan, the legs and tail are long, the body lightly built, and the forearms short. The teeth however are distinctive, being propprtionally smaller, and completely covered in enamel (rather than only on the grinding surfaces). This may have indicated a tougher more abrasive food source.

Othnielia rex Galton, 1977b

Comments: Closely resembles Hypsilophodon in the structure of the skeleton, but some features mean it has been placed in a separate family, although a sub-family ranking is more likely

Othnielia nisti (Bakker, Galton, Siegwarth, and Filla, 1990)

Comments: Presumably a relative or descendent of Othnielia rex, this little dinosaur is probably not different enough to deserve a new generic name. It had long spreading toes, indicating that it lived in swampy terrain. This small animal lived after the extinction of the Jurassic megafauna

Subfamily Uncertain

Leaellynasaura amicagraphica Rich and Rich, 1989

Comments: The best known of all the small Austraklian hypsilophodonts. Distinctive ridges on the unworn maxillary teeth different to those of the contemorary Atlascopcosaurus. The femur shows some primitive fabrosaur-like features. The eyes are very large, and the brain cavity shows an enlarged optic lobe, suggesting nocturnal vision. This animal lived in a polar environment, and hence needed to see during the long periods of darkness. If so, it was quite likely to have been endothermic (warm-blooded) as well. Even if the antarctic climate was not as severe as it is today, the temperature still frequently dropped below freezing, and it is difficult to see how a small cold-blooded animal could remain active in such an environment. As with many of these animals, the incomplete nature of the material means that Leaellynasaura's evolutionary relationships are unclear. It seems to have some similarities to the zephyrosaurines [ref Jaime Headden] or possibly the othnieliines [ref Justin Tweet - Thescelosaurus!] Stanley Friesen places it with Othnielia and Zephyrosaurus in an unnamed new family.
 Dann's Dinosaur Info: LEAELLYNASAURA

Subfamily Zephyrosaurinae

Although apparently less specialised than the Othnieliidae, these animals lived later (in the Cretaceous rather than the Jurassic period). This makes me wonder about the phylogentic analysis, especially in view of the fairly scanty material available. They are distinguished by specialised skull characters, such as a bony boss or expansion on the jugal (cheek-bone), which means they are probably a monophyletic side-branch from the main line of ornithopod evolution. These were fast running animals.

Zephyrosaurus schaffi Sues, 1980a

Comments: Distinguished by an unusual skull. For more info, see the DinoData

Orodromeus makelai Horner and Weishampel,1988

Comments: Features on the skull indicate a relationship with Zephyrosaurus. Teeth resemble those of Fabrosaurus. In contrast to Hypsilophodon, the tibia (shin bone) is considerably longer than the femur (thigh bone). Nests once assigned to this species may actually belong to a small theropod. Laosaurus minimus Gilmore, 1924b from the late Campanian of Alberta may be a related species

subfamily Hypsilophodontinae

Hypsilophodon foxii, life reconstruction. Illustration by Steve Kirk - Illustrated Encyclopedia of Dinosaurs and Prehistoric Animals, ed.. Barry Cox

As used in the present limited context, refers to a fairly advanced small to medium-sized lightly-built fast-running bipedal herbivores. Although Hypsilophodon is the only genus known for certain in this assemblage, the very early (middle Jurassic) Yandusaurus may also belong here. The late Cretaceous Parksosaurus may also be a Hypsilophodontine, although it displays more advanced dryosaur-like features. These animals, or creatures very like them, evolved into dryosaurs.

Yandusaurus hongheensis He, 1979

Comments: This early form may belong in the Hypsilophodontinae

Hypsilophodon foxii skeleton, image from Professor Paul Olsen

Hypsilophodon foxii Huxley, 1869

Comments: A number of partial and complete skeletons, mostly juveniles. It was at one time believed that this animal was arboreal, and there is a famous painting by Neave Parker showing it perched on a large branch. It was later realised that this was a purely ground living, active cusorial (running) animal.

Parksosaurus warrenae. Illustration by Steve Kirk - Illustrated Encyclopedia of Dinosaurs and Prehistoric Animals, ed.. Barry Cox

Parksosaurus warrenae Sternberg, 1937

Comments: A late, advanced form, with a distinctive skull. More advanced than Hypsilophodon in some features, while more primitive in some features of the skull. The large eyes indicate well developed vision, possibly nocturnal or in dim light. Like other hypsilophodonts, this animal probably foraged among the forest undergrowth.

Family Tenontosauridae

Tenontosaurus tillettorum, life reconstruction. Illustration by Steve Kirk - Illustrated Encyclopedia of Dinosaurs and Prehistoric Animals, ed.. Barry Cox

Some of the five middle and late Cretaceous forms shown here have been previously considered Iguanodontids, others Hypsilophodontids. Now it seems some or all of them may be neither.  Jaime Headden   suggests some Australian species may be included with Tenontosaurus and Rhabdodon under a clade "Rhabdomorpha". Stanley Friesen Linenan dinosaur arrangement places them in the family Tenontosauridae. As yet a lot the material is too fragmentary to tell for certain. But if this suggestion is correct than these are all forms that serve as a late transitional taxon between the hypsilophontia and the iguanodontia. While the more primitive genera Qantassaurus and Atlascopcosaurus remained small, bipedal, and typical hypsilophodontian in form, in other members of this group their developed Iguanodontian features such as large size, quadrapedal posture, enlargement of the external nares, loss of premaxillary teeth, and typically iguanodontian skull and skeletal proportions. For this reason these animals are often placed in cladograms between the hypsilophodonts and the Dryosauridae, usually as basal members of the clade Iguanodontia. However, in as much as these animals lived some sixty or seventy million of years after the first dryosaurs and captosaurs - a period of time equal to the entire Cenozoic (age of mammals)!) they cannot be ancestral to the latter. Rather they are seem to have evolved advanced characteristics independently of the Iguanodontian ornithopods, in response to similar environmental conditions. This is exactly the same phenomenon as occured with the various therapsid lineages during the Permian and Triassic, which indepependently evolved mamal-like characteristics (so much so that some paleontologists believed that mammals eveolved from several different groups of therapsids)

Subfamily "Muttaburrasaurinae"

Migrating Muttaburrasaurus herd.
from Walking with Dinosaurs © 1999 ABC, BBC

This is not an official subfamily. It is based on the assumption that first Muttaburrasaurus is a hypsilophodont, and secondary, that the large Muttaburrasaurus and the small Atlascopcosaurus are related. Whether this is true or not, there is no doubt that a number of unique hypsilophodontians populated south-east Gondwanaland, at the time right on the south pole. This assemblage features a greater diversity of hypsilophodonts then anywhere else in the world.

Qantassaurus intrepidus Rich and Vickers-Rich, 1999

Comments: Differs from all other known hypsilophodonts in having only 12 teeth in the lower jaw, indicating a a shorter, deeper face. I have followed Stanley Friesen in including it under the family Tenontosauridae.
 Dann's Dinosaur Info: QANTASSAURUS

Atlascopcosaurus loadsi Rich and Rich, 1989

Comments: Resembles zephyrosaurus in the structure of its unworn maxillary teeth except for a more pronounced primary ridge (ref Sues &Norman) It is also suggested (ref Thescelosaurus!) that Muttaburrasaurus may be closely related to it, despite the latter's much larger size. I have followed Stanley Friesen in including it under the family Tenontosauridae. Some isolated femorii found at the same locality may or may not belong to this species

Muttaburrasaurus langdoni Bartholomai and Molnar, 1981

Comments: Perhaps Australia's best known dinosaur; certainly it represents one of the most complete skeletons of any dinosaur from this region. Originally included under the Iguanodontidae which it resembles in size and body proportions, and compared to Camptosaurus (similar cranial proportions), it is now thought to be more primitive than either. Muttaburrasaurus was most probably quadrapedal, witha broad low skull bearing a remarkable hollow chamber on the snout, remiscent of Altirhinus (Iguanodont) and Kritosaurus (Hadrosaurid). This was probably a resonating chamber, although it may also have enhanced the sense of smell (both options are not exclusive, e.g. calls during the mating season, and smell to detect a mate). Unlike all other ornithopods, Muttaburrasaurus had a very powerful, ceratopsian-like shearing, rather than a grinding, dentition. [ref. Molnar]. This seems to represent an approach to increased chewing efficency distinct from the Iguanodontian one. Although it has been suggested that Muttaburrasaurus may have been partially carnivorous, this seems a misinterpraetation of its unique and obviously efficient oral-processing mechanism. The original identification of an Iguanodon-style thumb-spike also appears to be in error. A second skull is known, from a slightly earlier form, which appears to represent a more primitive (possibly ancestral?) species. I have followed Stanley Friesen in including it under the family Tenontosauridae, but it (and related smaller forms) may well belong in their own family.
 Dann's Dinosaur Info: MUTTABURRASAURUS  Best on the Web

Subfamily Tenontosaurinae

I have included here the Laurasian members of the family. If Mutaburrasaurus and related forms are Tenontosaurs then the two lineages, geographically isolated, followed very different evolutionary paths, although both paralleled the contemporary camptosaurids and iguanodontids, and doubtless fulfileld a similar ecological role. Although the Tenontosaurines are similar in overall size, and in skull proportions, to the Iguanodontids, the structure and ararngement of the teeth place them among the Hypsilophodontoidea.

image from Professor Paul Olsen

Tenontosaurus tillettorum Ostrom, 1970a

Comments: distinguished by its unusually long tail (which may or may not have been an aid in swimming), Tenontosaurus has the misfortune to be continually portayed by dinosaur artists as being torn apart by a pack of hungry Deinonychus. It has been various identified as a hypsilophodont, an iguanodont, and a seperate taxa of its own. As the material spans some ten million years or more, it is not unlikely that several species are included here.

Tenontosaurus dossi Winkler, Murray, and Jacobs, 1997

Comments: Apparently more primitive than T. tillettorum, it is also larger

Rhabdodon priscus Matheron, 1869

Comments: A fairly common but poorly known late Cretaceous European Ornithopod, for a long time considered under the Iguanodontidae, it is now seen as an advanced proto-iguanodontian. I have followed Stanley Friesen in including it under the family Tenontosauridae. R. septimanicus Buffetaut and Le Loeuff, 1991 (France) and R. robustus (Nopcsa, 1900) (Maastrichtian of Romania) may or may not be seperate species. In any case, given the wide stratiigraphic and geographic range, it is likely that more than one species are included here

Books, DVDs and Web Links

Peer Reviewed References and Books (whether Peer Reviewed or not)

Sues, H.-D., & Norman, D. B., 1990: Hypsilophodontidae, Tenontosaurus, Dryosauridae. 498-509 The Dinosauria. University of California Press, Berkley, Los Angeles, Oxford, 1990

Norman, D. B., & Weishampel, D. B., 1990: Iguanodontidae and related ornithopods. 510-533.The Dinosauria. University of California Press, Berkley, Los Angeles, Oxford, 1990

Ralph E. Molnar, Fossil Reptiles of Australia, pp.605-689, in Vertebrate Paleontology of Australia  ed. P. Vickers-Rich, J.M. Monaghan, R.F.Baird & T.H.Rich, p.581

Mass estimates for dinosaur genera - the data set used in Damuth, J. 1994. Potential energetic fitness: why the dinosaurs were so large.

Web Sites

Earth History Portal

Hypsilophodontidae - Palaeos - detailed technical diagnosis, lots of links

Hypsilophodontidae - Fred Bervoets' DinoData - lists every genus and species

- Mikko Haaramo's phylogeny pages

Other

Ornithopoda - Dinosauricon - cladogram of known genera, also short notes suggesting that the Hypsilophodontia are a paraphyletic sequence of ancestors to higher ornithopods.

Ornithopoda - Justin Tweet's Thescelosaurus! - good short summary of each genus and species, including poorly known forms

Suborder Ornithopoda - Linnean (non cladistic) classification, including several hypsilophodont families

Dann's Dinosaur Info: HYPSILOPHODONTIDS

Lecture 20 - The Early Cretaceous - Europe, the Cloverly Formation, Montana, Mongolia and North China an informative page on early and middle Cretaceous dinosaurs, includes material on Hypsilophodon; by Professor Paul Eric Olsen, part of DINOSAURS AND THE HISTORY OF LIFE - GEOLOGY V1001x

Spirits of the Ice Forest - from the popular BBC TV series, available in DVD.

Re: Revised Ornithischian Classification - Jaime A. Headden - some comments on relationships of primitive Ornithopod genera. From the archives of the Dinosaur mailing list