The ornithopods are a diverse group of ornithischians that include hadrosaurids, "iguanodonts," and "hypsilophodonts."  Hypsilophodonts probably do not fall under one monophyletic group, but are more likely spread out through several. However, as the name is useful in describing small, bipedal, cursorial (running), basal ornithopods, I will use it as an adjective. Similarly, iguanodont refers to animals more bulky than hypsilophodonts, often with spiked thumbs and an ability to walk on all fours, but not having the characters that define hadrosaurids. Hadrosaur-something-or-other (some people use Hadrosauridae, some Hadrosauroidea, and some use Hadrosauria) is the group known informally as "duck-bills." Hadrosaurids proper are distinguished from the more basal iguanodonts and hypsils by their closely packed dental batteries (although 2000 teeth is a fallacy) and their loss of the first digit of the hand (our thumb), among other characters. They are further divided into the "flat-headed" saurolophines and the crested lambeosaurines, with several divisions within this. Some researchers have proposed dividing Hadrosauridae into two families, with the saurolophids descended from Iguanodon-type iguanodonts, and the lambeosaurids descended from Ouranosaurus-type iguanodonts, but this has not caught on.

Hypsilophodonts have historically attracted little attention, and the experienced aficionado will encounter a fair number of undescribed taxa and additional undescribed fossils of named forms. Unsurprisingly, the relationships of hypsils vary between authors, although there is some consensus now that Agilisaurus, Hexinlusaurus ("Yandusaurus" multidens), and Othnielosaurus are basal to ornithopods, and there are a couple of knots among the basal ornithopods. These include the jeholosaurs, long-skulled East Asian forms; the zephyro-"burrowers"; and the bulky thescelosaurs. A 2013 analysis finds these three groups to be allied, forming a Thescelosauridae (it leaves Jeholosauridae a bit awkward, but I suppose they can always call the clade Jeholosauria or something). One question left is what is going on with Gasparinisaura, which sometimes joins the party as a relative of Parksosaurus, and sometimes stays in Iguanodontia. I'm keeping it and its ilk on this page, in the interest of not switching them around after every new paper, and perhaps spurring some kind of resolution based on the principle that if I start walking away from the bus stop, the bus will show up. Of course, the flip side of this principle is that I will be too far away from the bus to go back, which in this case means that Gasparinisaura will end up as an iguanodont, but sometimes you just want closure.

      |    |--Orodrominae
      |    |    |--Oryctodromeus
      |    |    `--+--Albertadromeus
      |    |         |--Orodromeus
      |    |         `--Zephyrosaurus
      |    `--Thescelosaurinae
      |         |--Parksosaurus
      |         `--+--Jeholosauridae? Jeholosauria?
      |              |    |--Haya
      |              |    `--+--Changchunsaurus
      |              |         `--Jeholosaurus
      |              `--Thescelosaurus

Ornithopoda, including Thescelosauridae :  In the past, these animals would have just been referred to Hypsilophodontidae and left there.
    Several ornithopods show possible evidence for cartilaginous scapular extensions, like lizards and horses. These include Thescelosaurus and Hypsilophodon, which both have roughened dorsal margins to their scapulae, and Parksosaurus, which has an actual ossified suprascapular bone. Additionally, a variety of basal ornithopods including Hypsilophodon, Parksosaurus, Thescelosaurus, basal iguanodontians Macrogryphosaurus and Talenkauen, and ornithopod well-wisher Othnielosaurus appear to have unusual mineralized (not necessarily bone) plates located alongside the ribs, with an unknown function.

Taxon or Taxa: Time/Place: Comments:
Oryctodromeus cubicularis Varricchio, Martin, and Katsura, 2007  Cenomanian (LK) of Montana AKA the "burrowing hypsilophodont", the type specimen and paratype juveniles of Oryctodromeus were found in a sandstone-filled burrow. Because of the adult's skeletal specializations, the size of the burrow, and the preservation of the remains, Oryctodromeus appears to have been burrowing. It may have been doing so to raise its young in safety. It appears to be closest to Orodromeus and Zephyrosaurus, both also from Montana. They too may have been burrowers, as they share some of its specializations, and Orodromeus remains have been found preserved in a similar way. Unlike other ornithopods, Oryctodromeus did not have an extensive lattice of ossified tendons on the tail, which would make turning around in a burrow much easier.
Albertadromeus syntarsus Brown, Evans, Ryan, and Russell, 2013 middle-late Campanian (LK) of Alberta Albertadromeus is known from a handful of associated bones, including a cervical rib, two dorsals, a caudal, some shin and foot bones, and the inevitable ossified tendon scraps. The shin bones are the most distinctive; probably most noticeably, the fibula is reduced and partially fused to the tibia. 
Orodromeus makelai Horner and Weishampel, 1988 middle-late Campanian (LK) of Montana Orodromeus is known from material pertaining to several individuals. Nests once assigned here now appear to belong to a theropod, although apparently not Troodon formosus, as had been suggested.  Orodromeus had a body plan well suited for running. It appears to have had a jugal boss like Zephyrosaurus. Like Oryctodromeus, it may have made burrows.
Zephyrosaurus schaffi Sues, 1980 late Aptian-early Albian (EK) of Montana Zephyrosaurus is based on a partial skull and some verts. The skull shows an odd jugal expansion, giving this animal a prominent "cheek" bump. More material has been assigned to this taxon, including most of the limbs. This material indicates it had very small hands compared to the feet. 
Parksosaurus warrenae Sternberg, 1937 (originally Thescelosaurus warreni Parks, 1926) late Campanian-middle Maastrichtian (LK) of Alberta This animal is known from an incomplete skeleton and skull. The legs are suited for running, and the skull is distinctive. An unusual bone, a "suprascapula" is preserved above the scapula, a rare occurrence. After its long sojourn in "Hypsilophodontidae," new studies are reuniting Parksosaurus with its original genus Thescelosaurus, although no one is suggesting making them the same genus again.
Haya griva Makovicky, Kilbourne, Sadleir, and Norell, 2011 ?Santonian (LK) of Mongolia Third exemplar of an apparent East Asian Cretaceous radiation of hypsil-types, Haya is represented by at least eight specimens (including a quite lovely articulated postcranium) that together cover essentially the entire specimen. The tail seems to have lacked ossified tendons, and one specimen includes a mass of pebbles thought to represent gastroliths.
Changchunsaurus parvus Zan, Chen, Jin, and Li, 2005 Aptian-Cenomanian (EK-LK) of China This cerapod is based on a partial skeleton (mostly presacral, lacking forearms and hands) with skull, and a couple of referred skull fragments. It had five premaxillary teeth, a jugal boss, relatively short toothless gaps at the tip of the premaxilla and between the premax and maxilla, and premax bottom margin at about the same height as that of the maxilla. The long predentary and form of the quadrate remind me of Thescelosaurus. Changchunsaurus appears to be very comparable to Jeholosaurus.
Jeholosaurus shangyuanensis Xu, Wang, and You, 2000 Barremian-early Aptian (EK) of China A Yixian dinosaur, this appears to be something similar to a hypsilophodont, perhaps a representative of a localized east Asian offshoot. It is based on a nearly complete but dorsally compressed skull, along with postcranial remains, with several other skulls and skeletons referred.
Thescelosaurus (including Bugenasaura Galton, 1995): Gilmore, 1913 T. neglectus (type) Gilmore, 1913 late Maastrichtian (LK) of Wyoming; Thescelosaurus skeletons not assigned to a species known from Alberta and South Dakota One of the last dinosaurs, and my favorite, Thescelosaurus has also been a bit of a wanderer among the hypsilophodonts. It was a long (over three meters in some cases), low, heavily-built animal, with short, stout limbs, and may have come down on all-fours at times. Honestly, it's not something you notice until you get a chance to handle bones, but it's astonishing just how robust it was. It has always been seen as a little different from the other hypsils, or at least it has since there've been other hypsils, sometimes thought of as very derived among that group (1970s, now), sometimes thought of as very basal (1990s). Thescelosaurus doesn't seem to be as well-suited for speed as other hypsilophodonts, but would have had a tight turning radius, due to its low center of gravity. Its basal status is corroborated by its premaxillary teeth and the fact it still had four full toes per foot, along with other characters. It is known from several partial skeletons and at least one that includes a skull, which is very long and low with a pointy snout. 
Armor has been reputed to have been found with one specimen, above the first cervicals, although this claim has not really been considered much since it was published (Morris, 1976). Other material found with the new "Willo" specimen (see below) has put bony(?) plates along the ribcage, but these were exceedingly thin (3 mm or so at their thickest) and not suitable for defense. They may be related to the uncinate processes found in birds and a variety of reptiles, but at this point they are most like similar broad thin ribcage plates in Talenkauen, a basal iguanodontian.  
Its name comes from external circumstances; the type specimen was first excavated in 1891, but was only brought to light in the 1910s, thus the specific name neglectus, for neglected. Its generic name, which means surprising, wonderful, or marvelous lizard or reptile, comes from the fact its type was a mostly complete skeleton, missing only the skull and neck.
There may be sexual dimorphism in this animal; a specimen named T. edmontonesis (Sternberg, 1940) is somewhat more heavily built than the type. This difference could mean that this specimen comes from the gender opposite of the type.  
Remains attributed to this genus have been found from New Mexico to Alaska throughout the Maastrichtian, but all of the most diagnostic remains referable to Thescelosaurus have so far come from late Maastrichtian age formations. It was a characteristic Lancian dinosaur.
Back in 2000, we were treated to thescelosaur specimen "Willo", the dinosaur with a "heart of stone". Included within its chest area is a large concretion, which was first described as a possible heart. Almost immediately this was challenged as a more mundane concretion that happened to be found in the heart region. In 2011, a team of researchers published a study on the results of unleashing numerous investigative techniques on the object, including some not possible in 2000. They concluded that it was most likely an inorganic concretion.
Like most North American hypsils, there is a fair amount of useful material that has been assigned to Thescelosaurus than has not been described (see also: Drinker, Orodromeus, honorary hypsil Othnielosaurus, Zephyrosaurus). There may be multiple taxa concealed in these fossils, which wouldn't surprise me, given: we've already got at least two species as it is; isolated pachycephalosaurian limb bones and post-cervical verts are pretty similar to corresponding thescelosaur elements; and for some reason researchers have historically been hypsil-averse.
Second species T. garbanii was named for hindlimb remains and verts, from an animal larger than the other best specimens of Thescelosaurus. It differs from T. neglectus in ankle structure. Between 1995 and 2009 it was usually represented as a second species of Bugenasaura, with the possibility of being something more exotic like postcranial fossils of the similarly-sized pachycephalosaurid Stygimoloch (subadult Pachycephalosaurus). An additional unnamed species may also be present.
Bugenasaura infernalis was based mostly on a partial skull noted for the heavy ridges on the upper and lower jaws. Further discoveries of thescelosaur skulls with postcrania showed that B. infernalis is essentially the same as the others, and it is now regarded as T. sp.
T. assiniboiensis is based on a small (possibly immature) partial skeleton from Saskatchewan. It differs from the other species by the form of the squamosal and the possession of a small hole in the upper rear of the braincase, and it lacks the fancier ankle of T. garbanii.
T. assiniboiensis Brown, Boyd, and Russell, 2011 late Maastrichtian (LK) of Saskatchewan
T. garbanii Morris, 1976 late Maastrichtian (LK) of Montana
Hypsilophodon foxii Huxley, 1869 late Barremian (EK) of England Hypsilophodon is known from a lot of good material, including several skeletons; interestingly, it is essentially limited to one prolific locality in the upper Wessex Formation. It was very much an average hypsilophodont, although most restorations understate the size, as much of the known material is juvenile.
In the past, it was common to show this animal perched in trees, because it was thought to have had numerous climbing adaptations, but it is now understood that it was better adapted to a running life on the ground.
Supposed armor has turned out to be the remnants of mineralized intercostal (laying on the ribcage, essentially) plates, like those of Talenkauen and Thesceloasurus.

Ornithopoda i.s.:

Taxon or Taxa: Time/Place: Comments:
Atlascopcosaurus loadsi (N.D.) Rich and Rich, 1989 late Aptian-early Albian (EK) of Australia Atlascopcosaurus may have some similarities to the orodromines ("zephyrosaurs"). It is based on a partial upper jaw.
Eucercosaurus tanyspondylus (N.D.) Seeley, 1879 late Albian (EK) of England This taxon is sometimes considered a junior synonym of Acanthopholis, but it appears that it is actually an ornithopod.
Fulgurotherium australe (N.D.) Huene, 1932 late Aptian-Albian (EK) of Australia This animal spent a while as a dubious coelurosaurian before its true affinities were discovered. It currently could best be called a complex, because bones referred to it may pertain to up to four types of hypsilophodonts and basal iguanodonts. It might be a basal iguanodont, although the iguanodont-noniguanodont boundary is a bit unsettled at this time.
Gasparinisaura cincosaltensis Coria and Salgado, 1996 Santonian-early Campanian (LK) of Argentina Gasparinisaura was described as a dryosaur-like small iguanodontian, although some recent analyses indicates that it was closer to or less derived than Hypsilophodon. It is known from over 15 individuals, including the type specimen, a mostly-complete juvenile skeleton. As with many small ornithopods, it appears that the known skeletal sample is of partially-grown individuals.
Gideonmantellia amosanjuanae Ruíz-Omeñaca, Canudo, Cuenca-Bescós, Cruzado-Caballero, Gasca, and Moreno-Azanza, 2012 early Barremian (EK) of Spain Gideonmantellia can be understood as the Spanish "Hypsilophodon" of previous references. It is known from a partial juvenile specimen mostly comprising 33 dorsal, sacral, and caudal vertebrae, and pelvic and hindlimb material.
"Hypsilophodon" wielandi (N.D.) Galton and Jensen, 1979 Barremian (EK) of South Dakota This species, based on a femur, cannot be confidently assigned to Hypsilophodon, and is best considered a dubious hypsilophodont-type ornithopod.
Koreanosaurus boseongensis Huh, D.-G. Lee, J.-K. Kim, J.-D. Lim, and Godefroit, 2010 ?Santonian-Campanian (LK) of South Korea After having spent many years as the unofficial cognomen of a maniraptoran femur, Koreanosaurus finally officially enters the literature for a probably burrowing hypsil. It is based on most of the pre-pelvic half of an articulated skeleton (no head), but hips and legs were found 2 m away and likely belong to the same individual. Should this be so, Koreanosaurus joins a few sauropods (think Brachiosaurus and Giraffatitan) among classic dinosaurs with humeri longer than femurs; its scapula is also longer than the femur. The arms and shoulders are heavily built, and it may well have been quadrupedal.
Laosaurus celer (N.D.) Marsh, 1878 Kimmeridgian (LJ) of Wyoming The vertebrae that this taxon is based on could come from just about any hypsilophodont.
"Laosaurus" minimus (N.D.) Gilmore, 1924 middle-late Campanian (LK) of Alberta Based on a partial hindlimb and some verts, this species has been sometimes suggested as pertaining to Orodromeus, although the material is too sparse to be certain.
Leaellynasaura amicagraphica Rich and Rich, 1989 late Aptian-early Albian (EK) of Australia This hypsilophodont has some similarities to the "othnieliids". It is known from material including a skull, which has very large eyes and suggests both youth and an ability to see better when Australia, which was then much closer to the South Pole, was plunged into periods of very short days. Its tail was exceptionally long, on the order of three times the length of the rest of the body.
Notohypsilophodon comodorensis Martinez, 1999 Cenomanian (LK) of Argentina The affinities of this animal are not yet completely clear. Based on an incomplete skeleton of a partially-grown individual, its remains include an assortment of verts from the entire body, a partial shoulder girdle, and partial fore and hindlimbs.
Qantassaurus intrepidus Rich and Vickers-Rich, 1999 Albian (EK) of Australia Qantassaurus is named for Qantas, an Australian airline. It is based on jaw material with teeth superficially similar to those of Zalmoxes.
Trinisaura santamartaensis Coria, Moly, Reguero, Santillana, and Marenssi, 2013 Campanian (LK) of Antarctica Trinisaura extends the hypsil empire to Antarctica, if by hypsil we accept all of those basal ornithopod-like dinosaurs. It was probably more derived than Hypsilophodon; think Gasparinisaura-grade. It is known from the partial skeleton of a subadult, including caudals, partial pectoral and pelvic girdles (including a scapula with a large spiny process), a humerus, and most of a hindlimb.
Yandusaurus hongheensis He, 1979 Oxfordian (LJ) of China Yandusaurus is known from decent material, which shows it to be a fairly average hypsilophodont, but as far as press goes, in this group of obscurities, it's one of the most obscure.
Yueosaurus tiantaiensis Zheng W., Jin X., Shibata, Azuma, and Yu F., 2011 Aptian-Cenomanian (EK-LK) of China Yueosaurus is a basal ornithopod known from a partial postcranial skeleton. It will be interesting to see how it compares to other East Asian basal ornithopods, such as Changchunsaurus, Haya, and Jeholosaurus

Iguanodontia: From here, the ornithopods get larger, more and more heavily built, more quadrupedal, and less suited for swift running.

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