ornithopoda

Ornithopoda

The ornithopods are a diverse group of ornithischians that include hadrosaurids, "iguanodonts," and "hypsilophodonts." Hypsilophodonts probably do not fall under one monophyletic group (although this idea hasn't really caught on yet), 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" hadrosaurines and the crested lambeosaurines, with several divisions within this. Some researchers divide Hadrosauridae into two families, and have the hadrosaurids descended from Iguanodon-type iguanodonts, and the lambeosaurids descended from Ouranosaurus-type iguanodonts. A lot of the various hypsilophodont groupings are based on tooth characters, which are sometimes very esoteric.

This following diagram is very sketchy, given that comparatively few people are currently publishing on "hypsils" and so far they're contradictory even on the question of a monophyletic Hypsilophodontidae. For example, Thescelosaurus is bouncing all over the place, from basal ornithopod to hypsilophodontid to more derived than Hypsilophodon. The following is based mostly on Norman, Sues, Witmer, and Coria (2004; their Dinosauria paper), and Weishampel, Jianu, Csiki, and Norman (2003; description of Zalmoxes), where I used an unscientific reduction of the two. Agilisaurus, Hexinlusaurus ("Yandusaurus" multidens), and Othnielosaurus tend to show up outside of the hypsils, as I have them now, with Orodromeus next, followed by a knot of hypsils that may form a Hypsilophodontidae, depending on what the next reports suggest, followed by an unnamed knot of hypsils that correspond to Thescelosauridae. There really hasn't been enough work to assign these animals to higher levels than genera yet. Interestingly, the former paper finds Heterodontosauridae to end up with the marginocephalians fairly often in the analyses (traditionally, it's been thought of as the most basal group of ornithopods).

Ornithopoda: In the past, these animals would have just been referred to Hypsilophodontidae and left there. The "zephyrosaurids" above may be linked by skull characters, including a strange bump on the jugal.
    Several ornithopods show possible evidence for cartilagenous 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.
    Juvenile hypsilophodont material, sometimes referred to Hypsilophodon, is known from the late Hauterivian-early Barremian (EK) of Spain.

Taxon or Taxa:	Time\Place:	Comments:
Orodromeus makelai Horner and Weishampel, 1988	late middle 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.
Hypsilophodon foxii Huxley, 1869	Barremian (EK) of England	Hypsilophodon is known from a lot of good material, including several skeletons. It was a common Wealden Formation dinosaur, and 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. It may have been lightly armored.
Gasparinisaura cincosaltensis Coria and Salgado, 1996	Santonian-early Campanian (LK) of Argentina	Gasparinisaura was described as a dryosaur-like small iguanodontian, although recent analysis indicates that it was closer to Hypsilophodon. It is known from over 15 individuals, including the type specimen, a mostly-complete juvenile skeleton.
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.

Ornithopoda i.s.:

Taxon or Taxa:	Time\Place:	Comments:
Atlascopcosaurus loadsi Rich and Rich, 1989	late Aptian-early Albian (EK) of Australia	Atlascopcosaurus may have some similarities to the "zephyrosaurids". It is based on a partial upper jaw. Muttaburrasaurus may be related to it.
?Changchunsaurus parvus Zan, Chen, Jin, and Li, 2005	?Aptian (EK) of China	This basal ornithopod is based on a skeleton 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 premaxillary teeth, short toothless sections, and even margins suggest a pretty basal ornithopod, as basal ornithopods go. The long predentary and form of the quadrate remind me of Thescelosaurus.
?Drinker nisti Bakker, Galton, Siegwarth, and Filla, 1990 (?Othnielosaurus)	Kimmeridgian (LJ) of Wyoming	Closely resembling Othnielosaurus, Drinker is named after Edward Drinker Cope, contrasting Othnielosaurus, which is named after his arch-rival, Othniel Charles Marsh. It may end up as a species of Othnielosaurus. Drinker had unusual long spreading toes, suggesting it could negotiate swampy terrain well. Bob Bakker claims to have found the remains of over thirty individuals in what might have been a burrow.
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 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 it may include remains from up to four types of hypsilophodonts.
"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.
?Jeholosaurus shangyuanensis Xu, Wang, and You, 2000	Valanginian-early Barremian (EK) of China	A new Yixian dinosaur, this appears to be some sort of hypsilophodont, or a representative of a localized east Asian offshoot. It is based on a nearly complete but dorsally compressed skull, along with postcranial remains, with another skull and additional postcranial remains referred.
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	late middle 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 poor 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.
Loncosaurus argentinus (N.D.) Ameghino, 1898	Santonian-early Campanian (LK) of Argentina	Long misidentified due to theropod teeth mixed in with the type, this animal is some sort of ornithopod.
?Nanosaurus agilis Marsh, 1877	Kimmeridgian (LJ) of Colorado and Utah	At one time considered a "fabrosaur"-type ornithischian, Nanosaurus is most likely a basal ornithopod, although it could be related to the Fruita heterodontosaurid. It is based on molds of its teeth, with other material, including an ilium, femorae, tibiae, and a fibula, referred to it.
"Nanosaurus" rex (N.D.) Marsh, 1877	Kimmeridgian (LJ) of Colorado	It was a nice run while it lasted. Marsh named this species for a femur, that Galton later deemed worthy of its own genus which, when combined with a couple of good partial skeletons, ruled the Morrison of our imaginations for nearly three decades as Othnielia (and you thought I was going to say Allosaurus). However, the femur wasn't that distinct, while the skeletons were, so a Lagosuchus\Marasuchus has been pulled, with the skeletons transfered to Othnielosaurus consors (from an obscure species of Laosaurus that had better remains for a type specimen), leaving Othnielia, and "Nanosaurus" rex, to the dustbin of history (which is too bad, because it was always funny to have an ornithopod with rex as its species name; it's like naming a tiny dog Rex).
Notohypsilophodon comodorensis Martinez, 1999	late Cenomanian-early Turonian (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.
Oryctodromeus cubicularis Varricchio, Martin, and Katsura, 2007	Cenomanian (LK) of Montana	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.
Phyllodon henkeli (?N.D.) Thulborn, 1973	early Kimmeridgian (LJ) of Portugal	This is an ornithopod tooth taxon of uncertain affiliation. It could be a valid genus akin to Drinker.
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.
Siluosaurus zhangqiani Dong, 1997	Barremian-Albian (EK) of China	Siluosaurus, based on two teeth, is possibly the smallest known ornithopod.
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.

Thescelosauridae: It had been in the past that all late Maastrichtian western North American "hypsil" remains were referred to Thescelosaurus, but with the appearance of Bugenasaura and ?B. garbanii, this is no longer justified unless skull or hindlimb material is found for comparison. Thescelosaurid material is actually not that uncommon, but has suffered from a dearth of study. These animals differ from more basal hypsilophodonts by their longer skulls and increased number of teeth.

Taxon or Taxa:		Time\Place:	Comments:
Bugenasaura: Galton, 1995	B. infernalis (type) Galton, 1995	late Maastrichtian (LK) of South Dakota	B. infernalis is based on a partial skull belonging to a basal ornithopod. Its specific name, infernalis, refers to the Hell Creek Formation, where it was found. Additional material is known but not yet described.
Bugenasaura: Galton, 1995	?B. garbanii (Morris, 1976 [originally Thescelosaurus])	late Maastrichtian (LK) of Montana	The remains referred here, including hindlimb remains and verts, could belong to Bugenasaura, or a similar basal ornithopod, or alternately could represent the body of the similarly-sized pachycephalosaurid Stygimoloch. The remains pertain to an animal larger than the best specimens of Thescelosaurus.
?Parksosaurus warrenae Sternberg, 1937 (originally Thescelosaurus warreni Parks, 1926)		early 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.
Thescelosaurus neglectus Gilmore, 1913		late Maastrichtian (LK) of Alberta, Saskatchewan, Colorado, Montana, South Dakota, and Wyoming	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. 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), and later referred to the type, 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 remains referable to T. neglectus have so far come from late Maastrichtian age formations. It was a characteristic Lancian dinosaur. Recently, it was revealed that one specimen may preserve the remains of a heart. Nicknamed "Willo," the specimen is a large, mostly-complete skeleton. Included within its chest area is a large concretion, which when subjected to scanning appears to show two ventricles and one aorta, suggesting the animal possessed a four-chambered, single-aorta heart. This interpretation, if correct, would indicate Thescelosaurus had high blood pressure, a possible sign of endothermy. There may be other organs associated with this specimen as well. Alternately, it could just be oddly-hardened mud. 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: Bugenasaura, 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 Bugenasaura; 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

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

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