Thescelosaurus

*Thescelosaurus neglectus*

* Timothy Bradley Artwork
* Joe Tucciarone Artwork

***Thescelosaurus*** was a genus of small ornithopod dinosaur that appeared at the very end of the Late Cretaceous period in North America. It was a member of the last dinosaurian fauna before the Cretaceous–Paleogene extinction event around 66 million years ago. The preservation and completeness of many of its specimens indicate that it may have preferred to live near streams.

This bipedal ornithopod is known from several partial skeletons and skulls that indicate it grew to between 2.5 and 4.0 meters (8.2 to 13.1 ft) in length on average. It had sturdy hind limbs, small wide hands, and a head with an elongate pointed snout. The form of the teeth and jaws suggest a primarily herbivorous animal. This genus of dinosaur is regarded as a specialized basal ornithopod, traditionally described as a hypsilophodont, but more recently recognized as distinct from *Hypsilophodon*. Several species have been suggested for this genus. Three currently are recognized as valid: the type species *T. neglectus*, *T. garbanii* and *T. assiniboiensis*.

The genus attracted media attention in 2000, when a specimen unearthed in 1993 in South Dakota, United States, was interpreted as including a fossilized heart. There was much discussion over whether the remains were of a heart. Many scientists now doubt the identification of the object and the implications of such an identification.

## Description

*Thescelosaurus* was a heavily built bipedal animal, probably herbivorous, but potentially not. There was a prominent ridge along the length of both maxillae (the tooth-bearing "cheek" bones), and a ridge on both dentaries (tooth-bearing bone of the lower jaw). The ridges and position of the teeth, deeply internal to the outside surface of the skull, are interpreted as evidence for muscular cheeks. Aside from the long narrow beak, the skull also had teeth in the premaxilla, or upper beak (a primitive trait among ornithopods). Long rod-like bones called palpebrals were present over the eyes, giving the animal heavy bony eyebrows. Its teeth were of two types: small pointed premaxillary teeth, and leaf-shaped cheek teeth. Six small teeth were present in both premaxillae, with a toothless section at the tip of the beak.

*Thescelosaurus* had short, broad, five-fingered hands, four-toed feet with hoof-like toe tips, and a long tail braced by ossified tendons from the middle to the tip, which would have reduced the flexibility of the tail. The rib cage was broad, giving it a wide back, and the limbs were robust. The animals may have been able to move on all fours, given its fairly long arms and wide hands, but this idea has not been widely discussed in the scientific literature, although it does appear in popular works. Charles M. Sternberg reconstructed it with the upper arm oriented almost perpendicular to the body, another idea that has gone by the wayside. As noted by Peter Galton, the upper arm bone of most ornithischians articulated with the shoulder by an articular surface that consisted of the entire end of the bone, instead of a distinct ball and socket as in mammals. The orientation of the shoulder's articular surface also indicates a vertical and not horizontal upper arm in dinosaurs.

Large thin flat mineralized plates have been found next to the ribs' sides. Their function is unknown; they may have played a role in respiration. However, muscle scars or other indications of attachment have not been found for the plates, which argues against a respiratory function. Recent histological study of layered plates from a probable subadult indicates that they may have started as cartilage and became bone as the animal aged. Such plates are known from several other ornithopods and their cerapodan relatives.

The nature of this genus' integument, be it scales or something else, is currently unknown, although potential evidence exists: Charles Gilmore described patches of carbonized material near the shoulders as possible epidermis, with a "punctured" texture, but no regular pattern, and William J. Morris suggested that armor was present, in the form of small scutes he interpreted as located at least along the midline of the neck of one specimen. Scutes have not been found with other articulated specimens of *Thescelosaurus*, though, and Morris's scutes could be crocodilian in origin.

Overall, the skeletal anatomy of this genus is well documented, and restorations have been published in several papers, including skeletal restorations and models. The skeleton is known well enough that a detailed reconstruction of the hip and hindlimb muscles has been made. The animal's size has been estimated in the 2.5–4.0 m range for length (8.2–13.1 ft) for various specimens, and a weight of 200–300 kilograms (450–660 pounds), with the large type specimen of *T. garbanii* estimated at 4–4.5 meters (13.1–14.8 feet) long. The dinosaur may have been sexually dimorphic, with one sex larger than the other. Juvenile remains are known from several locations, mostly based on teeth.

## Classification

*Thescelosaurus* has generally been allied to *Hypsilophodon* and other small ornithopods as a hypsilophodontid, although recognized as being distinct among them for its robust build, unusual hindlimbs, and, more recently, its unusually long skull.[3][5][9][25] Peter Galton in 1974 presented one twist to the classic arrangement, suggesting that because of its hindlimb structure and heavy build (not cursorial, or built for running, by his definition), it should be included in the Iguanodontidae. This has not been followed, with Morris arguing strongly against Galton's classification scheme.[7] At any rate, Galton's Iguanodontidae was polyphyletic and not a natural group, and so would not be recognized under modern cladistic usage.

Although Hypsilophodontidae was interpreted as a natural group in the early 1990s,[25][26] this hypothesis has fallen out of favor and Hypsilophodontidae has been found to be an unnatural family composed of a variety of animals more or less closely related to Iguanodontia (paraphyly), with various small clades of closely related taxa.[1][3][27][28][29] "Hypsilophodontidae" and "hypsilophodont" are better understood as informal terms for an evolutionary grade, not a true clade. *Thescelosaurus* has been regarded as both very basal[26] and very derived[3] among the hypsilophodonts. One issue that has potentially interfered with classifying *Thescelosaurus* is that not all of the remains assigned to *T. neglectus* necessarily belong to it.[30] Clint Boyd and colleagues found that while the clade *Thescelosaurus* included the genus *Bugenasaura* and the species that had been assigned to that genus, there were at least two and possibly three species within *Thescelosaurus*, and several specimens previously assigned to *T. neglectus* could not yet be assigned to a species within the genus.[1] It appears to be closely related to *Parksosaurus*.[1][3][28][31][32]

The dissolution of Hypsilophodontidae has been followed by the recognition of the distinct family Thescelosauridae. This area of the dinosaur family tree has historically been complicated by a lack of research, but papers by Clint Boyd and colleagues[1] and Caleb Brown and colleagues[31][32] have specifically addressed these dinosaurs. Boyd et al. (2009) and Brown et al. (2011) found North American "hypsilophodonts" of Cretaceous age to sort into two related clusters, one consisting of *Orodromeus*, *Oryctodromeus*, and *Zephyrosaurus*, and the other consisting of *Parksosaurus* and *Thescelosaurus*.[1][31] Brown et al. (2013) recovered similar results, with the addition of the new genus *Albertadromeus* to the *Orodromeus* clade and several long-snouted Asian forms (previously described under Jeholosauridae)[27] to the *Thescelosaurus* clade. They also formally defined Thescelosauridae (*Thescelosaurus neglectus*, *Orodromeus makelai*, their most recent common ancestor, and all descendants) and the smaller clades Orodrominae and Thescelosaurinae.

## JPInstitute.com Description

A small, agile plant-eater, Thescelosaurus was among the last of the dinosaurs and was probably wiped out in the final extinction 65 million years ago. It was also one of those "basement discoveries," dug up in 1891 and stored away for more than 20 years before it was re-discovered in its packing crate.

In 1993 a very fine specimen was discovered in South Dakota by amateur paleontologist Mike Hammer. This skeleton had a complete skull that showed a very efficient set of at least three different types of teeth, and contained evidence that this dinosaur had cheeks that would have improved its food processing efficiency. Bipedal, with a relatively long tail, its front legs were sturdy, but not particularly long. It had fairly long front claws, which may have been useful for digging. Its hind legs were not built for the speed evidenced by other members of the hypsilophodont family.

One of the most unusual aspects of the 1993 specimen is that it seems to have a fossilized heart. This was widely publicized, and following studies conducted by Dr. Dale Russell, it was concluded that the heart's structure suggests an intermediate form somewhere between crocodiles and birds. Many scientists feel that this is evidence that some dinosaurs had high metabolic rates, suggesting that they were warm-blooded. One of the most spectacular fossil finds of all time, the specimen is on display at the North Carolina Museum of Natural Sciences

## Dinosaur Field Guide Description

hescelosaurus
("marvelous
lizard") was originally collected
in
Wyoming in 1891 by legendary field
paleontologist John Bell Hatcher. He
shipped it back to his employer, Pro-
fessor Othniel C. Marsh, at Yale Univer-
sity. It stayed packed up and ignored
until the turn of the century, It was then
given to the Smithsonian Institution as a
part of the Marsh Collection. And there
it remained, still packed up, until 1913.
When Charles Gilmore finally unpacked
it, he immediately knew the specimen
was a new dinosaur. He named it
Thescelosaurus neglects, or "marvelous
but neglected lizard"!
Thescelosaurus is part of the
ornithopod family Hypsilophodontidae.
All its members have relatively small
arms, well-muscled legs, and a stiffened
tail. Most known specimens are from
juveniles.
One rare adult Thescelosaurus spec-
imen made worldwide headlines in
2000. It was sold by a commercial col-
lector and nicknamed "Willo" It has a
nodule inside the chest cavity that a few
scientists believe is a fossilized heart.
The specimen is now under intense
study.

### Fun Facts

Skin
samples (which are
very rare!) are also known
for this dinosaur

### Trivia

The original
specimen is on display at the
Smithsonian Institution in
Washington, D.C.

## Links

http://web.archive.org/web/20040806050104fw\_/http://www.jpinstitute.com/dinopedia/dinocards/dc\_thesc.html#