Thescelosaurus

Thescelosaurus (pronounced /ˌθɛsɨlɵˈsɔrəs/ the-SEL-ə̇-SOR-əs, from the Greek θεσκελο-/thescelo- meaning "godlike", "marvelous", or "wondrous" and σαυρος/saurus "lizard") 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-Tertiary extinction event around 65.5 million years ago. The preservation and completeness of many of its specimens indicate that it may have preferred to live near streams.

Description
Thescelosaurus was a heavily built bipedal animal, probably herbivorous,[3] but possibly omnivorous.[4] It would have browsed in the first meter or so from the ground, feeding selectively,[3] with food held in the mouth by cheeks while chewing.[5] There was a prominent ridge along the length of both maxillae (the tooth-bearing "cheek" bones),[1] and a ridge on both dentaries (tooth-bearing bone of the lower jaw).[6] The ridges[6] and position of the teeth, deeply internal to the outside surface of the skull, are interpreted as evidence for muscular cheeks.[7] Aside from the long narrow beak, the skull also had teeth in the premaxilla, or upper beak (a primitive trait among ornithopods).[8] Long rod-like bones called palpebrals were present over the eyes, giving the animal heavy bony eyebrows.[8] Its teeth were of two types: small pointed premaxillary teeth, and leaf-shaped cheek teeth.[9] Six small teeth were present in both premaxillae, with a toothless section at the tip of the beak.[1]

Thescelosaurs 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.[10] The rib cage was broad, giving it a wide back, and the limbs were robust.[9] The animals may have been able to move on all fours, given its fairly long arms and wide hands,[5] but this idea has not been widely discussed in the scientific literature, although it does appear in popular works.[11][12] Charles M. Sternberg reconstructed it with the upper arm oriented almost perpendicular to the body,[9] 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.[13] Thescelosaurus was probably slower than other hypsilophodonts, because of its heavier build and leg structure. Compared to them, it had unusual hindlimbs, because the upper leg was longer than the shin, the opposite of Hypsilophodon and running animals in general.[9] One specimen is known to have had a bone pathology, with the long bones of the right foot fused at their tops, hindering swift movement.

Large thin flat mineralized plates have been found next to the ribs' sides.[15] Their function is unknown; they may have played a role in respiration.[16] 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.[17] 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,[10] 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.[7] Scutes have not been found with other articulated specimens of Thescelosaurus, though, and Morris's scutes could be crocodilian in origin.[18]

Overall, the skeletal anatomy of this genus is well documented, and restorations have been published in several papers, including skeletal restorations[1][5][10][19] and models.[9][10] The skeleton is known well enough that a detailed reconstruction of the hip and hindlimb muscles has been made.[20] The animal's size has been estimated in the 2.5–4.0 m range for length (8.2–13.1 ft)[5] for various specimens, and a weight of 200–300 kilograms (450–660 pounds),[21] with the large type specimen of T. garbanii estimated at 4–4.5 meters (13.1–14.8 feet) long.[7] As discussed more fully under "Discovery, history, and species", it may have been sexually dimorphic, with one sex larger than the other.[5] 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.