Thescelosaurus

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Thescelosaurus
Fossil range: Late Cretaceous
Thescelosaurus neglectus
Thescelosaurus neglectus
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Sauropsida
Superorder: Dinosauria
Order: Ornithischia
Suborder: Cerapoda
Infraorder: Ornithopoda
Family: Hypsilophodontidae
Subfamily: Thescelosaurinae
Genus: Thescelosaurus
Gilmore, 1913
Species
  • T. neglectus Gilmore, 1913 (type)

Thescelosaurus (pronounced /ˌθɛsɪləˈsɔrəs/, from the Greek θεσκελο-/thescelo- meaning "godlike", "marvelous", or "wondrous" and σαυρος/saurus "lizard")[1] 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.

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, a head with an elongate pointed snout, and possibly small armor scutes along the midline of the back. This genus of dinosaur is regarded as a specialized hypsilophodont and a herbivore. Several species have been suggested for this genus, but only one, T. neglectus, is currently recognized; the others have been given their own genera, or are believed to be the same as T. neglectus (although there may be more than one species represented by the various fossils classified as Thescelosaurus).

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

Contents

[edit] Description

The size of Thescelosaurus compared to a human.
The size of Thescelosaurus compared to a human.

Thescelosaurus was a heavily-built bipedal animal, probably herbivorous,[2] but possibly omnivorous.[3] It would have browsed in the first meter or so from the ground, feeding selectively,[2] with food held in the mouth by cheeks while chewing.[4] Aside from the long narrow beak, the skull also had teeth in the premaxilla, or upper beak (a primitive trait among ornithopods), and long rod-like bones called palpebrals over the eyes, giving it heavy bony eyebrows.[5] Its teeth were of two types: small pointed premaxillary teeth, and leaf-shaped cheek teeth.[6] The exact number of teeth is unknown, as complete jaws have not been described.

Thescelosaurus at the Burpee Museum of Natural History in Rockford, Illinois.
Thescelosaurus at the Burpee Museum of Natural History in Rockford, Illinois.

It 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 back to the tip, which would have reduced the flexibility of the tail.[7] The rib cage was broad, giving it a wide back, and the limbs were robust.[6] This animal may have been able to move on all fours, given its fairly long arms and wide hands,[4] but this idea has not been followed up in the scientific literature (although it does appear in popular works).[8][9] Charles M. Sternberg reconstructed it with the upper arm sticking out almost perpendicular to the body, another idea that has gone by the wayside. 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.[6] 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.[10]

Head and arms of Thescelosaurus, Rocky Mountain Dinosaur Resource Center.
Head and arms of Thescelosaurus, Rocky Mountain Dinosaur Resource Center.

Large thin flat mineralized plates similar to those in Talenkauen have been found next to the ribs' sides.[11] Their function is unknown; they may have played a role in respiration.[12] 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.[13] The nature of this genus' integument, be it scales or something else, is currently unknown, although potential evidence is known; Charles Gilmore described patches of carbonized material near the shoulders as possible epidermis, with a "punctured" texture but no regular pattern,[7] and William J. Morris suggested that armor was present, in the form of small scutes present at least along the midline of the neck.[14]

Overall, the skeletal anatomy of this genus is well-documented (except for the head), and restorations have been published in several papers, including skeletal restorations[7][4][15] and models.[7][6] The skeleton is known well enough that a detailed reconstruction of the hip and hindlimb muscles has been made.[16] The animal's size has been estimated in the 2.5–4.0 m range for length (8.2–13.1 ft)[4] for various specimens, and a weight of 200–300 kilograms (450–660 pounds).[17] As discussed more fully under "Discovery, history, and species", it may have been sexually dimorphic, with one sex larger than the other.[4] Juvenile remains are known from several locations, mostly based on teeth.[18][19]

[edit] Classification

 
Euornithopoda 

Orodromeus


Othnielosaurus

unnamed
unnamed

Gasparinisaura


Hypsilophodon


Zephyrosaurus


unnamed
unnamed

?Bugenasaura


Parksosaurus


Thescelosaurus



Iguanodontia





This cladogram is based on Norman et al. (2004),[2] with the results of the very similar cladogram from Weishampel et al. (2003)[20] used to clarify the position of Iguanodontia, which was left out of Norman et al. The group consisting of Bugenasaura, Parksosaurus, and Thescelosaurus is roughly what modern informal usage of Thescelosaurinae corresponds to.

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.[6][4][21][2] 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.[14] At any rate, Galton's Iguanodontidae was not a natural group due to polyphyly, and so would not be recognized under modern cladistic usage.

Although little tested by cladistics, it is currently thought that Bugenasaura and Thescelosaurus are closely related,[22] or that Thescelosaurus belongs in its own family or subfamily, Thescelosauridae or Thescelosaurinae.[23][24][19] Two recent studies have found it to be a close relative of Parksosaurus, although neither named a specific clade, and one of the studies (Norman et al., 2004) is difficult to interpret because it did not include Iguanodontia in its diagrams.[20][2] This area of the dinosaur family tree is complicated by a lack of research, with some papers finding Hypsilophodontidae to be a natural group,[21][25] and others finding it to be a unnatural family leading into Iguanodontia (paraphyly).[20][2][26] The "natural group" hypothesis has been falling out of favor since the mid to late 1990s, and thus a paraphyletic Hypsilophodontidae is shown here. Oddly, Thescelosaurus has been regarded as both very basal[25] and very derived among the hypsilophodonts.[2] One issue specifically concerning Thescelosaurus is that not all of the remains assigned to T. neglectus necessarily belong to it.[27]

[edit] Discovery, history, and species

Charles Gilmore's 1915 reconstruction of Thescelosaurus.
Charles Gilmore's 1915 reconstruction of Thescelosaurus.

The type specimen of Thescelosaurus (USNM 7757) was discovered in 1891 by paleontologists John Bell Hatcher and William H. Utterback, from beds of the late Maastrichtian-age Upper Cretaceous Lance Formation of Niobrara County, Wyoming, USA. The skeleton, however, remained in its shipping crates for years until Charles W. Gilmore of the Smithsonian Institution' National Museum of Natural History had it prepared and described it in a short paper in 1913, naming it T. neglectus (neglectus: "neglected"). At the time, he thought it was related to Camptosaurus.[28] He provided a detailed monograph in 1915, describing the well-preserved skeleton. The type specimen was found largely in natural articulation and was missing only the head and neck, which were lost due to erosion.[7] The name comes from the surprise Gilmore felt at finding such a good specimen that had been unattended to for so long. He considered it to be a light, agile creature, and assigned it to the Hypsilophodontidae, a family of small bipedal dinosaurs.[7]

Other remains of similar animals were found throughout the late 1800s and 1900s, although they did not receive much attention. Another well-preserved skeleton from the slightly older Horseshoe Canyon Formation, in Alberta, Canada, was named T. warreni by William Parks in 1926.[29] This skeleton had notable differences from T. neglectus, and so Charles M. Sternberg placed it in a new genus, Parksosaurus, in 1937.[30] Sternberg also named an additional species, T. edmontonensis, based on another articulated skeleton, this time including a partial skull (NMC 8537), and drew attention to the genus' heavy build and thick bones; due to these differences from the regular light hypsilophodont build, he suggested that the genus warranted its own subfamily, Thescelosaurinae.[6] T. edmontonensis has, since Peter Galton's 1974 review, generally been considered a more robust individual (possibly the opposite sex of the type individual)[4] of T. neglectus.[21][2] The only sticking point has been the ankle of T. edmontonensis, which Galton claimed was damaged and misinterpreted, but which was accepted by William J. Morris (1976) as truly different from T. neglectus.[14]

Thescelosaurus reconstruction
Thescelosaurus reconstruction

In his paper, Morris described a partial skull with heavy ridges on the lower jaw and cheek (SDSM 7210) as an unidentified species of Thescelosaurus, from the late Maastrichtian-age Hell Creek Formation of Harding County, South Dakota, USA.[14] This skull was recognized as an unnamed hypsilophodont for many years,[21] until Galton assigned it the name Bugenasaura infernalis.[31] Morris also named a new species, on the basis of vertebrae and limb remains (LACM 33542) from the Hell Creek Formation of Garfield County, Montana, USA: T. garbanii. T. garbanii would have been about 4.5 m long (15 feet), larger than average specimens of T. neglectus. Because Morris believed that the ankles of T. garbanii compared favorably to those of T. edmontonensis, he assigned it to Thescelosaurus.[14] However, the scientific literature has favored Galton's view that T. edmontonensis was not different from T. neglectus (see above). To better accommodate this species, Galton in 1995 suggested that it belonged to his new genus Bugenasaura as B. garbanii (although noting that it could also be a leg of the similarly sized pachycephalosaurid Stygimoloch). As a result, only one valid species of Thescelosaurus is currently recognized: T. neglectus.[31] More study, though, could again split the known material into two or more species.[27]

[edit] Paleoecology

[edit] Temporal and geographic range

True Thescelosaurus remains are known definitely only from late Maastrichtian-age rocks, from Alberta (Scollard Formation) and Saskatchewan (Frenchman Formation), Canada, and Wyoming (Lance Formation), South Dakota (Hell Creek Formation), Montana (Hell Creek), and Colorado (Laramie Formation), USA.[2] With the exception of birds, it was one of the last extant genera of dinosaurs, its remains being found as close as 3 meters to the boundary clay containing the iridium layer that closes the Cretaceous.[32] There are reports of teeth from older, Campanian-age rocks, particularly from the Dinosaur Park Formation of Alberta,[33] but these specimens are not from Thescelosaurus and are much more likely those of Orodromeus.[31] More specimens are known than have been officially described for this genus, such as "Willo", with its complete skull, and the Triebold specimen,[3] which has been the source of several skeletal casts for museums.

[edit] Habitat

Two Thescelosaurus skeletons, North American Museum of Ancient Life.
Two Thescelosaurus skeletons, North American Museum of Ancient Life.

Conflicting reports have been made as to its preferred habitat; one paper suggests it preferred channels to floodplains,[34] but another suggests it preferred the opposite.[24] No bonebeds or accumulations of multiple individuals have yet been reported. Dale Russell, in a popular work, noted that Thescelosaurus was the most common small herbivore in the Hell Creek Formation of the Fort Peck area. He described the environment of the time as a flat floodplain, with a relatively dry subtropical climate that supported a variety of plants ranging from angiosperm trees, to cedar and bald cypress, to ferns and ginkgos. Although most dinosaur skeletons from this area are incomplete, possibly due to the low preservation potential of forests, Thescelosaurus skeletons are much more complete, suggesting that this genus frequented stream channels. Thus when a Thescelosaurus died, it may have been in or near a river, making it easier to bury and preserve for later fossilization. Russell tentatively compared it to the capybaras and tapirs.[35] Other dinosaurs that shared its time and place include Bugenasaura, the ceratopsids Triceratops and Torosaurus, hadrosaurids Edmontosaurus and Anatotitan, ankylosaurid Ankylosaurus, pachycephalosaurians Pachycephalosaurus and Stygimoloch, and the theropods Ornithomimus, Troodon, and Tyrannosaurus.[36][37] Thescelosaurus was also abundant in the Lance Formation. Toe bones from this genus are the most common finds after fossils of Triceratops and Edmontosaurus, and it may have been the most common dinosaur, if the Lance Formation had a preservation bias against small animals.[38]

[edit] Paleobiology

[edit] "Heart of stone"

The possible heart of "Willo" (center).
The possible heart of "Willo" (center).

In 2000, a skeleton of this genus informally known as "Willo", now on display at the North Carolina Museum of Natural Sciences, was described as including the remnants of a four-chambered heart and an aorta. It had been originally unearthed in 1993 in northwestern South Dakota. The authors had found the internal detail through computed tomography imagery. They suggested that the heart had been saponified (turned to grave wax) under airless burial conditions, and then changed to goethite, an iron mineral, by replacement of the original material. The authors interpreted the structure of the heart as indicating an elevated metabolic rate for Thescelosaurus, not reptilian cold-bloodedness.[11]

Their conclusions have been disputed; other researchers published a paper where they assert that the heart is really a concretion. As they note, the anatomy given for the object is incorrect (for example, the "aorta" narrows coming into the "heart" and lacks arteries coming from it), it partially engulfs one of the ribs and has an internal structure of concentric layers in some places, and another concretion is preserved behind the right leg.[39] The original authors defended their position; they agreed that it was a type of concretion, but one that had formed around and partially preserved the more muscular portions of the heart and aorta.[40] The question of how this find reflects on metabolic rate and dinosaur internal anatomy may be moot, though, regardless of the object's identity. Both modern crocodilians and birds, the closest living relatives of dinosaurs, have four-chambered hearts (albeit modified in crocodilians), so dinosaurs probably had them as well; the structure is not necessarily tied to metabolic rate.[41]

[edit] In popular culture

The low build and short legs of Thescelosaurus may have given it a small turning radius.
The low build and short legs of Thescelosaurus may have given it a small turning radius.

Thescelosaurus, despite being known from better remains than many dinosaurs, is an uncommon genus in children's books and general dinosaur books, although it may be highlighted in more comprehensive field guides and encyclopedias due to its differences from other hypsilophodonts.[42][8] Books since the scientific publication of "Willo"'s possible heart have incorporated the initial reports, but because these events are still relatively recent, the entire controversy is not necessarily included.[43]

Despite its reputation among professionals for lacking speed and agility,[6][4] Thescelosaurus has been a featured animal in a mathematical modeling problem given to undergraduate students, based around the question of what the best hunting strategy is for one or two pursuing Velociraptor hunting a Thescelosaurus which, due to its bony structure, has a much shorter turning radius.[44][45]

[edit] References

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