Thescelosaurus
Thescelosaurus is a genus of ornithischian dinosaur that lived during the Late Cretaceous period in western North America. It was named and described in 1913 by the paleontologist Charles W. Gilmore; the type species is T. neglectus. Two other species, T. garbanii and T. assiniboiensis, were named in 1976 and 2011, respectively. Additional species have been suggested but are currently not accepted. Thescelosaurus is the eponymous member of its family, the Thescelosauridae. Thescelosaurids are either considered to be basal ornithopods, or are placed outside of this group within the broader group Neornithischia.
Adult Thescelosaurus would have measured roughly long and probably weighed. It moved on two legs, and its body was counter-balanced by its long tail, which made up half of the body length and was stiffened by rod-like ossified tendons. The animal had a long, low snout that ended in a toothless. It had more teeth than related genera, and the teeth were of different types. The hand bore five fingers, and the foot four toes. Thin plates are found next to the ribs' sides, the function of which is unclear. Scale impressions are known from the leg of one specimen. An herbivore, Thescelosaurus was likely a selective feeder, as indicated by its teeth and narrow snout. Its limbs were robust, and its was longer than its , suggesting that it was not adapted to running. Its brain was comparatively small, possibly indicating small group sizes of two to three individuals. The senses of smell and balance were acute, but hearing was poor. It might have been burrowing, as acute smell and poor hearing are typical for modern burrowing animals. Burrowing has been confirmed for the closely related Oryctodromeus, and might have been widespread in thescelosaurids. The genus attracted media attention in 2000, when a specimen unearthed in 1999 was interpreted as including a fossilized heart, but scientists now doubt the identification of the object.
Thescelosaurus has been found across a wide geographic range across western North America. The first specimens were discovered in the Lance Formation of Wyoming, but subsequent discoveries have been made in North Dakota, South Dakota, Montana, Alberta, and Saskatchewan, in geological formations including the Frenchman Formation, Hell Creek Formation, and Scollard Formation. It was relatively common, and may have been the most common dinosaur in the Frenchman Formation. Living during the late Maastrichtian age, it was among the last of the non-avian dinosaurs before the entire group went extinct during the Cretaceous–Paleogene extinction event around 66 million years ago.
Discovery and history
''T. neglectus'' and its type specimen
The first specimens of what would later be named Thescelosaurus were discovered during the Bone Wars, a heated rivalry between the paleontologists Edward Drinker Cope and Othniel Charles Marsh. In July 1891, the fossil hunter John Bell Hatcher, who had been hired by Marsh, and his assistant William H. Utterback discovered a near-complete skeleton of a small herbivorous dinosaur along Doegie Creek in Niobrara County, Wyoming, in rocks of the Lance Formation. The skeleton was found lying on its left side and largely in natural, with only the head and neck lost to erosion. It was taken to the Smithsonian Institution's National Museum of Natural History, where it remained in its original, unlabelled packing box. In 1903, the USNM hired the paleontologist Charles W. Gilmore to work on the extensive collection that had been amassed under the direction of Marsh, who had died in 1899. It was not before 1913 that Gilmore opened the box and, to his surprise, found the skeleton of a new species of dinosaur. In 1913, Gilmore published a preliminary description naming the new genus and species Thescelosaurus neglectus. In addition to Hatcher's specimen, which became the type specimen of the new species, Gilmore assigned a second, more fragmentary skeleton from Lance Creek, also in Niobrara County, to the species. The generic name derives from the Greek words θέσκελος,, and σαυρος or. The specific name, neglectus, is Latin for or, as the type specimen had been unattended to for so long.File:Gilmorethesc.jpg|thumb|right|Charles Gilmore's 1915 skeletal reconstruction of the T. neglectus type specimen with head and neck after Hypsilophodon
Gilmore published a comprehensive description in 1915 after the type specimen was fully prepared. He identified six more specimens, including a shoulder blade with coracoid, a neck vertebra, and a toe bone, as well as three partial skeletons that had been collected by Barnum Brown and were stored in the American Museum of Natural History. The neck and skull remained unknown, however, and Gilmore restored these missing parts based on Hypsilophodon, which he considered a close relative, in his skeletal and life reconstructions. For the museum display of the type specimen, Gilmore maintained its original posture and incompleteness. Only the right leg, which was slightly dislocated, was adjusted in position, and some minor damage to the bones was restored, but painted lighter than the original bones so that the real and reconstructed parts could be distinguished visually. In 1963, the display was included in a wall mount alongside the ornithischians Edmontosaurus and Corythosaurus and the theropod Gorgosaurus. In 1981 the display was rearranged, placing Thescelosaurus higher and more out-of-sight. Renovations of the exhibit from 2014 to 2019 removed the Thescelosaurus and other dinosaurs on display, replacing them with plaster casts so that the original fossils could be further prepared and studied.
''T. edmontonensis'', revision, and ''T. garbanii''
In 1926, William Parks described the new species T. warreni from a well-preserved skeleton from Alberta, Canada, from what was then known as the Edmonton Formation. This skeleton had notable differences from T. neglectus, and so Charles M. Sternberg placed it in a new genus, Parksosaurus, in 1937. In 1940, Sternberg named an additional species, T. edmontonensis, based on another articulated skeleton that he had discovered in the Edmonton Formation of Rumsey, Alberta. Sternberg had already mentioned this specimen in 1926, though it was still unprepared at that time. It preserves most of the vertebral column, pelvis, legs, scapula, coracoid, arm, and, most significantly, multiple bones of the skull roof and a complete mandible, the first known from Thescelosaurus. Newer geology has separated the Edmonton Formation into four formations, with Parksosaurus from the older Horseshoe Canyon Formation and Thescelosaurus edmontonensis from the younger Scollard Formation.In 1974, Peter M. Galton revised Thescelosaurus and described additional specimens, resulting in a total of 15 specimens known. These include four specimens from the Hell Creek Formation collected by Barnum Brown in Montana in 1906 and 1909, some of which had already been mentioned by Gilmore in 1915; one specimen found in 1892 by Wortman and Peterson at an uncertain location; two specimens found in 1921 by Levi Sternberg in the Frenchman Formation of Rocky Creek, Saskatchewan; and two isolated bones, also from Saskatchewan. One of Browns specimens, AMNH 5034, was found just below the Fort Union Formation, at the youngest locality from which dinosaurs were found. Galton concluded that T. edmontonensis was simply a more robust individual of T. neglectus.
William J. Morris described three additional partial skeletons in 1976, two found in the Hell Creek Formation of Garfield County, Montana by preparator Harli Garbani, and one from an unknown location in Harding County, South Dakota. The first specimen preserves parts of the vertebral column and pelvis in addition to bones of the skull not yet known from Thescelosaurus such as the jugals and braincase. The second specimen includes vertebrae from the neck and back, and a nearly complete lower leg with a partial femur. Morris concluded that its ankle anatomy and larger size was unique, and therefore named the new species Thescelosaurus garbanii, in honor of the discoverer Garbani. Morris also argued that the ankle of T. edmontonensis, which Galton claimed was damaged and misinterpreted, was truly different from T. neglectus and more similar to T. garbanii. Therefore, he suggested that T. edmontonensis and T. garbanii may eventually be separated from Thescelosaurus as a new genus. The third specimen includes a large part of the skull, some partial vertebrae from the back and two bones of the fingers, parts that do not overlap with the diagnostic regions of the T. neglectus type specimen, preventing comparisons. Morris provisionally assigned the specimen to Thescelosaurus, but suggested that it could represent a new species; this potential species has later been called the "Hell Creek hypsilophodontid".
''Bugenasaura'' and the "Willo" specimen
Galton revised Thescelosaurus for a second time in 1995. He argued that the supposedly diagnostic traits of the ankle of the T. edmontonensis specimen are the result of breakage, as indicated by the previously undescribed left ankle of that specimen that showed the same anatomy as T. neglectus. Consequently, he synonymized T. edmontonensis with T. neglectus. Galton determined that Morris correctly interpreted the ankle of T. garbanii and suggested that the species could be elevated to a genus of its own. There was also the possibility that the hindlimb of T. garbanii did instead belong to the pachycephalosaurid Stygimoloch, which is also known from the Hell Creek Formation and for which the hindlimb was unknown. Galton also concluded that the skull of SDSM 7210, the third of the specimens described by Morris, was distinct from Thescelosaurus, and therefore named the new taxon Bugenasaura infernalis. The name is a combination of the Latin bu, and gena,, as well as the Ancient Greek saura,. The specific name, from the Latin infernalis,, is a reference to the lower levels of the Hell Creek Formation from which it is known. Galton also tentatively assigned LACM 33543, the type of T. garbanii, to the new species, noting that additional material is necessary to determine if the referral is correct, and that the name garbanii should have priority if this turns out to be the case.In his 1995 revision, Galton also reassigned isolated teeth from the Campanian Judith River Formation of Montana to the related genus Orodromeus. These teeth had been assigned to Thescelosaurus cf. neglectus by Ashok Sahni in 1972, which would have been the oldest occurrence of Thescelosaurus. In a 1999 study on the anatomy of Bugenasaura, Galton assigned a tooth in the collection of the University of California Museum of Paleontology to the latter. Significantly, this tooth reportedly came from the Late Jurassic Kimmeridge Clay Formation of Weymouth, England, and therefore is roughly 70 million years older than the Bugenasaura type specimen and from another continent. Galton argued that it had possibly been mislabelled and was actually from the Lance Formation of Wyoming, but the tooth was first collected before the museum was active in the Lance region. The lack of diagnostic features led Paul M. Barrett and Susannah Maidment to classify the tooth as an indeterminate ornithischian in 2011.
After the discovery of additional specimens of Thescelosaurus preserving both the skull and skeleton, Clint Boyd and colleagues reassessed the historic and current species of Thescelosaurus in 2009. One of the new specimens was found in the Hell Creek of Montana and preserves a nearly complete skull and skeleton. The researchers also identified previously overlooked skull material of the T. neglectus paratype USNM 7758, which allowed comparisons of the diagnostic regions of the skull and ankle across multiple specimens and species. The key specimen, however, was NCSM 15728, nicknamed "Willo", which was found in the upper Hell Creek Formation in Harding County, South Dakota by Michael Hammer in 1999. This specimen preserves most of the skeleton and a mass in the chest cavity that was initially interpreted as a heart. "Willo" also includes a complete skull, showing that it was much lower and longer than previously thought. "Willo" and the other new specimens made it clear that Bugenasaura infernalis must be assigned to Thescelosaurus. By reassigning the species, Boyd and colleagues created the new combination T. infernalis which they considered undiagnostic.