Thescelosaurus ( THESS-el-oh-sore-us) 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 ("primitive") ornithopods, or are placed outside of this group within the broader group Neornithischia. Adult Thescelosaurus would have measured roughly 3–4 m (10–13 ft) long and probably weighed 200 to 300 kg (440 to 660 lb). 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 beak. 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 femur (thigh bone) was longer than its tibia (shin bone), 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 articulation, with only the head and neck lost to erosion. It was taken to the Smithsonian Institution's National Museum of Natural History (USNM), 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 (USNM 7757), 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 (paratype, USNM 7758). The generic name derives from the Greek words θέσκελος (theskelos), 'marvelous', and σαυρος (sauros) 'reptile' or 'lizard'. The specific name, neglectus, is Latin for 'neglected' or 'overlooked', as the type specimen had been unattended to for so long.
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 (AMNH). 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.
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