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Thescelosaurus ( ) is a genus of Ornithischia dinosaur that lived during the Late Cretaceous period in western North America. It was named and described in 1913 by the Paleontology 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") , or are placed outside of this group within the broader group Neornithischia.
Adult Thescelosaurus would have measured roughly long and probably weighed . It biped, 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 heterodont. 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 (upper thigh bone) was longer than its (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 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.
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 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 so that the original fossils could be further prepared and studied.
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 (possibly the opposite sex of the type individual).
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 (LACM 33543) preserves parts of the vertebral column and pelvis in addition to bones of the skull not yet known from Thescelosaurus such as the and braincase. The second specimen (LACM 33542) 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 (SDSM 7210) 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".
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 (UCMP 49611) 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 (MOR 979) 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.
Specimens can only be directly compared if they preserve the same bones, but overlapping material is often not available – the assignment of most Thescelosaurus specimens to any of the three recognized species therefore remained uncertain. This situation improved in 2014, when Boyd and colleagues reported a new specimen from the Hell Creek Formation of Dewey County, South Dakota (TLAM.BA.2014.027.0001), that was collected from private lands by Bill Alley before being donated to the Timber Lake and Area Museum. This specimen had yet to be fully prepared but includes a mostly complete but slightly crushed skull and much of the skeleton. This find allowed the assignment of this specimen and the "Willo" specimen to T. neglectus. In 2022, news media reported that a specimen of Thescelosaurus was found at the Tanis fossil site in North Dakota, which is thought to show direct signs of the Chicxulub crater in the Gulf of Mexico that resulted in the K-Pg extinction.
The skull had a long, low snout that ended in a toothless . As in other dinosaurs, it was perforated by several , or skull openings. Of these, the (eye socket) and the (behind the orbit) were proportionally large, while the (nostril) was small. The external naris was formed by the (the front bone of the upper jaw) and the , while the (the tooth-bearing "cheek" bone) was excluded. Another fenestra, the antorbital fenestra, was in-between the external naris and the orbit and contained two smaller internal fenestrae. Long rod-like bones called palpebrals were present above the eyes, giving the animal heavy bony eyebrows. The palpebral was not aligned with the upper margin of the orbit as in some other ornithischians, but protruded across it. The , which form the above the orbit, were widest at the level of the middle of the orbit and narrower at their posterior (rear) ends – an autapomorphy of Thescelosaurus.
There was a prominent ridge along the length of both maxillae; a similar ridge was also present on both (the tooth-bearing bone of the lower jaw). The ridges and position of the teeth, deeply internal to the outside surface of the skull, have been interpreted as evidence for muscular cheeks. The morphology of the ridge on the maxilla, which is very pronounced and has small and oblique ridges covering its posterior end, is an autapomorphy of the genus. The teeth were of heterodont: small pointed premaxillary teeth, and leaf-shaped cheek teeth that differed between the maxilla and the dentary. The premaxillae had six teeth each, a primitive trait among ornithischians that is otherwise only found in much earlier and more basal forms such as Lesothosaurus and Scutellosaurus. Immature individuals may have had less than six premaxillary teeth. Unlike many other basal ornithischians, the premaxillary teeth lacked (small protuberances on the cutting edges). Both the maxilla and the dentary had up to twenty cheek teeth on each side, which is again similar to basal ornithischians and unlike other , which had a reduced tooth count. The cheek teeth themselves likewise showed primitive features, such as a constriction that separated the tooth crown from their tooth root, and a (bulge surrounding the tooth) above the constriction. The front bone of the lower jaw was the , which was unique to ornithischians. When seen from below, the posterior end of the predentary was bifurcated, which is a derived feature.
Boyd and colleagues, in 2014, listed seven skull features that separate T. assiniboiensis from T. neglectus, most of which are found in the at the back of the skull. These include, amongst others, a (small opening) piercing the roof of the braincase (absent in T. neglectus); the flattened anterior (front) surface of the bone (V-shaped in T. neglectus); and the trigeminal foramen (the opening for the trigeminal nerve) piercing both the and bones (restricted to the prootic in T. neglectus).
The limbs were robust. The (upper thigh bone) was longer than the (shin bone), which distinguishes the genus from closely related genera. Thescelosaurus had short, broad, five-fingered hands. The second digit was the longest, and the fifth digit was strongly reduced in size. Only the first three digits ended in hooflike . There were two phalanx bones (finger bones) in the first digit, three in the second, four in the third, three in the fourth, and two in the fifth. The foot had five , though only the first four carried digits, with the fifth metatarsal being vestigial (reduced to a small splint). The first metatarsal was only half the length of the third, and its digit might not have regularly touched the ground. Most of the animal's weight was therefore supported by the center three , of which the middle (third) was the longest. The first digit had two phalanges, the second had three, the third had four, and the fourth had five. The digits were shorter than the metatarsals, and their phalanges were distinctly flattened. The species T. garbanii differs from the other species in its unique ankle, with the calcaneus being reduced and not contributing to the midtarsal joint.
Hypsilophodontidae only included four genera in 1940: Hypsilophodon, Thescelosaurus, Parksosaurus, and Dysalotosaurus. In 1966,Alfred Sherwood Romer assigned most small ornithopods to the family, which was followed by Galton and later authors, though Thescelosaurus was not always included in the family. As a result, Hypsilophodontidae included 13 genera in the first edition of the book The Dinosauria in 1990. This concept of Hypsilophodontidae as an inclusive monophyletic (natural) group was supported by the early cladistic studies of Paul C. Sereno, David B. Weishampel, and Ronald Heinrich, who found Thescelosaurus to be the most basal hypsilophodontid. The analysis of Weishampel and Heinrich in 1992 can be seen below.
The concept of Hypsilophodontidae as a monophyletic group then fell out of favor. Rodney Sheetz suggested in 1999 that "hypsilophodontids" were simply the primitive forms of ornithopods, the larger grouping to which they were commonly assigned. Scheetz found Thescelosaurus, Parksosaurus and Bugenasaura to be successively closer to Hypsilophodon and later ornithopods, but not a group of their own. Other studies had similar results, with Thescelosaurus or Bugenasaura as early ornithopods close to the origin of the group, sometimes forming a clade with Parksosaurus. An issue with T. neglectus prior to the revision by Boyd and colleagues in 2009 was the uncertainty about the assigned specimens, including the separation of Bugenasaura and the unresolved question of whether T. edmontonensis was distinct or not. Following their taxonomic revision, the systematic relationships of Thescelosaurus and "hypsilophodonts" have become clearer, and Boyd and colleagues found support for a larger group of early ornithopods consisting of Thescelosaurus, Parksosaurus, Zephyrosaurus, Orodromeus and Oryctodromeus. Brown and colleagues, while describing T. assiniboiensis in 2011, came to similar results. The same authors confirmed these results again in 2013, prompting them to reintroduce the name Thescelosauridae for the entire group, which was divided into the revised subfamily Thescelosaurinae and the new subfamily Orodrominae.
Other studies did not find Parksosaurus to be closely related to Thescelosaurus, and instead proposed that it was related to the South American Gasparinisaura. However, Boyd argued that the anatomy of Parksosaurus had been misinterpreted, and that Parksosaurus and Thescelosaurus were very closely related if not each other's closest relatives. The clades Thescelosauridae (or, alternatively, Parksosauridae) and Thescelosaurinae have been confirmed by numerous phylogenetic analyses, though not by all. There is also disagreement about whether Thescelosaurus and thescelosaurids are members of Ornithopoda or more basal. Boyd highlighted in 2015 that many phylogenetic studies that included Thescelosaurus either do not include or are unresolved, so there was no definitive evidence that Thescelosaurus was an ornithopod. In his analysis, Thescelosaurus and Thescelosauridae were outside Ornithopoda, instead forming an expansive clade of non-ornithopod neornithischians. Some studies agree with this placement for thescelosaurids, while others support Thescelosaurus as an ornithopod, and others are unresolved. Fonseca and colleagues gave the name Pyrodontia to the clade uniting Thescelosaurus with more derived ornithischians when Thescelosauridae is outside Ornithopoda, referencing the early and rapid diversification of Thescelosauridae, Marginocephalia and Ornithopoda. The thescelosaurid results of Fonseca and colleagues in 2024 can be seen below.
The earliest-known thescelosaurids, Changchunsaurus and Zephyrosaurus, are from the middle Cretaceous, roughly 40 million years younger than when the group would have evolved, suggesting a long ghost lineage (a period of geologic time during which a group existed but left no fossil evidence). In 2024, André Fonseca and colleagues recovered the Late Jurassic Nanosaurus as the earliest thescelosaurid, which would shorten the ghost lineage. Boyd concluded in 2015 that the split between Orodrominae and Thescelosaurinae took place in North America by the Aptian stage, with Orodrominae diversifying within North America. Thescelosaurinae might have diversified either in North America or Asia; the genus Fona, described in 2024, suggests that Thescelosaurinae was already established in North America at the beginning of the Late Cretaceous.
A 2023 study by David Button and Lindsay Zanno concluded that Thescelosaurus was less adapted for running than other thescelosaurids but nonetheless showed two traits that are common in runners. The first of these is the fourth trochanter, a bony crest on the femur that anchored the main locomotory muscle. This crest was relatively proximal (closer to the upper end of the bone), allowing for faster movements at the expense of power. The second trait is found in the inner ear, which contains the three semicircular canals that house the sense of balance: one of these canals, the anterior semicircular canal, was greatly enlarged, suggesting acute balance sensitivity, which in turn might suggest high agility but could also be explained by possible Fossorial.
In Sternberg's 1940 model, the upper arm was horizontal and almost perpendicular to the body. Peter Galton pointed out in 1970 that the (upper arm bone) of most ornithischians was articulated to the shoulder by an articular surface consisting of the entire end of the bone, rather than a distinct ball and socket as in mammals, and that the humerus would not have spread sidewards as in Sternberg's model. Senter and Mackey found that the humerus could swing forward to a vertical position, but not much beyond that point.
The semicircular canals may allow for reconstructing the habitual posture of the head. In modern animals, one of the canals, the lateral semicircular canal, is typically horizontal when the head is in an "alert" posture. Button and Zanno argued that the head of Thescelosaurus would be slightly up-tilted when oriented such that the canal is horizontal. This is similar to Dysalotosaurus, but contrasts with the down-tilted alert postures hypothesized for many other ornithischians including , , and .
It had poor hearing, with an estimated hearing range between around 296 and 2150 Hertz, which is narrower than that of related genera such as Dysalotosaurus. The sense of smell, in contrast, was acute, as indicated by the large of the brain, which are around 3% of the entire volume of the endocast. This is comparable to modern and and more than in birds. Poor hearing and an acute sense of smell are commonly found in modern animals that create burrows, leading Button and Zanno to suggest that Thescelosaurus may have been fossorial. The animal might have dug for food such as roots and tubers, which can be detected by smelling. Some anatomical features of the skeleton could also be related to digging, such as the robust forelimbs and the premaxillae that were fused together towards their tips, reinforcing the tip of the snout to aid in digging. Furthermore, the shoulder blade was broad, possibly to provide a larger attachment surface for muscles important for scratch-digging. The relatively large size of Thescelosaurus does not necessarily preclude burrowing behaviour, as tunnels have been associated with the only slightly smaller Oryctrodromeus and with much larger mammals.
Button and Zanno alternatively suggested that Thescelosaurus could have inherited its burrowing adaptations from burrowing ancestors, while not burrowing itself. This idea is supported by the lack of some of the burrowing adaptations seen in the closely related Oryctodromeus. Burrowing might have been widespread in thescelosaurids and other basal neornithischians.
In 2001, Timothy Rowe and colleagues commented that the anatomy of the object is inconsistent with a heart – for example, the supposed heart partially engulfs one of the ribs and has an internal structure of concentric layers in some places. Instead, they suggested that the structure is an ironstone concretion; such concretions are common in similar sediments, and another concretion is preserved behind the right leg. The original authors defended their position, arguing that the concretion is unique and has formed around the actual heart.
In 2011, researchers supervised by Mary Schweitzer applied multiple lines of inquiry to the question of the object's identity, including more advanced CT scanning, histology, X-ray diffraction, X-ray photoelectron spectroscopy, and scanning electron microscopy. The team found that the object's internal structure does not include chambers but is made up of three unconnected areas of lower density material, and is not comparable to the structure of an ostrich's heart. The "walls" are composed of sedimentary minerals not known to be produced in biological systems, such as goethite, feldspar minerals, quartz, and gypsum, as well as some plant fragments. Carbon, nitrogen, and phosphorus, chemical elements important to life, were lacking in their samples, and cardiac cellular structures were absent. There was one possible patch with animal cellular structures. The authors found their data supported identification as a geologic concretion, not the heart, with the possibility that isolated areas of tissues were preserved.
The disproportional presence of Thescelosaurus and in sandstone, versus in mudstone, could suggest that Thescelosaurus preferred the habitat along channel margins rather than floodplains, but the possible presence in the Laramie Formation would imply Thescelosaurus preferred a low coastal environment. Alternatively, these supposed habitat preferences may simply be a result of Thescelosaurus fossils being more readily preserved in some environments than in others. Thescelosaurus would have inhabited an ecomorphospace different from that of other dinosaurs including the similarly sized and built pachycephalosaurids.
Many fossil vertebrates are found in the Scollard Formation alongside Thescelosaurus, including Chondrichthyes and Osteichthyes such as Palaeospinax, Myledaphus, Lepisosteus and Cyclurus, amphibians like Scapherpeton, turtles including Compsemys, indeterminate champsosaurs, crocodilians, pterosaurs and birds, a variety of theropod groups including , , the tyrannosaurid Tyrannosaurus, and ornithischians including Leptoceratops, pachycephalosaurids, Triceratops and Ankylosaurus. Mammals are also very diverse, with multituberculates, deltatheridiidae, the marsupials Alphadon, Pediomys, Didelphodon and Eodelphis, and the insectivorans Gypsonictops, Cimolestes and Batodon. Within the Hell Creek Formation of Montana, Thescelosaurus lived alongside dinosaurs including Leptoceratops, pachycephalosaurids Pachycephalosaurus, Stygimoloch and Sphaerotholus, the Edmontosaurus and possibly Parasaurolophus, like Triceratops and Torosaurus, the Nodosauridae Edmontonia and ankylosaurid Ankylosaurus, multiple and troodontids, the ornithomimid Ornithomimus, the caenagnathid Elmisaurus, including Tyrannosaurus, an alvarezsaurid, and the bird Avisaurus. The dinosaur fauna of the Frenchman Formation is similar, with the presence of pachycephalosaurids, Edmontosaurus, Triceratops, Torosaurus, ankylosaurids, dromaeosaurids, troodontids, ornithomimids, caenagnathids, and Tyrannosaurus, as well as the intermediate theropod Richardoestesia.
The Lance Formation contains one of the best known faunas from the Late Cretaceous, with a diverse assemblage of cartilaginous and bony fishes, frogs, , turtles, champsosaurs, lizards, , crocodilians, pterosaurs, mammals, and birds such as Potamornis and Palintropus. The dinosaurs of the Lance Formation include troodontids such as Pectinodon and Paronychodon, dromaeosaurids, the ornithomimid Ornithomimus, the caenagnathid Chirostenotes, the tyrannosaurid Tyrannosaurus, the pachycephalosaurids Pachycephalosaurus and Stygimoloch, the hadrosaurid Edmontosaurus, ankylosaurs such as Edmontonia and Ankylosaurus, and ceratopsians such as Leptoceratops, Triceratops, and Torosaurus. Small tyrannosaurids, large dromaeosaurids and other trophic level predators likely targeted Thescelosaurus and other smaller ornithischians and theropods, with very young ornithischians also preyed on by smaller dromaeosaurids and troodontids, with crocodilians, lizards and mammals as opportunistic lower trophic level hunters and scavengers.
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