Thescelosaurus

Thescelosaurus Temporal range: Late Cretaceous, 69.42–66.04 Ma PreꞒ Ꞓ O S D C P T J K Pg N ↓
Reconstructed skeleton
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Dinosauria
Clade:	†Ornithischia
Clade:	†Neornithischia
Family:	†Thescelosauridae
Subfamily:	†Thescelosaurinae
Genus:	†Thescelosaurus Gilmore, 1913[1]
Type species
†Thescelosaurus neglectus Gilmore, 1913
Other species
†T. garbanii Morris, 1976[2] †T. assiniboiensis Brown et al., 2011[3]
Synonyms
Bugenasaura Galton, 1995[4]

Thescelosaurus (/ˌθɛsɪləˈsɔːrəs/ THESS-il-ə-SOR-əs; ancient Greek θέσκελος- (theskelos-) meaning "marvelous", and σαυρος (sauros) "lizard") is an extinct genus of thescelosaurid neornithischian dinosaur that lived at the end of the Late Cretaceous period in North America. It was a member of the last dinosaurian fauna before the Cretaceous–Paleogene extinction event around 66 million years ago.

This bipedal neornithischian 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, and a head with an elongate pointed snout. The form of the teeth and jaws suggest a primarily herbivorous animal. This genus of dinosaur is regarded as a specialized neornithischian, traditionally described as a hypsilophodont, but more recently recognized as distinct from Hypsilophodon. Several species have been suggested for this genus. Three currently are recognized as valid: the type species T. neglectus, T. garbanii and T. assiniboiensis.

Like other ornithischians, Thescelosaurus was probably a herbivore that browsed on vegetation. The preservation and completeness of many of its specimens may indicate that it preferred to live near streams.

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

In July of 1891 American paleontologists John Bell Hatcher and William H. Utterback discovered a partial skeleton of a small ornithopod in Wyoming. The specimen was found in Doegie Creek, Niobrara County, which was at the time part of Converse County, and was nearly complete and articulated, lacking only the skull and neck, and parts of the arm bones. It was taken to the United States National Museum and accessioned as specimen USNM 7757, where it remained in the original packing boxes until close to 1913 when it was identified as a new taxon by American paleontologist Charles W. Gilmore.^[1][5] When the skeleton was found, it was lying on its left side with nearly all bones articulated, but the skull, neck, and parts of the pectoral girdle had been eroded away, though it was likely complete when buried. This posture was maintained for the exhibit of the skeleton, with only the right leg, which was slightly dislocated, being adjusted in position. Some minor restoration of damage to the bones was restored, but painted lighter than the original bones so that the real and reconstructed parts could be distinguished visually.^[5] From the legs, pelvis and hands of the skeleton, Gilmore found it unique from all other members of the ornithopod family Camptosauridae, so in 1913 he published a preliminary description naming the specimen Thescelosaurus neglectus; the genus name derived from the Greek words θέσκελος (theskelos), "marvelous", and σαυρος (sauros) "reptile" or "lizard".^[1][6]

As well as the type specimen USNM 7757, Gilmore referred USNM 7758 to Thescelosaurus in 1913, a partial skeleton including vertebrae from the neck, back, and tail, parts of the pectoral girdle and arm, and a partial leg. It was collected in 1889 by Olof August Peterson at Lance Creek, Niobrara County.^[1] Both Lance Creek and Doegie Creek localities are part of the Lance Formation, which is a Maastrichtian deposit that spans from 69.42 million years ago until the end of the Cretaceous.^[7] Preparation of the type specimen of Thescelosaurus was completed between 1913 and 1915, at which point Gilmore published a full monograph of the taxon, and identified six more specimens in the collections of the USNM and the American Museum of Natural History that could be identified as the same species. These additional specimens include the scapula and coracoid USNM 7760 found in 1891 by Hatcher in Deer Ears Buttes in Butte County, South Dakota, the neck vertebra USNM 7761 found in 1891 by Hatcher, Sullins and Burrell in Beecher's Quarry in Niobrara County, the phalanx of the foot USNM 8065 found in 1890 by Hatcher in Niobrara County, and three undescribed partial skeletons at the AMNH found in Dawson County, Montana. Thescelosaurus was found to be more similar to Hypsilophodon than Camptosaurus by Gilmore in 1915, from which he reconstructed the neck and skull to illustrate the complete skeleton of Thescelosaurus neglectus.^[5]

Charles Gilmore's 1915 skeletal and life reconstruction of the T. neglectus type specimen with head and neck after Hypsilophodon

The type skeleton of Thescelosaurus was first displayed in the Hall of Extinct Monsters of the Smithsonian National Museum of Natural History (formerly United States National Museum), but was then redisplayed in 1963 more prominently as a wall-mount alongside the ornithischians Edmontosaurus and Corythosaurus and the theropod Gorgosaurus, a collection of dinosaurs that did not live at the same time or in the same place. 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 remounted casts so that the original fossils could be further prepared and studied. The duplication and repreparation of Thescelosaurus was done by Research Casting International, who replicated the ossified tendons and cartilage of the original fossil in the mount, and updated the skull to a Thescelosaurus specimen described in 2014.^[8]

A skeleton of a similar ornithopod to Thescelosaurus neglectus was found in 1922 by an expedition of the University of Toronto to the Edmonton Formation of Alberta. This specimen, collected and taken to the Royal Ontario Museum as ROM 804, was found around 0.5 mi (0.80 km) east of the Red Deer River, 100 ft (30 m) above the water level.^[9][10] The similarity to Thescelosaurus prompted Canadian paleontologist William A. Parks to describe ROM 804 in 1926 as the new species Thescelosaurus warreni, named after ROM Board of Trustees member H.D. Warren, as the first member of Hypsilophodontidae from Canada and the first specimen of Thescelosaurus to preserve a skull.^[9] Also in 1926, Canadian paleontologist Charles Mortram Sternberg noted the discovery of a new specimen of Thescelosaurus, also from the Edmonton Formation, 7 mi (11 km) northwest of Rumsey, Alberta and 260 ft (79 m) above the water level of the Red Deer River. The specimen, found within the last three field seasons by Sternberg, was not yet prepared, but did include parts of the skull.^[11] As there were substantial differences between T. warreni, T. neglectus and the other Edmonton specimen, Sternberg placed T. warreni in the new genus Parksosaurus in 1937, and proposed the new family Thescelosauridae to unite the two genera.^[12] Newer geology has separated the Edmonton Formation into four formations as the Edmonton Group, with Parksosaurus from the Tolman Member of the Horseshoe Canyon Formation between 70.896 and 69.6 million years old, and Thescelosaurus from the Scollard Formation between 66.88 million years old and the end of the Cretacous.^[13] Sternberg described the Thescelosaurus specimen, accessioned in the Canadian Museum of Nature as CMN 8537, in 1940 as a new species, T. edmontonensis, known from 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. Sternberg also reconsidered the separation of Thescelosauridae, instead proposing the subfamily Thescelosaurinae to house Thescelosaurus separately from Parksosaurus, Hypsilophodon and Dysalotosaurus within Hypsilophodontidae.^[14]

Thescelosaurus was reviewed by American paleontologist Peter M. Galton in 1974, including the first description of the material in the AMNH mentioned by Gilmore in 1915. At the time of Galton's review, 15 specimens of Thescelosaurus were available to be described, and additional material was being worked on by American paleontologist William J. Morris. In addition to the specimens previously described, there was AMNH 117 found in 1892 by Wortman and Peterson at an uncertain location, AMNH 5031, 5034 and 5052 found by American paleontologists Barnum Brown and Peter Kaisen in 1909 from localities of the Hell Creek Formation around Limas, Montana, AMNH 5889 found by Brown in 1806 in Hell Creek, Montana, CMN 9493 and 9507 found in 1921 by Canadian paleontologist Levi Sternberg in the Frenchman Formation of Rocky Creek, Saskatchewan, and CMN 9143 and 9534 also from Saskatchewan. AMNH 5034 was found only 5 ft (1.5 m) below the Fort Union Formation, as the youngest locality from which dinosaurs were found. From this material, Galton could not support the identification of T. edmontonensis as a separate species, instead considering all the specimens to represent T. neglectus. The few differences identified between T. edmontonensis and all the other material were considered to represent either individual or sexual variation, not significant enough to justify a separate species.^[15]

Morris published his description of three Thescelosaurus specimens in 1976, two found in the Hell Creek Formation of Garfield County, Montana by Harli Garbani and stored in the Los Angeles County Museum of Natural History, and one found in an unknown location within Harding County, South Dakota and stored in the South Dakota School of Mines and Technology Geology Museum. The first specimen, LACM 33543, preserved parts of the vertebral column and pelvis in addition to bones of the skull now yet known from Thescelosaurus such as the jugals and braincase. Morris referred this specimen to T. neglectus, and also noted that two small scutes were found above the neck vertebrae of this individual.^[2] These scutes were suggested to be crocodilian and not from Thescelosaurus by Galton in 2008, and no other specimens that preserve the neck suggest osteoderms were present.^[16]

The second specimen described by Morris, LACM 33542, includes vertebrae from the neck and back, and a nearly complete lower leg with a partial femur. Morris identified that the larger size and ankle of this specimen was unique, with the reduction of the calcaneum bone, and as such he named it Thescelosaurus garbanii, in honor of the discoverer and preparator Garbani. Morris also suggested that the ankle of T. edmontonensis had been previously misinterpreted and was more similar to T. garbanii in the reduction of the calcaneum, and that the two species may eventually be separated from Thescelosaurus as a new genus. The third specimen, SDSM 7210, was provisionally referred to Thescelosaurus as it includes a large part of the skull, some partial vertebrae from the back and two bones of the fingers, preventing comparison with the diagnostic regions of the Thescelosaurus species. Morris chose not to name the specimen due to this lack of overlapping material, but regarded the specimen as a new species nonetheless.^[2] American paleontologist Hans-Dieter Sues agreed with retaining SDSM 7210 as unnamed in his description of the hypsilophodontid Zephyrosaurus, informally referring to it as the Hell Creek hypsilophodontid.^[17]

The species of Thescelosaurus were reviewed again by Galton in 1995. The diagnostic ankle of T. edmontonensis identified by Morris was reinterpreted as the result of breakage on the right ankle, with the previously undescribed left ankle showing the same anatomy as T. neglectus. As a result, Galton synonymized T. edmontonensis with T. neglectus again. Galton determined that Morris correctly interpreted the ankle of T. garbanii, preventing it from being a synonym of T. neglectus, but suggesting it is different enough to be an entirely separate genus of hypsilophodontid from Thescelosaurus. There was also the possibility that the hindlimb of T. garbanii was instead not a hypsilophodontid, but the hindlimb of the pachycephalosaurid Stygimoloch also known from the Hell Creek Formation, for which the hindlimb was unknown. Following the reassessments of T. edmontonensis and T. garbanii, Galton concluded that the skull of SDSM 7210 was distinct from all other hypsilophodontids including Thescelosaurus, and named the new taxon Bugenasaura infernalis. The name was a combination of the Latin bu, "large", gena, "cheek", and saura, "lizard", with the species name as a reference to the lower levels of the Hell Creek Formation from which it is known. Galton also tentatively referred 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 if it is confirmed the species name garbanii should have priority.^[4]

Isolated teeth from the Campanian Judith River Formation of Montana that were referred to Thescelosaurus cf. neglectus by Indian paleontologist Ashok Sahni in 1972, which would be the oldest occurrence of Thescelosaurus, were reassigned to Orodromeus in the same 1995 review.^[4][18] In a 1999 study on the anatomy of Bugenasaura, Galton referred two isolated teeth to the genus, one (Yale Peabody Museum 8098) collected by Hatcher in 1889 from the Lance Formation of Lusk, Wyoming, was referred to B. infernalis, while another, University of California Museum of Paleontology 49611 was referred to cf. Bugenasaura, as it showed some anatomical differences, but most significantly was listed as being from the Late Jurassic Kimmeridge Clay Formation of Weymouth, England, roughly 70 million years older than Bugenasaura and from another continent. Galton questioned whether the origins of the tooth were correct, as it had possibly been mislabelled and was actually from the Lance Formation of Wyoming, but the tooth was first collected before the UCMP was actively in the Lance region.^[19] The lack of diagnostic features led British paleontologists Paul M. Barrett and Susannah Maidment to classify UCPM 49611 as an indeterminate ornithischian in 2011.^[20]

With the discovery of additional specimens of Thescelosaurus preserving both the skull and skeleton, American paleontologist Clint Boyd and colleagues reassessed the historic and current species of Thescelosaurus in 2009.^[10] One of the new specimens, housed in the North Carolina State Museum of Natural Sciences (NCSM 15728), was found in the upper Hell Creek Formation in Harding County, South Dakota by Michael Hammer in 1999, and preserves a complete skull, most of a skeleton, and a mass in the chest cavity that was originally interpreted as a heart.^[10][21][22] Another, Museum of the Rockies specimen MOR 979, was collected from the Hell Creek of Montana, first published by American paleontologist John R. Horner, and preserves a nearly complete skull and skeleton. Boyd and colleagues 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.^[10]

Based on the limitations of overlapping material, T. neglectus was restricted to only the types USNM 7757 and 7758, and T. garbanii was maintained as a species of Thescelosaurus but limited to its type LACM 33542, all other specimens, including the types of T. edmontonensis and Bugenasaura infernalis, were referred to Thescelosaurus incertae sedis, as they showed features characteristic of Thescelosaurus in the skull and hindlimb, but either did not preserve the squamosal bone of the skull for comparison to T. neglectus, or the ankle for comparison to T. garbanii. With the referral of Bugenasaura to Thescelosaurus, Boyd and colleagues created the new combination T. infernalis, but could not identify features of the skull to distinguish the species from others, considering it undiagnostic. Two specimens were noted as having anatomy slightly different from T. neglectus, the specimens NCSM 15728 and Royal Saskatchewan Museum specimen RSM P 1225.1 (previously described by Galton in 1995 and 1997 as T. neglectus^[4][23]). However, the ankle of NCSM 15728 is unknown, preventing separation from T. garbanii, and the skull of RSM P 1225.1 showed some similarity to T. edmontonensis, which would be the proper name for the species if further study showed both specimens could be distinguished from T. neglectus. Parksosaurus was retained as a separate genus.^[10]

RSM P 1225.1 was first found on June 19th, 1968 by Albert Swanson of the Saskatchewan Museum of Natural History (now the Royal Saskatchewan Museum), who collected the specimen on July 17th. The originally reported location was incorrect, revisiting of the Frenchman River valley by Tim Tokaryk in the 1980s found that the excavation, identifiable by bone and plaster remnants, was from the northwest side of a butte on the north side of the valley, approximately halfway up the exposed claystone. This places the specimen in the Frenchman Formation, which was deposited in the last half million years before the end of the Cretaceous.^[3] The specimen, described as T. neglectus by Galton, and an indeterminate species requiring further study by Boyd and colleagues, was fully studied by Canadian paleontologist Caleb M. Brown and colleagues in 2011, where it was determined that it represented a new species they named T. assiniboiensis. The species name derives from the historic District of Assiniboia that covered the southern Saskatchewan region where the Frenchman Formation is exposed, which in turn takes its name from the Assiniboine peoples. The presence of a foramen in the braincase not found in any other specimens of Thescelosaurus, including CMN 8537 (type of T. edmontonensis) and NCSM 15728.^[3] The separation of T. assiniboiensis and T. neglectus was further supported by Boyd in his 2014 description of the skull of NCSM 15728 and Timber Lake and Area Museum specimen TLAM.BA.2014.027.0001, discovered and collected from private lands by Bill Alley before being donated to the museum, which had yet to be fully prepared but includes a mostly complete but slightly crushed skull and much of the skeleton.^[22]

Thescelosaurus is known from multiple specimens found throughout Alberta, Saskatchewan, Wyoming, North Dakota, South Dakota and Montana, with T. neglectus from the Lance and Hell Creek Formations, T. assiniboiensis from the Frenchman Formation, T. garbanii from the Hell Creek Formation, and other indeterminate specimens across all localities.^[10][22] In April 2022, news media reported that a specimen of Thescelosaurus was found at the Tanis fossil site in North Dakota. This site is thought to show direct signs of the Chicxulub asteroid impact in the Gulf of Mexico that resulted in the K-Pg extinction.^[24][25] The fossil, consisting of a leg, was found to have particularly well-preserved skin.^[26]

Thescelosaurus was a heavily built bipedal animal.^[27] Overall, the skeletal anatomy of this genus is well documented, and restorations have been published in several papers, including skeletal restorations^{[10][5][15][28]} and models.^[14][5] The skeleton is known well enough that a detailed reconstruction of the hip and hindlimb muscles has been made.^[29] The animal's size has been estimated in the 2.5–4.0 m range for length (8.2–13.1 ft)^[15] for various specimens, and a weight of 200–300 kilograms (450–660 pounds),^[30] with the large type specimen of T. garbanii estimated at 4–4.5 meters (13.1–14.8 feet) long.^[2] It may have been sexually dimorphic, with one sex larger than the other.^[15] Juvenile remains are known from several locations, mostly based on teeth.^[31][32]

Skull of T. neglectus NCSM 15728 ("Willo") in right, left, and top view

The skull is best known from T. neglectus, mostly thanks to an almost complete example (specimen NCSM 15728) that has been CT-scanned to reveal its internal details. A fragmentary skull is also known from T. assiniboiensis (RSM P 1225.1). Most autapomorphies – distinguishing features that are not found in related genera – are found in the skull, as are most of the features that separate T. neglectus from T. assiniboiensis. The skull also shows many plesiomorphies, "primitive" features that are typically found in ornithischians which are geologically much older, but also shows derived (advanced) features.^{[22]: 1–9}

The skull had a long, low snout that ended in a beak with a toothless tip.^[27][23] As in other dinosaurs, it was perforated by several fenestrae, or skull openings. Of these, the orbit (eye socket) and the infratemporal fenestra (behind the orbit) were proportionally large, while the external naris (nostril) was small.^[27] The external naris was formed by the premaxilla (the front bone of the upper jaw) and the nasal bone, while the maxilla (the tooth-bearing "cheek" bone) was excluded.^[22]: 18 Another fenestra, the antorbital fenestra, was in-between the external naris and the orbit and contained two smaller internal fenestrae.^[22]: 20 Most groups of dinosaurs also had a fenestra just behind the dentary (the tooth-bearing portion of the lower jaw) called the mandibular fenestra. In Thescelosaurus and most other ornithischians, this fenestra is closed,^[16] though Thescelosaurus shows a small opening (foramen) that could have been a remnant of it.^[27] Long rod-like bones called palpebrals were present above the eyes, giving the animal heavy bony eyebrows.^[23] The palpebral was not aligned with the upper margin of the orbit as in some other ornithischians, but protruded across it.^[22]: 55 The frontal bones, which form the skull roof above the orbit, were widest at the level of the middle of the orbit and narrower at their rear ends – an autapomorphy of Thescelosaurus.^[22]: 6

There was a prominent ridge along the length of both maxillae; a similar ridge was also present on both dentaries (the tooth-bearing bone of the lower jaw).^[19] The ridges and position of the teeth, deeply internal to the outside surface of the skull, are interpreted as evidence for muscular cheeks.^[19][2] The morphology of the ridge on the maxilla, which is very pronounced and has small and oblique ridges covering its rear end, is an autapomorphy of the genus.^[22]: 7 The teeth were of two types: small pointed premaxillary teeth (in the premaxilla, or upper beak), and leaf-shaped cheek teeth (in the maxilla and the dentary).^[14] 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 had less than six premaxillary teeth. Unlike many other basal ornithischians, the premaxillary teeth lacked serrations (small protuberances on the cutting edges).^[22]: 63 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 neornithischians, which had a reduced tooth count. The cheek teeth themselves likewise showed primitive features, such as a constriction that separated the crowns from their roots, and a cingulum (bulge surrounding the tooth) above the constriction. The lower beak was formed by the predentary, a bone unique to ornithischians. When seen from below, the rear end of the predentary was bifurcated, which is a derived feature.^{[22]: 63–64}

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.^[5] The rib cage was broad, giving it a wide back, and the limbs were robust.^[14] The animals may have been able to move on all fours, given its fairly long arms and wide hands,^[15] but this idea has not been widely discussed in the scientific literature, although it does appear in popular works.^[33][34] Charles M. Sternberg reconstructed it with the upper arm oriented almost perpendicular to the body,^[14] 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.^[35]

Large thin flat mineralized plates have been found next to the ribs' sides.^[21] Their function is unknown; they may have played a role in respiration.^[36] 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.^[37] Such plates are known from several other cerapodans.^[16]

For most of its history, the nature of this genus' integument, be it scales or something else, remained unknown. Charles Gilmore described patches of carbonized material near the shoulders as possible epidermis, with a "punctured" texture, but no regular pattern,^[5] while 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.^[2] Scutes have not been found with other articulated specimens of Thescelosaurus, though, and Morris's scutes could be crocodilian in origin.^[16] In 2022, news media reported that the Tanis specimen preserves skin impressions on a leg that show that parts of the animal were covered in scales.^[24]

While Thescelosaurus was originally considered a member of Camptosauridae by Gilmore alongside Hypsilophodon, Dryosaurus and Laosaurus, he revised his opinion following further study to place the taxon within Hypsilophontidae (a misspelling of Hypsilophodontidae) alongside only Hypsilophodon.^[1][5] Many authors followed the classification of Thescelosaurus within Hypsilophodontidae, but not all.^[38] Hungarian paleontologist Franz Nopcsa and German paleontologist Friedrich von Huene retained Thescelosaurus (misspelled "Thescelesaurus" by Nopcsa) as a relative of Camptosaurus.^[39][40] Sternberg at first separated Thescelosaurus and related Parksosaurus into Thescelosauridae, before considering both members of Hypsilophodontidae with Thescelosaurus separated from the other genera as a member of Thescelosaurinae.^[12][14] Russian paleontologist Anatoly Konstantinovich Rozhdestvensky and Australian paleontologist Richard A. Thulborn retained Thescelosauridae as a separate family.^[41][42] Galton argued against the inclusion of Thescelosaurus within Hypsilophodontidae originally, instead emphasizing the hindlimb proportions in classifying it as a member of Iguanodontidae. Iguanodontidae was not considered to have evolved from a single evolutionary source, but instead composed a polyphyletic group of ornithopods with similar convergent bauplans.^[15] However, he revised his taxonomy of Thescelosaurus in 1995, returning to a hypsilophodontid classification.^[4]

Hypsilophodontidae only included four genera when its classification was assessed by Sternberg in 1940, Hypsilophodon, Thescelosaurus, Parksosaurus, and Dysalotosaurus.^[14] The content of Hypsilophodontidae was expanded by American paleontologist Alfred Sherwood Romer to include most small ornithopods by 1966, which was followed by Galton (though Thescelosaurus was removed) and later authors, with Hypsilophodontidae including 13 genera in the first edition of the book The Dinosauria in 1990.^[43][15][44] This concept of Hypsilophodontidae as an expansive natural group has been recovered by the early cladistic studies of American paleontologists Paul C. Sereno, and David B. Weishampel and Ronald Heinrich, with Thescelosaurus as the most primitive hypsilophodontid outside the remaining taxa. The analysis of Weishampel and Heinrich in 1992 can be seen below.^[45][46]

The concept of Hypsilophodontidae as a monophyletic group then fell out of favor, instead it was suggested in 1999 by American paleontologist Rodney Sheetz that "hypsilophodontids" were simply the primitive form of ornithopods. Scheetz found Thescelosaurus, Parkosaurus and Bugenasaura to be successively closer to Hypsilophodon and later ornithopods, but not a group of their own.^[47] Such a result became common, with Thescelosaurus or Bugenasaura as an early ornithopod close to the origins of the group, sometimes forming a clade with Parksosaurus.^[48][49][50] An issue with Thescelosaurus neglectus prior to the review of Boyd and colleagues in 2009 was the uncertainty about the referred materials, including the separation of Bugenasaura and lack of clear synonymy of T. edmontonensis.^[50] Following the taxonomic revision, the systematic relationships of Thescelosaurus and "hypsilophodonts" have become clearer, with Boyd and colleagues finding Thescelosaurus and Parksosaurus to unite with Zephyrosaurus, Orodromeus and Oryctodromeus as a larger clade of early ornithopods.^[10] In the description of T. assiniboiensis, identical results were recovered.^[3] Brown and colleagues found similar results within Ornithopoda again in 2013, prompting them to reintroduce the name Thescelosauridae for the total group, which could be divided into the revised subfamily Thescelosaurinae and the new subfamily Orodrominae. Thescelosaurus and Parksosaurus constituted Thescelosaurinae alongside the Asian taxa Haya griva, Jeholosaurus and Changchunsaurus that had been found to be related previously, with Zephyrosaurus, Orodromeus, Oryctodromeus, the new genus Albertadromeus, and an unnamed specimen forming Orodrominae.^[51][52]

Some studies separate from Boyd and Brown did not find Parksosaurus to be closely related to Thescelosaurus, instead forming a relationship with South American Gasparinisaura. However, this relationship is due to the anatomy of Parksosaurus being misinterpreted, and the anatomy of Thescelosaurus showing some variation, with Boyd demonstrating that Parksosaurus and Thescelosaurus were very closely related if not each others closest relatives.^[22] The clades Thescelosauridae (or alternatively Parksosauridae) and Thescelosaurinae have been found by numerous phylogenetic analyses,^{[51][38][53][54][55]} though not all agree.^[56][57] There is also disagreement about whether Thescelosaurus and thescelosaurids are members of Ornithopoda, or more basal. Boyd highlighted in 2015 that many phylogenetic studies to include Thescelosaurus either do not include marginocephalians or are unresolved, so there was not definitive evidence that Thescelosaurus was an ornithopod. In his analysis, Thescelosaurus and Thescelosauridae were outside Ornithopoda, instead being an expansive clade of non-ornithopod neornithischians.^[38] Some studies agree with this placement for thescelosaurids,^[53][56] while others support Thescelosaurus as an ornithopod,^[57] and others are unresolved.^[54][55] 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.^[53]

From the phylogenetic results, the origins of Ornithopoda, Thescelosauridae, and Thescelosaurus can be inferred. The earliest-known thescelosaurids, Changchunsaurus and Zephyrosaurus, are from the middle Cretaceous, roughly 40 million years younger than when the group would have evolved.^[38] However, it has been suggested by some studies that Nanosaurus, from the Late Jurassic of North America, is the earliest thescelosaurid, and the recovery of Asian genera Jeholosaurus and Changmiania as thescelosaurids further shortens the ghost lineage of thescelosaurid evolution.^[53][56] Reconstructions suggest that the split between Orodrominae and Thescelosaurinae took place in North America by the Aptian stage, with Orodrominae diversifying within North America while Thescelosaurinae is uncertain. The presence of both North American and Asian taxa in Thescelosaurinae results in the group diversifying either in North America or Asia under different inference methods, and the presence of some South American taxa under the results of Boyd cause further uncertainty. The presence of large ghost lineages around the origins of thescelosaurids show there is still much to learn about their evolution and diversification.^[38]

Like other ornithischians, Thescelosaurus was probably herbivorous.^[27] Thescelosaurus would have browsed in the first meter or so from the ground, feeding selectively,^[27] with food held in the mouth by cheeks while chewing.^[15] 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.^[14] 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.^[58] Examinations of the teeth of Thescelosaurus and comparisons with the contemporary pachycephalosaur Stegoceras suggest that Thescelosaurus was a selective feeder, while Stegoceras was a more indiscriminate feeder, allowing both animals to share the same environment without competing for food.^[59]

Examinations of the skull of Thescelosaurus, specifically the "Willo" specimen by Button and Zanno et al., published in 2023, suggest its brain was small compared to other neornithischian dinosaurs, suggesting its cognitive abilities were within the range of modern reptiles. Additional analysis suggests it had poor hearing and was equipped with a highly-developed sense of smell and balance. Combined with its post-cranial skeletal features such as its strong forelimbs, it is suggested that Thescelosaurus may have been possibly semi-fossorial, perhaps digging burrows underground. Alternatively, the analysis could suggest phylogenetically inherited traits from burrowing ancestors.^[60][61]

In 2000, a skeleton of this genus (specimen NCSM 15728) 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 (CT) 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.^[21]

Their conclusions have been disputed; soon after the initial description, other researchers published a paper where they asserted that the heart is really a concretion. As they noted, 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.^[62] 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.^[63]

A study published in 2011 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. From these methods, the authors found the following: 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 concretion of sand from the burial environment, not the heart, with the possibility that isolated areas of tissues were preserved.^[64]

The question of how this find reflects metabolic rate and dinosaur internal anatomy is moot, though, regardless of the object's identity.^[64] Both modern crocodilians and birds, the closest living relatives of non-avian dinosaurs, have four-chambered hearts (albeit modified in crocodilians), so non-avian dinosaurs probably had them as well; the structure is not necessarily tied to metabolic rate.^[65]

Thescelosaurus is only known from Maastrichtian deposits of western North America, with suggested occurrences in the Campanian Judith River and Fruitland Formations and possibly Late Jurassic strata of Weymouth, England, being reassigned to Orodromeus or indeterminate Ornithischia.^[4][20][66] T. neglectus is known from the Lance Formation of Wyoming and the Hell Creek Formation of South Dakota, T. garbanii is known from the Hell Creek Formation of Montana, and T. assiniboiensis is known from the Frenchman Formation of Saskatchewan.^[2][3][22] An additional definitive Thescelosaurus specimen that cannot be assigned to a diagnostic species, the type of T. edmontonensis, is known from the Scollard Formation of Alberta.^[22][67] Equivocal material of Thescelosaurus has also been reported from the Horseshoe Canyon Formation of Alberta, the Hell Creek Formation of North Dakota, Laramie Formation of Colorado, the Ferris, Medicine Bow, and Almond Formations of Wyoming, the Willow Creek Formation of Montana, and the Prince Creek Formation of Alaska. All of these localities are of similar late Maastrichtian age to those bearing clear Thescelosaurus material, except the Horseshoe Canyon and Prince Creek Formations.^[67] The presence of Thescelosaurus in those would extend the known range of the genus into the middle or early Maastrichtian, but they have since been reassigned as probable Parksosaurus specimens.^[13][68]

Limited to the late Maastrichtian, Thescelosaurus would have lived at the very end of the Cretaceous, with the Lance and Scollard Formations being 69.42 and 66.88 million years old at the latest respectively, and lasting until the Cretaceous-Paleogene Extinction Event at 66.043 million years ago.^[7][13] The Frenchman and Hell Creek Formations can be closely correlated to the Battle and Scollard Formations, with the Frenchman Formation being no older than the base of the Scollard at 66.8 million years old to the end of the Maastrichtian, and the Hell Creek Formation spanning at least the duration of both the Battle and Scollard Formations from 67.2 mya to 66.043 million years ago. Only the upper third of the Hell Creek Formation is of the same age as the Scollard, with the middle third overlapping both the Scollard and Battle Formations and the lower third being the same age as the Battle Formation or older.^[69]

Palaeoenvironments of the Scollard and Hell Creek formation show that the very end of the Cretaceous was intermediate between semi-arid and humid, with both formations showing braided streams and floodplains and meandering river channels, that shifted to become more humid and wetland following the Cretaceous-Paleogene extinction event.^[70] The formations where Thescelosaurus fossils have been found represent different sections of the western shore of the Western Interior Seaway dividing western and eastern North America during the Cretaceous, a broad coastal plain extending westward from the seaway to the newly formed Rocky Mountains. These formations are composed largely of sandstone and mudstone, which have been attributed to floodplain environments.^[71][72][73] While slightly older floras were codominated by cycad-palm-fern meadows, by the time of the Hell Creek angiosperms were dominant in a forested landscape of small trees.^[74] The floral assemblages of the Frenchman Formation show that southern Saskatchewan at the end of the Cretaceous was a subtropical to warm temperate environment, with seasons and an average mean temperature of 54–57 °F (12–14 °C). The paleoenvironment would have been a swampy to lowland forest with a tree canopy of conifers and a diverse angiosperm-dominated mid-canopy and understory. There is also indications of forest fire, known to be widespread throughout the Late Cretaceous, with one site having a mature forest while another was in the stage of recovering from a fire.^[75]

Thescelosaurus, while historically thought to be a relatively uncommon in its paleoenvironments, is now known to have been one of the most abundant dinosaurs.^[76] Assessing only body fossils and not isolated teeth, Thescelosaurus can range between being absent from a site of the Hell Creek Formation, to comprising 22.7% of all dinosaur bones, and can be interpreted as a major component of the Hell Creek and Lance ecosystems.^[77][78][79] One multi-individual site of Thescelosaurus is known from the Hell Creek Formation, where at least seven individuals are known, the greatest number being subadults with all ages present. Material of the taxon was typically better-preserved than those of other dinosaurs at the site, lacking many insect borings, scratch or bite marks, or weathering, suggesting they were buried quickly and formed a local community.^[78][79] The disproportional presence of Thescelosaurus and hadrosaurs in sandstone, versus ceratopsians 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.^[80][81] These supposed habitat preferences may be a result of taphonomy rather than environmental effects, but Thescelosaurus would have inhabited an ecomorphospace different from even the similarly-sized and built pachycephalosaurids regardless.^[82]

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 troodontids, ornithomimids, the tyrannosaurid Tyrannosaurus, and ornithischians including Leptoceratops, pachycephalosaurids, Triceratops and Ankylosaurus. Mammals are also very diverse, with multituberculates, deltatheridiids, the marsupials Alphadon, Pediomys, Didelphodon and Eodelphis, and the insectivorans Gypsonictops, Cimolestes and Batodon.^[73] Within the Hell Creek Formation of Montana, Thescelosaurus lived alongside the dinosaurs including Leptoceratops, pachycephalosaurids Pachycephalosaurus, Stygimoloch and Sphaerotholus, the hadrosaurid Edmontosaurus and possibly Parasaurolophus, ceratopsians like Triceratops and Torosaurus, the nodosaurid Edmontonia and ankylosaurid Ankylosaurus, multiple dromaeosaurids and troodontids, the ornithomimid Ornithomimus, the caenagnathid Elmisaurus, tyrannosaurids 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.^[67]

The Lance Formation contains one of the best known faunas from the Late Cretaceous, with a diverse assemblage of cartilaginous and bony fishes, frogs, salamanders, turtles, champsosaurs, lizards, snakes, crocodilians, pterosaurs, mammals and birds such as Potamornis and Palintropus.^[67][72] The dinosaurs of the Lance Formation and questionable lancian deposits include Richardoestesia, troodontids including Pectinodon, Stenonychosaurus and Paronychodon, dromaeosaurids, the ornithomimid Ornithomimus, the caenagnathid Chirostenotes, the tyrannosaurids Albertosaurus and Tyrannosaurus, the pachycephalosaurids Pachycephalosaurus and Stygimoloch, the hadrosaurid Edmontosaurus, the ankylosaurs Edmontonia and Ankylosaurus, and the ceratopsians Leptoceratops, Triceratops, Torosaurus and also Nedoceratops.^[67] Small tyrannosaurids, large dromaeosaurids and other second tier predators likely targeted Thescelosaurus and other smaller ornithischians and theropods, with very young ornithischians also fed on by smaller dromaeosaurids and troodontids, with crocodilians, lizards and mammals as opportunistic lower trophic level hunters and scavengers.^[79]

• Willo, the Dinosaur with a Heart – The official site for "Willo", from the North Carolina Museum of Natural Sciences.