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Thescelosaurus

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Thescelosaurus
Temporal range: Late Cretaceous, 69.42–66.04 Ma
↓
Reconstructed skeleton
Scientific classification Edit this 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
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 neornithischian dinosaur that lived during the Late Cretaceous period in North America. It was among the last of the non-avian dinosaurs to appear before the entire group went extinct during the Cretaceous–Paleogene extinction event around 66 million years ago. The genus Thescelosaurus is the type genus and also the largest member of the eponymous Thescelosauridae. Adult Thescelosaurus would have measured roughly 3–4 metres (10–13 ft) long and probably weighed several hundred kilograms. 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 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 attached to its ribs, the function of which is unknown. It is also unknown to what extent the body was feathered, but at least parts of the legs appear to have been covered in scales.

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 1993 was interpreted as including a fossilized heart. Scientists now doubt the identification of the object and the implications of such an identification.

Thescelosaurus was named and described in 1913 by the paleontologist Charles W. Gilmore; the type species is T. neglectus. Numerous specimens have since been referred to it. Initially, it was identified as a "hypsilophodont" — a wastebasket taxon that included a variety of unrelated, but superficially similar bipedal ornithischians. Modern taxonomists consider thescelosaurids to be either stem-neornithischians, or basal members of Ornithopoda, although their precise affinities remain uncertain. The genus contains at least two other valid species: T. garbanii and T. assiniboiensis, which were named in 1976 and 2011, respectively, and are each known from a single specimen. Additional species have been suggested, but these are not widely accepted. Thescelosaurus has been found across a wide geographic range across North America. The type specimen was 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.

Discovery and history

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T. neglectus holotype USNM 7757 on display in 1963 with reconstructed head and neck, and below as remounted

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),[a] 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.[1][6][7] 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 (USNM 7758) from Lance Creek, also in Niobrara County, to the species. The generic name derives from the Greek words θέσκελος (theskelos), 'marvelous', and σαυρος (sauros) 'reptile' or 'lizard'.[1][8] The specific name, neglectus, is Latin for 'neglected' or 'overlooked', as the type specimen had been unattended to for so long.[7] Gilmore considered it to be a light, agile creature, and assigned it to the Hypsilophodontidae, a family of small bipedal dinosaurs.[6]

Gilmore published a comprehensive description in 1915 after the type specimen was fully prepared.[1][6] 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 the palaeontologist 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 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.[6] In 1963, the display was included in a wall mount alongside the ornithischians Edmontosaurus and Corythosaurus and the theropod Gorgosaurus, a selection of dinosaurs that did not live at the same time. 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.[9]

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

Additional species

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Another well-preserved skeleton from the slightly older Horseshoe Canyon Formation, in Alberta, Canada, was named T. warreni by William Parks in 1926.[10] This skeleton had notable differences from T. neglectus, and so Charles M. Sternberg placed it in a new genus, Parksosaurus, in 1937, and proposed the new family Thescelosauridae to unite the two genera.[11] In 1940, Sternberg named an additional species, T. edmontonensis, based on another articulated skeleton (CMN 8537) that he had discovered in the slightly younger Scollard Formation northwest of Rumsey, Alberta. Sternberg had already mentioned this specimen in 1926, though it was still unprepared at that time.[12][2] 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. Sternberg also reconsidered the separation of Thescelosauridae, instead proposing the Thescelosaurinae as a subfamily of Hypsilophodontidae, which is separate from the Hypsilophodontinae that contained Parksosaurus, Hypsilophodon and Dysalotosaurus.[2]

Thescelosaurus neglectus paratype USNM 7758 as stored in the Smithsonian

In 1974, the American paleontologist 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 and two specimens found in 1921 by the Canadian paleontologist Levi Sternberg in the Frenchman Formation of Rocky Creek, Saskatchewan, amongst others. One of Browns specimens, AMNH 5034, was found just 5 ft (1.5 m) 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).[13]

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 jugals 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. He suggested that both 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 Thescelosaurus, preventing comparisons. Morris provisionally referred the specimen to Thescelosaurus, but suggested that it could represent a new species; this potential species has later been called the "Hell Creek hypsilophodontid".[3][14]

Bugenasaura and synonymy

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Cast of CMN 8537, the type specimen of T. edmontonensis

Galton revised Thescelosaurus for a second time in 1995. He reinterpreted the diagnostic traits of the ankle of T. edmontonensis as the result of breakage, as the previously undescribed left ankle showed the same anatomy as T. neglectus. Consequently, Galton 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, within hypsilophodontids. There was also the possibility that the hindlimb of T. garbanii did not belong to a hypsilophodontid but to the pachycephalosaurid Stygimoloch, which is also known from the Hell Creek Formation and 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' and gena, 'cheek', as well as the Ancient Greek saura, "lizard". The specific name is 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 the name garbanii should have priority if this turns out to be the case.[4]

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 referred to Thescelosaurus cf. neglectus by Indian paleontologist Ashok Sahni in 1972, which would have been the oldest occurrence of Thescelosaurus.[4][15] In a 1999 study on the anatomy of Bugenasaura, Galton referred two isolated teeth to the genus. The first (Yale Peabody Museum 8098), collected by Hatcher in 1889 from the Lance Formation of Lusk, Wyoming, was referred to B. infernalis, while the second (University of California Museum of Paleontology 49611) was not referred to a particular species of Bugenasaura as it showed some anatomical differences. Significantly, this second tooth was listed as being from the Late Jurassic Kimmeridge Clay Formation of Weymouth, England, and therefore is 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 active in the Lance region.[16] The lack of diagnostic features led British paleontologists Paul M. Barrett and Susannah Maidment to classify UCPM 49611 as an indeterminate ornithischian in 2011.[17]

Thescelosaurus with skin impressions, Museum of the Rockies specimen 979

After 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.[18] 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.[18][19][20] 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.[18]

Because of 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 in the skull and hindlimb that were characteristic of Thescelosaurus, but do 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 had a slightly different anatomy from T. neglectus, namely NCSM 15728 and Royal Saskatchewan Museum specimen RSM P 1225.1 (previously described by Galton in 1995 and 1997 as T. neglectus[4][21]). 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.[18]

Skull and part of neck of T. neglectus specimen NCSM 15728 before complete preparation

RSM P 1225.1 was found on June 19, 1968, by Albert Swanson of the Saskatchewan Museum of Natural History (now the Royal Saskatchewan Museum), and collected on July 17. The location as originally reported was incorrect, as revisiting of the Frenchman River valley by Tim Tokaryk in the 1980s found that the excavation, identifiable by bone and plaster remnants, actually took place at 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.[5] 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 specific 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.[5] 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.[20] In April 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 asteroid impact in the Gulf of Mexico that resulted in the K-Pg extinction.[22][23]

Description

[edit]
Size of two Thescelosaurus species (right) compared to its relatives Parksosaurus (center) and Orodromeus (left), as well as a human

Thescelosaurus was a heavily built bipedal animal.[24] Overall, the skeletal anatomy of this genus is well documented, and restorations have been published in several papers, including skeletal restorations[18][6][13][25] and models.[2][6] The skeleton is known well enough that a detailed reconstruction of the hip and hindlimb muscles has been made.[26] The animal's size has been estimated in the 2.5–4.0 m range for length (8.2–13.1 ft)[13] for various specimens, and a weight of 200–300 kilograms (450–660 pounds),[27] with the large type specimen of T. garbanii estimated at 4–4.5 meters (13.1–14.8 feet) long.[3] It may have been sexually dimorphic, with one sex larger than the other.[13] Juvenile remains are known from several locations, mostly based on teeth.[28][29]

Skull

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Skull of T. neglectus NCSM 15728 ("Willo") in right, top, and hind 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.[20]: 1–9 

The skull had a long, low snout that ended in a beak with a toothless tip.[24][21] 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.[24] 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.[20]: 18  Another fenestra, the antorbital fenestra, was in-between the external naris and the orbit and contained two smaller internal fenestrae.[20]: 20  Long rod-like bones called palpebrals were present above the eyes, giving the animal heavy bony eyebrows.[21] The palpebral was not aligned with the upper margin of the orbit as in some other ornithischians, but protruded across it.[20]: 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.[20]: 6 

Snout areas and teeth of NCSM 15728

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).[16] The ridges and position of the teeth, deeply internal to the outside surface of the skull, are interpreted as evidence for muscular cheeks.[16][3] 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.[20]: 7  The teeth were of different types: small pointed premaxillary teeth (in the premaxilla, or upper beak), and leaf-shaped cheek teeth that differed between the maxilla and the dentary.[30][2] 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).[20]: 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.[20]: 63–64 

Vertebrae and limbs

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Illustrations of the lower arm, foot, and pelvis of the holotype, 1915

T. neglectus had 6 sacral ("hip") vertebrae and 27 presacral ("neck and back") vertebrae.[3][5] The holotype specimen of the species T. assiniboiensis appears to have had only five sacrals, but it is possible that this individual was not yet fully mature and that the last sacral was not yet fused to the other sacrals.[5] The tail was long and made up half of the total body length. It was braced by ossified tendons from the middle to the tip, which would have reduced the flexibility of the tail.[6] The rib cage was broad, giving it a wide back.[2] Large thin flat mineralized plates have been found next to the ribs' sides.[19] Their function is unknown; they may have played a role in respiration.[31] 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.[32] Such plates are known from several other cerapodans.[33] The front ribs were flattened and concave, and the rear margins of their lower ends had a rough surface. These features are autapomorphies of Thescelosaurus and are possibly adaptations that allow the plates to attach to the rib cage.[20]: 7 

The limbs were robust.[2] The femur (upper thigh bone) was longer than the tibia (shin bone), which distinguishes the genus from closely related genera.[20]: 7  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 unguals. There were two phalanges (finger bones) in the first digit, three in the second, four in the third, three in the fourth, and two in the fifth.[6] The foot had five metatarsals, 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 animals weight was therefore supported by the center three digits, 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.[5][6] 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.[20]: 8 [3]

Integument

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T. neglectus restored with protofeathers

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,[6] 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.[3] Scutes have not been found with other articulated specimens of Thescelosaurus, though, and Galton argued in 2008 that Morris's scutes are crocodilian in origin.[33] 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.[22]

Classification

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Gilmore initially considered Thescelosaurus to be a member of Camptosauridae, alongside Hypsilophodon, Dryosaurus and Laosaurus. He soon revised his opinion, however, and placed it instead within Hypsilophodontidae alongside only Hypsilophodon.[1][6] Many authors followed this classification within Hypsilophodontidae.[34] Hungarian paleontologist Franz Nopcsa and German paleontologist Friedrich von Huene instead retained Thescelosaurus as a relative of Camptosaurus.[35][36] Sternberg at first separated Thescelosaurus and the related Parksosaurus into a family of their own, Thescelosauridae, before considering both genera to be members of the subfamily Thescelosaurinae within Hypsilophodontidae.[11][2] Russian paleontologist Anatoly Konstantinovich Rozhdestvensky and Australian paleontologist Richard A. Thulborn retained Thescelosauridae as a separate family.[37][38] Galton initially classified it as a member of Iguanodontidae based on hindlimb proportions, but this family was found to be polyphyletic (not a natural group).[13] He revised his taxonomy of Thescelosaurus in 1995, returning to a hypsilophodontid classification.[4]

Historically posed skeletons of Thescelosaurus with head and neck reconstructed after Hypsilophodon, North American Museum of Ancient Life

Hypsilophodontidae only included four genera when its classification was assessed by Sternberg in 1940: Hypsilophodon, Thescelosaurus, Parksosaurus, and Dysalotosaurus.[2] The content of Hypsilophodontidae was expanded by American paleontologist Alfred Sherwood Romer in 1966 to include most small ornithopods, which was followed by Galton (though Thescelosaurus was removed from the family) and later authors. As a result, Hypsilophodontidae including 13 genera in the first edition of the book The Dinosauria in 1990.[39][13][40] This concept of Hypsilophodontidae as an expansive natural group has been recovered by the early cladistic studies of American paleontologists Paul C. Sereno, David B. Weishampel, and Ronald Heinrich, who found Thescelosaurus to be the most basal (primitive) hypsilophodontid. The analysis of Weishampel and Heinrich in 1992 can be seen below.[41][42]

Hypsilophodontidae
Mount restored with modern skull and posture

The concept of Hypsilophodontidae as a monophyletic group then fell out of favor. The American paleontologist 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, Parkosaurus and Bugenasaura to be successively closer to Hypsilophodon and later ornithopods, but not a group of their own.[43] 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.[44][45][46] An issue with Thescelosaurus neglectus prior to the revision by Boyd and colleagues in 2009 was the uncertainty about the referred specimens, including the separation of Bugenasaura and the unresolved question whether T. edmontonensis is distinct or not.[46] 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 ornithopod consisting of Thescelosaurus, Parksosaurus, Zephyrosaurus, Orodromeus and Oryctodromeus.[18] Brown and colleagues, while describing T. assiniboiensis, came to similar results.[5] 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. Thescelosaurus and Parksosaurus constituted Thescelosaurinae alongside the Asian taxa Haya, Jeholosaurus and Changchunsaurus, while Orodrominae consisted of Zephyrosaurus, Orodromeus, Oryctodromeus, the new genus Albertadromeus, and an unnamed taxon.[47][48]

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.[20] The clades Thescelosauridae (or alternatively Parksosauridae) and Thescelosaurinae have been confirmed by numerous phylogenetic analyses,[47][34][49][50][51] though not by all.[52][53] 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 marginocephalians 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.[34] Some studies agree with this placement for thescelosaurids,[49][52] while others support Thescelosaurus as an ornithopod,[53] and others are unresolved.[50][51] 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.[49]

Thescelosauridae
Orodrominae
Thescelosaurinae

Yueosaurus

Changmiania

Haya griva

RTMP 2008.045.0002

Parksosaurus

Thescelosaurus

CMN 8537 (T. edmontonensis)

T. neglectus

T. garbanii

T. assiniboiensis

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.[34] 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.[49][52] 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.[34] Haviv Avrahami and colleagues, in 2024, named a new, early-diverging thescelosaurine, Fona. Their analysis found that Thescelosaurinae consisted of Fona and Oryctodromeus, and Thescelosaurus, while Parksosaurus falls outside both Thescelosaurinae and Thescelosauridae.[54]

Paleobiology

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T. neglectus restored with scaly skin

Like other ornithischians, Thescelosaurus was probably herbivorous.[24] Thescelosaurus would have browsed in the first meter or so from the ground, feeding selectively,[24] with food held in the mouth by cheeks while chewing.[13] One specimen is known to have had a bone pathology, with the long bones of the right foot fused at their tops.[55] The different types of teeth, as well as the narrow snout, suggest that Thescelosaurus was a selective feeder. The contemporary pachycephalosaur Stegoceras, in contrast, was probably a more indiscriminate feeder, allowing both animals to share the same environment without competing for food.[30]

Posture and locomotion

[edit]

In his 1915 description, Gilmore suggested that Thescelosaurus was an agile animal that moved on two leggs and was adapted for running. He also created a model to depict its life appearance, showing a light and agile body built with slender hind limbs.[6] These ideas were contested by Sternberg in 1940, who argued that the skeleton, and especially the limbs, were robust. His own model, of the species T. edmontonensis, consequently showed limbs that were much more muscular.[2] Other subsequent studies disagreed with Gilmore idea of a proficient runner given the robust skeleton, the proportionally long femur, and the short lower leg bones.[56] Galton, in 1974, even suggested that Thescelosaurus could have occasionally moved on all all fours, given its fairly long arms and wide hands.[13] Phil Senter and Jared Mackey, in 2024, concluded that a four-legged posture would have been theoretically possible, as the spine of the back was bent down, allowing the hand to touch the ground even when the hind limbs were straight. However, in such a posture the fingers would have pointed towards the sides rather than front, and consequently could not have been used to propel the animal forward; four-legged locomotion therefore seems unlikely.[57]

Right pectoral girdle and forelimb bones of NCSM 15728 and motion at the shoulder

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. The 2024 study argued that the acute sense of balance could alternatively be explained by possible burrowing behavior, as this sense also tends to be acute in modern burrowing animals.[56]

In Sternbergs 1940 model, the upper arm was horizontal and almost perpendicular to the body.[2] 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 upper arm bone would not have spread sidewards as in Sternbergs model.[58] Senter and Mackey found that the upper arm bone could swing forward to a vertical position, but not much beyond that point.[57]

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 hypothezised for many other ornithischians including ceratopsians, ankylosaurs, and hadrosaurs.[56]

Senses, sociality, and possible burrowing behavior

[edit]
The broad shoulder blade is possibly an adaptation for burrowing behavior

Button and Zanno, in 2023, discussed the sensory and cognitive abilities of Thescelosaurus based on a CT scan of the skull of the "Willo" specimen (NCSM 15728). Even though the brain itself is not preserved, the skull vault that contained the brain, the endocast, can be studied. Overall, the brain was small compared to most other neornithischian dinosaurs, but similar in size to that of ceratopsids such as Triceratops. Its cognitive abilities were therefore likely within the range of modern reptiles. These limited cognitive abilities might suggest that social interactions were comparatively simple, or that it lived in smaller groups. In localities of the related Oryctodromeus, two to three individuals are usually found together, which could reflect the group size typical for that genus. Thescelosaurus might also have lived in such small groups, although Button and Zanno cautioned that the evidence for such claims remains weak.[56]

Distribution of characters associated with fossoriality within Thescelosauridae

It had poor hearing, with an estimated best hearing range between around 296 and 2150 Hz, which is narrower than that of related genera such as Dysalotosaurus. The sense of smell, in contrast, was acute, as indicated by the large olfactory bulbs of the brain, which are around 3% of the entire volume of the endocast. This is comparable to modern rodents and lagomorphs 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 semi-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.[56]

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.[56]

Supposed fossilized heart

[edit]
"Willo" specimen, with the possible heart left of the shoulder blade

In 2000, the "Willo" specimen (NCSM 15728) 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.[19]

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.[59] 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.[60]

"Willo"'s supposed "heart"

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.[61]

The question of how this find reflects metabolic rate and dinosaur internal anatomy is moot, though, regardless of the object's identity.[61] 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.[62]

Paleoecology

[edit]

Temporal and geographic range

[edit]
Thescelosaurus and other animals and plants that coexisted in North America at the end of the Cretaceous, Smithsonian

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][17][63] 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.[3][5][20] 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.[20][64] 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.[64] 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.[65][66]

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.[67][65] 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.[68]

Paleoenvironment

[edit]
Triceratops and Leptoceratops in the Hell Creek environment

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.[69] 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.[70][71][72] 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.[73] 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.[74]

Thescelosaurus, while historically thought to be a relatively uncommon in its paleoenvironments, is now known to have been one of the most abundant dinosaurs.[75] 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.[76][77][78] 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.[77][78] 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.[79][80] 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.[30]

Pie chart of the time averaged census for large-bodied dinosaurs from one section of the Hell Creek Formation[76]

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.[72] 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.[64]

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.[64][71] 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.[64] 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.[78]

Notes

[edit]
  1. ^ At the time known as "United States National Museum"

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