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. edmontonensis Sternberg, 1940[2] (nomen dubium) †T. garbanii Morris, 1976[3] †T. infernalis (Galton, 1995)[4] (nomen dubium) †T. assiniboiensis Brown et al., 2011[5]
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. Adult Thescelosaurus would have measured roughly 3–4 metres (10–13 ft) long and probably weighed several hundred kilograms. The genus Thescelosaurus is the type genus and also the largest member of the eponymous Thescelosauridae, which includes similarly-sized bipedal herbivores from the Late Cretaceous of Asia and North America such as Orodromeus, Parksosaurus, and Haya.

Thescelosaurus bore several of the characteristic traits of ornithischian dinosaurs including a beak, an inverted pubis, a relatively small antorbital fenestra, a closed mandibular fenestra, and five digits on the hands. However, the animals bore a mix of derived and basal traits when compared with similarly-sized ornithischians. The tip of the snout was edentulous, but the premaxillary, maxillary, and dentary bones bore both pointed and leaf-shaped serrated teeth. It also possessed tridactyl feet similar to those of theropods and ornithopods. The tail of Thescelosaurus bore rod-like ossified tendons, which aided in counterbalancing the weight of the animal. The ribs of Thescelosaurus were accompanied by thin plate-like mineralized tissues, the precise function of which is not confidently known. Thescelosaurus is relatively unique among thescelosaurids for having a relatively long femur in relation to the tibia, suggesting that it was probably not a very fast runner. All known ornithischians are presumed to have been primarily herbivorous and possessed the ability to chew, and Thescelosaurus conforms to this pattern. Analysis of their teeth suggests that they may have been more selective in feeding than similarly-sized pachycephalosaurids. Thescelosaurus also has a number of adaptations which have been interpreted as signs of fossoriality. These include robust arms, large olfactory bulbs, reduced hearing capacity, and shortened hindlimbs. There is no direct evidence that Thescelosaurus lived in burrows or utilized digging as a foraging strategy, but a closely-related animal, Oryctodromeus, was fossilized inside a burrow, so these behaviors are known to have existed among similar small ornithischians during the Late Cretaceous. Researchers have also used computed tomography to examine the internal structure of the skull, and the braincase of Thescelosaurus was relatively small for its size, suggesting that the animal was more comparable to squamates and crocodilians in intelligence than to other ornithischians.

The type species of Thescelosaurus, T. neglectus, was described in 1913 by scientists working for the Smithsonian Institution, and numerous specimens have been referred to the type species in the century since it was described. Initially, it was identified as a "hypsilophodont" — a wastebasket taxon that included a variety of unrelated, but superficially similar bipedal ornithischians. Modern taxonomists consider Thescelosaurus to be either a stem-neornithischian, or a basal member of Ornithopoda, although its precise affinities remain somewhat uncertain. The genus also 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 considered valid, and some of them are only referable to the genus because the fossils are too incomplete to preserve the characteristic traits of any particular species. Thescelosaurus has been found across a wide geographic range. The type specimen was discovered in the Lance Formation of Wyoming, but subsequent discoveries of new specimens have expanded the range of Thescelosaurus to include North Dakota, South Dakota, Montana, Alberta, and Saskatchewan. All of the places where Thescelosaurus remains have been found — the Frenchman Formation, Hell Creek Formation, Scollard Formation, and others — have been dated to the later part of the Maastrichtian (the very end of the Cretaceous Period). Relatives of Thescelosaurus lived earlier in the Cretaceous, but these animals were generally smaller and less robust than Thescelosaurus itself.

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.