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Thescelosaurus

Thescelosaurus, meaning “godlike”, “wondrous”, or “marvelous” and “lizard” was a genus of small ornithopod dinosaur that appeared at the very 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 65.5 million years ago. The completeness and preservation of many of its specimens illustrate that it might have preferred to live near streams.

This bipedal ornithopod is known from several incomplete skeletons and skulls that indicate it grew to between 8.2 to 13.1 feet in length on average. It has sturdy hind limbs, small wide hinds, and a head with an extended pointed snout. This genus of dinosaur is regarded as a specialized hypsilophodont and an herbivore. Several species have been suggested for this particular genus. Three are currently recognized as legitimate, the type species T. neglectus, as well as T. garbanii and T. assiniboiensis.

The genus attracted the medias attention in 2000, when a specimen unearthed in 1993 in South Dakota was interpreted as including a fossilized heart. There was much discussion over whether the remains were, in fact, a heart. May scientists now doubt the identification of the item and the implications of such identification.

This dinosaur was a heavily built bipedal animal, probably herbivorous, but possibly omnivorous. It would have browsed in the first meter or so from the ground, feeding selectively, with food held in the mouth by cheeks whilst chewing. There was a prominent ridge along the length of both maxillae, and a ridge on both dentaries. The ridges and position of the teeth deeply internal to the outside surface of the skull are seen as evidence for muscular cheeks. Aside from the long and narrow beak, the skull also had teeth in the premaxilla, or upper beak, and long rod-like bones called palpebrals over its eyes, giving it heavy bony eyebrows. Its teeth were of two types: small and pointed premaxillary teeth, and leaf-shaped cheek teeth. Six small teeth were present in both premaxillae, with a toothless section at the beaks tip.

It had broad, short, five-fingered hands, four toed feet with toe tips that were hoof-like, and a tail that was long and braced by ossified tendons from the middle back to the tip, which would have decreased the flexibility of the tail. While the rib cage was broad, the back was wide and the limbs were robust. This animal might have been able to move around on all fours, given its rather long arms and wide hands, but this idea hasn’t been followed up in the scientific literature, although it does appear in popular works. Charles M. Sternberg rebuilt it with the upper arm leaning almost at a 90 degree angle to the body, another idea that has gone by the wayside. The leaning of the shoulder articular surface also illustrates a vertical instead of horizontal upper arm in the dinosaurs. Thescelosaurus was mostly likely slower than the other hypsilophodonts, due to its heavier build and leg structure. Compared to them, the limbs were strange due to the upper leg was longer than the shin, which is the opposite of Hypsilophodon and animals than run in general. Large and thin mineralized plates have been found next to the ribs. Their function is not known, but they might have played in role in respiratory functions. Recent histological studies have suggested that the plates may have started with cartilage and became bone as the animal aged.

The nature of this genus’ integument, may it be scales or something else, is presently not known, although potential evidence is known; Charles Gilmore described some patches of carbonized material close to the shoulders as possible epidermis, with a punctured texture not no consistent pattern, and William J. Morris proposed that armor was present, in the appearance of small scutes present at least along the midline of the neck of one of the specimens. Scutes haven’t been found with other expressed specimens of Thescelosaurus, though, and Morris’s scutes might just be crocodilian in origin.

The skeleton is known well enough that a detailed reconstruction of the hind limb and hip muscles has been made. The animal’s size has been estimated at around 18.2 to 13.1 for length for various specimens, and a weight of around 450 to 660 pounds, with the large type specimen of T. garbanii estimated at 13.1 to 14.8 feet long. Under “Discovery, history, and species” it was discussed more fully that it may have been sexually dimorphic, with one sex larger than the other. The juvenile remains are known from several different locations, mainly based on teeth.

Thescelosaurus is frequently often informally given its own family or subfamily, Thescelosauridae or Thescelosaurinae. Three studies recently done have found it to be a close relative of Parksosaurus, although neither named a specific clade , and one of the studies is tricky to understand because it didn’t include Iguanodontia in its diagrams. This area of the dinosaur family tree is difficult by a lack of research. Oddly, Thescelosaurus has been considered as both very basal and very derived among the hypsilophodonts. One of the issues regarding Thescelosaurus is that not all of the remains that are assigned to T. neglectus necessarily belong to it.

The type specimens of the Thescelosaurus (USNM 7757) was discovered in the year 1891 by paleontologists John Bell Hatcher and William H. Utterback, from beds of the late Maastrichtian (http://en.wikipedia.org/wiki/Maastrichtian)-age Upper Cretaceious Lance Formation of Niobrara County, Wyoming, USA. The skeleton, however, stayed in its shipping crates for years until Charles W. Gilmore of the Smithsonian Institution’ National Museum of Natural History had it ready and described in a short paper in the year 1913, naming it T. neglectus. At that time, he though it was related to Camptosaurus. He supplied a detailed monogram in 1915, illustrating the well-preserved skeleton. The name comes from the surprise Gilmore felt at finding such a spectacular specimen that had been not tended to for so long. He thought it to be a light, agile creature, and assigned it to the Hypsilophodontidae, a family of small sized bipedal dinosaurs.

True Thescelosaurus remains are known definitely from late Maastrichtian-age rocks, from Alberta and Saskatchewan, Canada, and Wyoming, Montana, Colorado, and South Dakota USA. With the exception of birds, it was one of the last existing genera of dinosaurs, its remains being found as close as three meters to the boundary clay containing the iridium layer that closes the Cretaceous. There are documents of teeth from older, Campanian-age rocks, especially from the Dinosaur Park Formation in Alberta, but these specimens aren’t from Thescelosaurus and are probably those of Orodromeus. More specimens are known that have been formally described for this genus, such as the Triebold specimen, which has been the source of more than a couple skeletal casts for museums.

Conflicting accounts have been made as to its preferred habitat: two papers proposed that it preferred channels to floodplains, but another proposes it preferred the opposite. The possibility of a preference for channels is based on the relative abundance of thescelosaur fossils in sandstones, representing the floodplain environments. No bone beds or accumulations of multiple individuals have been reported. Dale Russell noted that Thescelosaurus was the most common small herbivore in the Hell Creek Formation of the Fort Peck location. He described the environment at that time to be a flat floodplain, with a quite dry subtropical climate that supported a variety of plants ranging from angiosperm trees, to bald cypress, ferns and ginkgos. Although most dinosaur skeletons that come from this area are incomplete, mostly likely because of the low preservation potential of the forests, Thescelosaurus skeletons are a lot more complete, proposing that this genus frequented stream channels. Thus, when a Thescelosaurus died, it might have been near or in a river, making it much easier to bury and preserve the dinosaur for later fossilization.

Image Caption: A Thescelosaurus at the Burpee Museum of Natural History in Rockford, Illinois, USA. Credit: Ben Jacobson/Wikipedia (CC BY-SA 3.0)

Thescelosaurus


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