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Superregnum: Eukaryota
Cladus: Unikonta
Cladus: Opisthokonta
Cladus: Holozoa
Regnum: Animalia
Subregnum: Eumetazoa
Cladus: Bilateria
Cladus: Nephrozoa
Superphylum: Deuterostomia
Phylum: Chordata
Subphylum: Vertebrata
Infraphylum: Gnathostomata
Megaclassis: Osteichthyes
Cladus: Sarcopterygii
Cladus: Rhipidistia
Cladus: Tetrapodomorpha
Cladus: Eotetrapodiformes
Cladus: Elpistostegalia
Superclassis: Tetrapoda
Cladus: Reptiliomorpha
Cladus: Amniota
Classis: Reptilia
Cladus: Eureptilia
Cladus: Romeriida
Subclassis: Diapsida
Cladus: Sauria
Infraclassis: Archosauromorpha
Cladus: Crurotarsi
Divisio: Archosauria
Cladus: Avemetatarsalia
Cladus: Ornithodira
Subtaxon: Dinosauromorpha
Cladus: Dinosauriformes
Cladus: Dracohors
Cladus: Dinosauria
Ordo: †Ornithischia
Cladus: †Genasauria
Cladus: †Neornithischia
Cladus: †Cerapoda
Cladus: †Euornithopoda
Taxon: †Hypsilophodontia

Familia: †Hypsilophodontidae
Genus: †Parksosaurus
Species: P. warreni
Name
Parksosaurus Sternberg, 1937

Parksosaurus (meaning "William Parks's lizard") is a genus of neornithischian dinosaur from the early Maastrichtian-age Upper Cretaceous Horseshoe Canyon Formation of Alberta, Canada. It is based on most of a partially articulated skeleton and partial skull, showing it to have been a small, bipedal, herbivorous dinosaur. It is one of the few described non-hadrosaurid ornithopods from the end of the Cretaceous in North America, existing around 70 million years ago.

Description
Size of Parksosaurus (center) compared to its relatives Thescelosaurus (right) and Orodromeus (left), as well as a human

Explicit estimates of the entire size of the animal are rare; in 2010 Gregory S. Paul estimated the length at 2.5 meters, the weight at forty-five kilograms.[1] William Parks found the hindlimb of his T. warreni to be about the same length overall as that of Thescelosaurus neglectus (93.0 centimeters (3.05 ft) for T. warreni versus 95.5 centimeters (3.13 ft) for T. neglectus), even though the shin was shorter than the thigh in T. neglectus, the opposite of T. warreni.[2] Thus, the animal would have been comparable to the better-known Thescelosaurus in linear dimensions, despite proportional differences (around 1 meter (3.3 ft) tall at the hips, 2-2.5 meters (6.56-8.2 ft) long).[2] The proportional differences probably would have made it lighter, though, as less weight was concentrated near the thigh. Like Thescelosaurus, it had thin partly ossified cartilaginous (intercostal) plates along the ribs.[3] The shoulder girdle was robust.[1] Parksosaurus had at least eighteen teeth in the maxilla and about twenty in the lower jaw; the number of teeth in the premaxilla is unknown.[4]
Discovery and history
Fossil, Royal Ontario Museum

Paleontologist William Parks described skeleton ROM 804 in 1926 as Thescelosaurus warreni, which had in 1922 been discovered in what was then called the Edmonton Formation near Rumsey Ferry on the Red Deer River. When found, it consisted of a partial skull missing the beak region, most of the left pectoral girdle (including a suprascapula, a bone more commonly found in lizards, but which is believed to have been present in cartilaginous form in some ornithopods due to the roughened ends of their scapulae),[5] the left arm except the hand, ribs and sternal elements, a damaged left pelvis, right ischium, the left leg except for some toe bones, articulated vertebrae from the back, hip, and tail, and a number of ossified tendons that sheathed the end of the tail. The body of the animal had fallen on its left side, and most of the right side had been destroyed before burial; in addition, the head had been separated from the body, and the neck lost. Parks differentiated the new species from T. neglectus by leg proportions; T. warreni had a longer tibia than femur, and longer toes.[2]

Charles M. Sternberg, upon the discovery of the specimen he named Thescelosaurus edmontonensis, revisited T. warreni and found that it warranted its own genus (it was named in an abstract, which is not typical, but the specimen had already been thoroughly described).[6] In 1940, he presented a more thorough comparison and found a number of differences between the two genera throughout the body. He assigned Parksosaurus to the Hypsilophodontinae with Hypsilophodon and Dysalotosaurus, and Thescelosaurus to the Thescelosaurinae.[7] The genus attracted little attention until Peter Galton began his revision of hypsilophodonts in the 1970s. Parksosaurus received a redescription in 1973, wherein it was considered to be related to a Hypsilophodon\Laosaurus\L. minimus lineage.[4] After this, it once again returned to obscurity.

George Olshevsky emended the species name to P. warrenae in 1992,[8] because the species name honors a woman (Mrs. H. D. Warren who financially supported the research), but outside of Internet sites, the original spelling has been preferred.[9]
Classification
Skull

Parksosaurus has been considered to be a hypsilophodont since its description.[2] Recent reviews have dealt with it with little comment,[10][9][11] although David B. Norman and colleagues (2004), in the framework of a paraphyletic Hypsilophodontidae, found it to be the sister taxon to Thescelosaurus,[9] and Richard Butler and colleagues (2008) found that it may be close to the South American genus Gasparinisaura.[11] However, basal ornithopod phylogeny is poorly known at this point, albeit under study. Like Thescelosaurus, Parksosaurus had a relatively robust hindlimb, and an elongate skull without as much of an arched shape to the forehead compared to other hypsilophodonts.[9] A 2015 study placed it as an intermediate member of Iguanodontia more derived than Elasmaria.[12]

Cladogram based in the phylogenetic analysis of Rozadilla et al., 2015:

Hypsilophodon

Thescelosaurus

Iguanodontia
Elasmaria

Gasparinisaura

Morrosaurus

Trinisaura

Macrogryphosaurus

Notohypsilophodon

Talenkauen

Anabisetia

Parksosaurus

Kangnasaurus

Rhabdodontidae

Tenontosaurus

Dryomorpha


The cladogram below results from analysis by Herne et al., 2019, which placed Parksosaurus as the most basal member of Ornithopoda.[13]
Ornithischia

Ornithischia

Heterodontosauridae

Eocursor

Thyreophora

Neornithischia

Lesothosaurus

Agilisaurus

Hexinlusaurus

Yandusaurus

Nanosaurus

Jeholosauridae

Haya

Jeholosaurus

Changchunsaurus

Thescelosauridae

Orodromeus

Koreanosaurus

Zephyrosaurus

Yueosaurus

Thescelosaurus

Cerapoda

Marginocephalia

Ornithopoda

Parksosaurus

Elasmaria

Talenkauen

Macrogryphosaurus

Gasparinisaura

Galleonosaurus

Leaellynasaura

Anabisetia

Diluvicursor

Clypeodonta

Hypsilophodon

Iguanodontia

Rhabdodontidae

Muttaburrasaurus

Tenontosaurus

Dryomorpha





Paleoecology and paleobiology
Life restoration of Parksosaurus warreni

Parksosaurus is known from the base of Unit 4 of the Horseshoe Canyon Formation,[14] which dates to about 69.5 million years ago.[15] Other dinosaur species from this same unit include the theropods Albertosaurus sarcophagus and Albertavenator curriei as well as the spike-crested hadrosaurid Saurolophus osborni, hollow-crested hadrosaurid Hypacrosaurus altispinus, and ankylosaurid Anodontosaurus lambei. Teeth of an unidentified ceratopsian species are known from the same stratigraphic level.[14] The dinosaurs from this formation are sometimes known as Edmontonian, after a land mammal age, and are distinct from those in the formations above and below.[16] The Horseshoe Canyon Formation is interpreted as having a significant marine influence, due to an encroaching Western Interior Seaway, the shallow sea that covered the midsection of North America through much of the Cretaceous.[16]

In life, Parksosaurus, as a hypsilophodont, would have been a small, swift bipedal herbivore. It would have had a moderately long neck and small head with a horny beak, short but strong forelimbs, and long powerful hindlimbs.[9] Paul in 2010 suggested that the long toes were an adaptation for walking over mud or clay near rivers and that the strong arms were used for burrowing.[1]
References

Paul, G.S., 2010, The Princeton Field Guide to Dinosaurs, Princeton University Press p. 277
Parks, William A (1926). "Thescelosaurus warreni, a new species of orthopodous dinosaur from the Edmonton Formation of Alberta". University of Toronto Studies (Geological Series). 21: 1–42.
Butler, Richard J.; Galton, Peter M. (2008). "The 'dermal armour' of the ornithopod dinosaur Hypsilophodon from the Wealden (Early Cretaceous: Barremian) of the Isle of Wight: a reappraisal". Cretaceous Research. 29 (4): 636–642. doi:10.1016/j.cretres.2008.02.002.
Galton, Peter M. (1973). "Redescription of the skull and mandible of Parksosaurus from the Late Cretaceous with comments on the family Hypsilophodontidae (Ornithischia)". Life Sciences Contribution, Royal Ontario Museum. 89: 1–21.
Gilmore, Charles W. (1915). "Osteology of Thescelosaurus, an orthopodus dinosaur from the Lance Formation of Wyoming" (PDF). Proceedings of the U.S. National Museum. 49 (2127): 591–616. doi:10.5479/si.00963801.49-2127.591.
Sternberg, Charles M. (1937). "Classification of Thescelosaurus, with a description of a new species". Geological Society of America Proceedings for 1936: 365.
Sternberg, Charles M. (1940). "Thescelosaurus edmontonensis, n. sp., and classification of the Hypsilophodontidae". Journal of Paleontology. 14 (5): 481–494.
Olshevsky, G. (1991). A Revision of the Parainfraclass Archosauria Cope, 1869, Excluding the Advanced Crocodylia. Mesozoic Meanderings No. 2. San Diego: Publications Requiring Research. p. 268.
Norman, David B.; Sues, Hans-Dieter; Witmer, Larry M.; Coria, Rodolfo A. (2004). "Basal Ornithopoda". In Weishampel, David B.; Dodson, Peter; Osmólska, Halszka (eds.). The Dinosauria (2nd ed.). Berkeley: University of California Press. pp. 393–412. ISBN 0-520-24209-2.
Sues, Hans-Dieter; Norman, David B. (1990). "Hypsilophodontidae, Tenontosaurus, Dryosauridae". In Weishampel, David B.; Dodson, Peter; Osmólska, Halszka (eds.). The Dinosauria (1st ed.). Berkeley: University of California Press. pp. 498–509. ISBN 0-520-06727-4.
Butler, Richard J.; Upchurch, Paul; Norman, David B. (2008). "The phylogeny of the ornithischian dinosaurs". Journal of Systematic Palaeontology. 6 (1): 1–40. doi:10.1017/S1477201907002271. S2CID 86728076.
Rozadilla, Sebastián (2016). "A new ornithopod (Dinosauria, Ornithischia) from the Upper Cretaceous of Antarctica and its palaeobiogeographical implications". Cretaceous Research. 57: 311–324. doi:10.1016/j.cretres.2015.09.009.
Herne, Matthew C.; Nair, Jay P.; Evans, Alistair R.; Tait, Alan M. (2019). "New small-bodied ornithopods (Dinosauria, Neornithischia) from the Early Cretaceous Wonthaggi Formation (Strzelecki Group) of the Australian-Antarctic rift system, with revision of Qantassaurus intrepidus Rich and Vickers-Rich, 1999". Journal of Paleontology. 93 (3): 543–584. doi:10.1017/jpa.2018.95.
Larson, D. W.; Brinkman, D. B.; Bell, P. R. (2010). "Faunal assemblages from the upper Horseshoe Canyon Formation, an early Maastrichtian cool-climate assemblage from Alberta, with special reference to the Albertosaurus sarcophagus bonebed This article is one of a series of papers published in this Special Issue on the theme Albertosaurus". Canadian Journal of Earth Sciences. 47 (9): 1159–1181. doi:10.1139/e10-005.
Arbour, Victoria (2010). "A Cretaceous armoury: Multiple ankylosaurid taxa in the Late Cretaceous of Alberta, Canada and Montana, USA". Journal of Vertebrate Paleontology. 30 (Supplement 2): 55A. doi:10.1080/02724634.2010.10411819. S2CID 220429286.
Dodson, Peter (1996). The Horned Dinosaurs: A Natural History. Princeton: Princeton University Press. pp. 14–15. ISBN 0-691-05900-4.

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