Wuerhosaurus
Wuerhosaurus is a genus of stegosaurid dinosaur from the Early Cretaceous Period of China and Mongolia. As such, it was one of the last genera of stegosaurians known to have existed, since most others lived in the late Jurassic. Discovery and species. "Wuerhosaurus homheni" is the type species, described by Dong Zhiming in 1973 from the Tugulu Group in Xinjiang, western China. The generic name is derived from the city of Wuerho. Three separate localities in the Wuerho Valley were discovered to contain material from the new stegosaur: , 64043 and 64045. The remains consisted of the holotype, Institute of  Vertebrate Paleontology and Paleoanthropology (IVPP) V.4006, a skull-less fragmentary skeleton, and the paratype IVPP V.4007. Holotype material includes a mostly complete pelvis and sacrum lacking the ischium, the first caudal vertebrae, two dorsal vertebrae, a scapulocoracoid, humerus and phalanx, as well as two dermal plates. Three posterior caudal vertebrae from the tail and a partial ulna of a second individual form the paratype, and Dong referred a partial ischium from a third locality to "Wuerhosaurus". A smaller stegosaur from the Ejinhoro Formation in the Ordos Basin in Inner Mongolia was found in 1988. When the specimen (IVPP V.6877) was  described by Dong in 1993, it was named "W. ordosensis", as it was from a similar age and had a similar anatomy. The holotype of the species includes a nearly complete torso, consisting of three cervical vertebrae, all eleven dorsal vertebrae (with attached ribs), a complete sacrum with a right ilium, and the first five caudal vertebrae, all articulated. An additional dorsal vertebra and dermal plate were referred to the taxon when it was named. In 2014 Ulansky named a new species of "Wuerhosaurus", "W. mongoliensis" for vertebrae and pelvic material, but the name is an invalid "nomen nudum". It  was formally described as "Mongolostegus" in 2018. Description. "Wuerhosaurus homheni" was probably a broad-bodied animal. Gregory S. Paul in 2016 estimated the length at and the weight at . Only a few scattered bones have been found, making a full restoration difficult. Its dorsal plates were at first thought to have been much rounder or flatter than other stegosaurids, but Maidment established this was an illusion caused by breakage: their actual form is unknown. "W. homheni" had a pelvis of which the front of the ilia strongly flared outwards indicating a very broad belly. The neural spines on the tail  base were exceptionally tall. "W. ordosensis" was estimated by Paul to have been long and weigh . It too has a broad pelvis but the neural spines are shorter. The neck seems to have been relatively long. Classification. Phylogeny. "Wuerhosaurus" is one of the most derived stegosaurians, being closely related to either "Dacentrurus" and "Hesperosaurus", or "Hesperosaurus" and "Stegosaurus", depending on phylogenetic analysis. Carpenter "et al." (2001) recovered "Wuerhosaurus" in the former relationship, close to "Hesperosaurus" and "Dacentrurus" as basal in Stegosauridae. "Wuerhosaurus" was recovered in a different position by Escaso "et al." (2007), still related to "Hesperosaurus", but basal to  a clade of "Lexovisaurus" and "Stegosaurus". Maidment "et al." (2008) recovered a different placement with "Wuerhosaurus" as being in a clade of taxa in derived Stegosaurinae, most closely related to "Hesperosaurus" and then "Stegosaurus". More recently, Maidment (2017) elaborated upon her earlier analyses, and instead resolved "Wuerhosaurus" as closest to "Stegosaurus", with "Hesperosaurus" being more closely related to "Miragaia". These results are shown below. Synonymy with "Stegosaurus". Maidment and colleagues proposed in 2008 that "Wuerhosaurus" was a junior synonym of "Stegosaurus", with "W. homheni" being renamed "Stegosaurus homheni", and "W. ordosensis" being a dubious taxon. "W. homheni" was synonymized because  the holotype that could be located was similar to "Stegosaurus", and because of its phylogenetic position. "Wuerhosaurus" placed between "Stegosaurus" and "Hesperosaurus", the latter being considered a species of "Stegosaurus" because of its age, location, and anatomy. Since "Wuerhosaurus" placed between two possible "Stegosaurus" species, Maidment "et al." synonymized the taxon as well. "Wuerhosaurus ordosensis" was considered to be a "nomen dubium" because the holotype could not be found in the IVPP collections. The original description did not mention any valid diagnostic traits, and no other description provided features either, so Maidment "et al." considered the taxon undiagnostic. This opinion  has been contested, however, by Carpenter (2010). He discussed how the diagnoses and features used by Maidment "et al." were inconsistent and generalized, with "Wuerhosaurus homheni" bearing numerous differences. As such, Carpenter advocated for the separation of both "Hesperosaurus" and "Wuerhosaurus" from "Stegosaurus", and the separation of the different "Stegosaurus" species from "S. armatus". Paleobiology. "Wuerhosaurus" was lower to the ground than most other stegosaurids; scientists believe that this was an adaptation to let it feed on low-growing vegetation. "Wuerhosaurus", like other stegosaurids, perhaps had a thagomizer on the end of its tail, like that of "Stegosaurus" which featured  four bony spikes that would most likely have been used for self-defense. A single spike was found but was seen by Dong as being positioned on the shoulder. Paleoecology. The type species, "W. homheni", is known from the Tugulu Group, while "W. ordosensis" was found in the Ejinhoro Formation. The approximate age of "Wuerhosaurus" is 130 mya, based on the approximate dating of the Tsaganstabian fauna, and thus the stegosaur would have lived in the Hauterivian era, which is roughly coeval with the Wealden group, from which other stegosaur material has been found. Stegosaur footprints from the Tugulu Group  in Xinjiang Province, China, have been attributed as registered by "W. homheni" individuals. These tracks include the world's smallest stegosaur tracks, measuring 5.7 cm in length, indicative of a dinosaur that was approximately cat-sized.