Jianchangosaurus is a genus of therizinosaurian dinosaur that lived approximately 126 million years ago during the early part of the Cretaceous Period from the Yixian Formation in what is now China. The nearly complete juvenile specimen was missing only the distal tail. Jianchangosaurus was a small, lightly built, bipedal, ground-dwelling herbivore, that could grow up to an estimated long and was high at the hips.
Discovery and naming
The holotype specimen, 41HIII-0308A, was discovered on the Yixian Formation of Jianchang County, in the western part of Liaoning Province and purchased by the Henan Geological Museum noting that some of the elements were repositioned during its preparation. The genus name Jianchangosaurus, means "Jianchang lizard", and is derived from "Jianchang", the name of the county of Liaoning Province, China, where the specimen was found, and the Greek word "" () meaning "lizard". The specific name yixianensis, refers to the Yixian Formation where the specimen was found, and the Latin suffix "-ensis" meaning "originating in". Jianchangosaurus was described and named by Hanyong Pu, Yoshitsugu Kobayashi, Junchang Lu, Li Xu, Yanhua Wu, Huali Chang, Jiming Zhang, and Songhai Jia in 2013 and the type species is Jianchangosaurus yixianensis.
Description
The holotype specimen of Jianchangosaurus 41HIII-0308A is a nearly complete skeleton of a single juvenile individual, missing only the distal portion of the tail. Jianchangosaurus was estimated to have been tall at the hips and approximately long. Gregory S. Paul estimated the weight of the holotype specimen around .
Five sacral vertebrae are present in this genus, a condition that is similar to that of other basal therizinosaurs. The humerus measures in length and is 7% shorter than the scapula. The ulna measures in length and is 78% of the humerus length - which approximates the ratio observed in the basal therizinosaur Falcarius (77%). The pubis is 20% longer than the ischium, and it projects anteroventrally and does not exhibit the opisthopubic condition. The tibia () is 1.5 times longer than the femur (), which is the highest ratio known in therizinosaurs, an adaptation which has been strongly correlated with the development of cursorial habits in dinosaurs.
Distinguishing anatomical features
According to Pu et al. 2013, Jianchangosaurus can be distinguished based on the presence of 27 tightly packed maxillary teeth; the dorsal border of the antorbital fenestra is formed by the maxilla, nasal, and lacrimal, but with the majority of the border formed by the nasal; there is no participation of jugal in the margin of the antorbital fenestra; a short diastema is present in the anterior tip of the dentary; dentary teeth have a concave labial surface and a convex lingual surface (this condition is present for all except six anterior teeth); the lack of prominent hypapophyses in the anterior dorsal vertebrae; the anterior caudal centra have an oval cross section and the articular facet is as tall as it is wide; the presence of weakly curved manual unguals with weak flexor tubercles positioned ventral to the articular facet; the ilium is shallow and elongated; the ridge bounding the cuppedicus fossa is confluent with acetabular rim; and there is extensive contact between the pubic apron.
Skull and dentition
The skull is in good preservation and is nearly complete, missing only the ventral lacrimal, the posterior jugal, the postorbital, the anterior edge of the quadrate, and the anterior surangular bones. Jianchangosaurus possesses 27 maxillary teeth and approximately 25 to 28 dentary teeth. The researchers observed, however, that at front of the upper jaw the premaxilla is edentulous and they hypothesized that it was covered by a rhamphotheca. This is also supported by the presence of a series of foramina along the buccal margin on the lateral surface of the premaxilla. By comparison, in most modern birds, the rhamphotheca grows, and is shed, continuously and in some species its color varies depending on the season.
The front portion of the lower jaw is down-turned and in combination with the rhamphotheca on the upper jaw, functioned to pluck food. The skull measures in length, and is 10% longer than the femur, a condition not shared by Beipiaosaurus. Derived features present in the skull of this genus strongly suggest adaptations for herbivory.