Jianchangosaurus

The skull of Jianchangosaurus' holotype is fairly well-preserved, lacking only a few elements. It measures 23 cm (9.1 in) in height, and 11 cm (4.3 in) in height. The premaxilla and the anterior (front) portion of the maxilla are edentulous, meaning they lacked teeth. The premaxilla is lined with small foramina, suggesting that a beak was present. The dorsal (top) border of the antorbital fenestra is formed by the lacrimal, maxilla, and nasal, with the majority being formed by the latter. The posterior (rear) portion of the nasal is wide transversely (from side to side), as in Falcarius. The jugal does not participate in the antorbital fenestra's margin, unlike many other coelurosaurs. The prefrontals are triangular, with a V-shaped anterodorsal region (at the top and towards the front) which contacts the lacrimal. The condyloid (articular) process of the quadrate, to which the lower jaw was articulated, was rounded. Jianchangosaurus' dentary was triangular in lateral view, with a downturned symphyseal region, as in other therizinosaurs (with the exception of Falcarius). At very front of the dentary was a toothless region called a diastema.^[1]

Jianchangosaurus had twenty-seven maxillary teeth, and an unknown number of dentary teeth (probably between twenty-five and twenty-eight). This tooth count is similar to that observed in other therizinosaurs, like Erlikosaurus, Falcarius, and Segnosaurus. The teeth were small and lanceolate (shaped like the tip of a lance). The teeth consistently have three denticles of equal size. In the anterior portion of the maxilla, before the edentulous region, the teeth were more densely packed. The anterior dentary teeth were similarly sized to those further back, unlike in Falcarius. The outside surface of each maxillary tooth was convex, while the outside surface of each dentary tooth was concave. A similar dental morphology is seen in ornithopods and ceratopsians, and likely maximised biting stress during occlusion, allowing Jianchangosaurus to more effectively slice plant fibres.^[1]

Ten cervical (neck) vertebrae are preserved in Jianchangosaurus, consisting of the atlas (first cervical vertebra), axis (second cervical vertebra), and all of the following vertebrae. All of the preserved cervical vertebrae are amphicoelous, meaning that they were concave at both ends, and were also highly pneumatised. The postzygapophysis of the atlas was short, with a rounded posterior end. The axis had longer postzygapophyses, which extended posterior to its short neural spine, and were longer than the prezygapophyses. The third to sixth cervicals were longer than those at the front. The neural arches of the middle and posterior cervicals were X-shaped in dorsal view, like other therizinosaurs, and like oviraptorosaurs. The dorsal (back) vertebrae of Jianchangosaurus had longer neural spines than any of the cervicals. The anterior dorsal vertebrae lacked prominent hypapophyses; finer details are difficult to discern, as in the holotype they are obscured by the ribs. The neural spines had rounded tips in the case of the anterior vertebrae, though in the middle to posterior dorsal vertebrae, they transition to a more square shape. At least sixteen gastralia, bones which would have supported the abdominal organs and served as muscle attachment sites, were present. They were spint-like in shape. Like other basal therizinosaurs, Jianchangosaurus had five sacral vertebrae. Eleven caudal (tail) vertebrae are preserved, consisting of the entire caudal column up to the eleventh. The neural spines of the caudal vertebrae were tilted posteriorly, with distal tips split into anterior and posterior alae.^[1]

La escápula y el coracoides de Jianchangosaurus no estaban fusionados, como en otros terizinosaurios basales, aunque esto puede deberse a su naturaleza juvenil . La glenoide de cada escápula miraba hacia atrás. La hoja escapular era recta y mantenía un ancho casi constante a lo largo de toda su longitud. Esto es diferente de la condición de otros terizinosaurios, que aumentaban o disminuían el ancho de la hoja escapular. El coracoides era similar al de los ornitomimosaurios, con un contorno cemicircular y un proceso posterior prominente. El húmero medía 158,5 mm (6,2 pulgadas) de longitud, ligeramente más corto que la escápula. Como en otros terizinosaurios basales, ambos extremos estaban moderadamente expandidos. La tuberosidad interna era grande y estaba separada de la cabeza humeral por una depresión, similar a la de Falcarius . A diferencia de otros terizinosaurios, el entepicóndilo estaba reducido. El cúbito medía aproximadamente el setenta y siete por ciento de la longitud del húmero, una proporción que también se observa en Falcarius . Su extremo proximal (cercano) era triangular, con un proceso olecraniano poco desarrollado en comparación con otros tericinosaurios. El radio era recto y carecía de tubérculo bicipital , de nuevo similar a Falcarius . Se conservan cuatro carpianos distales, incluido el carpo semilunar que caracteriza a los maniraptoranos . Se conservan todos los metacarpianos , excepto el tercer metacarpiano derecho. La falange conservada más larga ( hueso del dedo ; hueso del dedo, en este caso) es la primera, ligeramente más corta que el segundo metacarpiano. Las uñas de la mano (garras) eran curvas y estrechas transversalmente. Los tubérculos flexores de todas las uñas, a los que se unían los tendones flexores , eran redondeados. Un surco en la superficie medial de cada uña llega hasta la superficie dorsal en su extremo. ^{[ 1 ]}  

The pelvis of Jianchangosaurus bore several features that were primitive to Coelurosauia, though absent in more derived therizinosaurs. The ilium was low, with a nearly horizontal dorsal edge, a shallow process before the acetabulum (the preacetabular process), and a deep process after it (postacetabular process). The pubis is shorter than the ischium. It projects anteroventrally. As such, unlike more derived therizinosaurs, it is not opisthopubic, meaning that it was not oriented backwards. The femur was straight, with a cylindrical lesser trochanter that was separated from the greater trochanter by a cleft. The neck of the femoral head is weakly constructed. The fourth trochanter is not known. The tibia is around half as long again as the femur, the highest ratio seen in therizinosaurs;^[1] such an adaptation strongly correlates to cursorial habits in dinosaurs.^[1][4] The fibula is narrower than the tibia. It is not clear how many of Jianchangosaurus' metatarsals are real, as most appear to have been restored. Most pedal (foot) phalanges cannot be identified, as most are crushed, and elements from the manus (hand) may be mixed in. Four pedal unguals are known, all with poorly developed flexor tubercules. All of them are recurved and transversely compressed, similar to in Erlikosaurus.^[1]

The impressions of a series of wide and unbranched feathers surround the holotype of Jianchangosaurus. Only their distal ends are visible. The feathers, considered elongated broad filamentous feathers (EBFFs), are similar to those of Beipiaosaurus, from the same formation. The describers of Jianchangosaurus speculated that they may have been used for visual display.^[1]

Like the primitive Falcarius and Beipiaosaurus, Jianchangosaurus was classified as a basaltherizinosaur. Phylogenetic analysis suggests that it is more derived than Falcarius but more primitive than Beipiaosaurus. Jianchangosaurus is the only known therizinosaur that possesses a tail with caudal centra that are oval in shape.^[1]

Left cladogram is based on the phylogenetic analysis conducted by Pu et al. 2013, showing the relationships of Jianchangosaurus as a very primitive therizinosaur.^[1] Right cladogram is based on Hartman et al. 2019 which has corroborated the position of Jianchangosaurus:^[5]

The validity of Jianchangosaurus was called into question by Cau (2024), who considered it to be a junior synonym of the contemporary Beipiaosaurus since the diagnosis distinguishing the taxa was based on ontogenetically variable characters.^[6]