Changchunsaurus

The holotype JLUM L0403-j-Zn2 was found in the Quantou formation layer of the Sungiao oil and gas basin dated to the Aptian - Cenomanian tiers, about 126-113 million years ago. Includes partial postcranial skeleton and full skull. The JLUM L0204-Y-23 instance consists of a premaxillary bone (the left posterolateral process and the end of the dorsal process are lost); JLUM L0204-Y-23 consists of an almost complete right dentary ^[1] .

The postcranial skeleton includes an articulated series of 24 presacral, 2 sacral, incomplete cervical vertebrae, dorsal and sacral ribs, ossified tendons, partial left and right scapulocoracoids, thoracic plates, proximal left and right sternal bones, partial left ilium, ileal bone, proximal left tibia, right fibula and calcaneus (with a fragment of the right talus?), full right metatarsus and fingers and partial left metatarsus and fingers ^[2] .

Changchunsaurus is a small ornithopod about 1 m long, having a skull measuring 11.5 cm. A small preorbital window and large eye sockets, occupying up to 40% of the total size of the skull. The orbits have a posterolateral arrangement and a somewhat ellipsoidal shape. Palpebral bones are dense. The length of the lower jaw is 95–97 cm. There is a longitudinal depression in the midline of the nasal bone. There is no contact between the posterolateral process of the premaxillary bone and the lacrimal bone. The ventral margin of the premaxillary bone is at the same level as the jaw bone. On the dentistry and upper jaw, a deepening for the buccal muscle is noticeable. The zygomatic bone is relatively small. The square-zygomatic bone is more or less triangular in shape. A square bone hole is missing. The square bone is strong and tilted caudodorsally; has a wide and very shallow depression on the lateral surface of the bone along the midline. The mandibular joint is located below the chewing surface. The pre-articular process is well developed. High coronoid process. The predecessorium is long, its front part is sharp.

5 premaxillary teeth, they are fang-shaped and bent; on the crowns a thick layer of enamel. The enamel on the crowns of the teeth of the dentar and upper jaw is evenly distributed. Maxillary teeth in an amount of 16-17 pcs. squeezed between the cheek and tongue. The crowns of the maxillary teeth bear several small vertical ridges worn on the tongue surface. Dental teeth crowns (14 pcs.) Are somewhat rhomboidal in shape, have a pronounced vertical crest in the middle and several weaker secondary crests with wear on the buccal surface ^[1] .

In 2010, the cranial anatomy of Changchunsaurus was revised by Jin and others. The authors note that the original diagnosis of the dinosaur, performed by Zan and others in 2005, is a summary of cranial anatomy and does not attempt to subtly understand the features of autapomorphies. Jin and colleagues highlight three features that act as potential autapomorphies. These are: 1) a slit-like incisal opening in the midline of the premaxillary part of the palate, medial to the third and fifth crowns, with an opening located laterally from it on each side; 2) a thickened and slightly wrinkled dorsolateral surface of the dentar, adjacent to the first three teeth of the dentar, continuous with a beveled surface on the pretrium; 3) a groove passing rostromedially along the lateral surface of the caudal half of the dentarium and on the medial surface of the coronoid process ^[2] .

In the original work, only the skull is described. Changchunsaurus is a combination of advanced and basal traits. The presence of autapomorphies characteristic of ornithopods: a paroccipital process of the sickle shape, the jaw joint is displaced ventrally relative to the maxillary row of teeth. The presence of synapomorphies characteristic of euornitopods : a small preorbital window, no external mandibular window. Nevertheless, there is an assumption that Changchunsaurus is more primitive than most ornithopods and marginocephals due to the presence of five premaxillary teeth, a short toothless area in the anterior part of the maxillary bone, a short diastema between the premaxillary and maxillary teeth, evenly distributed enamel on the crowns of the dentarium and upper jaw etc. On the zygomatic bone near the ventral edge of the orbit there is a protrusion described by the authors as a “lumpy structure”, which was not previously seen in other ornithopods. Changchunsaurus had a long predentarium, which suggests a motionless symphysis of the lower jaw, as in ceratops ; ventral process of predentarium longer than ventral. The authors conclude that further work with the samples is necessary in order to find out the exact phylogenetic relations of this new ornithopod, but for now it is defined in the infraorder as incertae sedis ^[1] .

Jin and colleagues note that Changchunsaurus parvus is similar in general cranial anatomy to the similar in size and possibly its contemporary Jeholosaurus shangyuanensis . Both species possess the following features: a strongly wrinkled, coracoid margin of the premaxilla; fossa at the border of the maxillary and jawbone; knotted zygomatic bone ornament; possibly caudal zygomatic bifurcation (not confirmed by Changchunsaurus ); crest-shaped keel on the ventral surface of the occipital bone; elongated arrow-shaped predentarium with a sharp, upturned coracoid end, a deeply incised lateral groove and an elongated caudoventral process. Although all of these characters are found in other poultry-like ones, this combination of characters is unique to Changchunsaurus and Geholosaurus. Combined with their geographical and temporal proximity, this suggests that they are sister taxa and represent an endemic Asian treasure of the Early Cretaceous therapists . However, there are a number of significant differences between Changchunsaurus and the jecholosaur, supporting their separation into two different taxa ^[2] .

Butler and others in 2011 for the first time described in detail the postcranial skeleton and performed a large phylogenetic analysis, which confirmed the position of Changchunsaurus among the basal ornithopods in close connection with the jecholosaurus ^[3] , and later with the open in 2011 Haya ^[4] . Han and others conducted a cladistic analysis in 2012 based on 227 characters for 54 taxa, including Changchunsaurus , Geholosaurus and Haya , confirming their kinship. In this connection, the Jeholosauridae family was introduced for them ^[5] .

Cladogram based on analysis of Han and others, 2012:

According to the results of a cladistic analysis of Brown and others in 2013, Changchunsaurus falls into the same clade (the subfamily Thescelosaurinae ) with a geholosaur, Haya , teselosaurus and parksaosaurus ^[6] .