Study On Xinjiangtitan Shanshanensis From The Late Jurassic Of Shanshan,Xinjiang

China is an important source of Jurassic sauropod dinosaur fossils,and Mamenchisauridae is one of the most important group of Jurassic sauropod dinosaur in China.Xinjiangtitan shanshanensis was reported by Wu et al.(2013)and classified into Mamenchisauridae based on two caudal cervical vertebrae,complete dorsal vertebrae and sacral vertebrae,two anterior caudal vertebrae,and some elements of the appendicular skeletons.The initial description of Xinjiangtitan shanshanensis is preliminary and problematic,such as mistaking the last cervical vertebra for the first dorsal vertebra and regarding the 10 th and11th dorsal vertebrae as a single dorsal and so on.By re-excavation during 2014-2015,Xinjiangtitan shanshanensis(SSV12001)includes almost complete postcranial skeleton,which provided an important basis for the study of the morphology and evolution of this group.Here we redescribed its complete postcranial skeleton in detail,and believed that some identifications described by Wu et al.(2013)were incorrect: the number of cervical vertebrae should be 18 instead of 17 as described in the original text;a ventral keel is developed in the front portion of the ventral surface of the penultimate cervical centrum(actually the Cv16)instead of the 1/4 of the ventral surface,and the posterior articular surface of the penultimate cervical vertebra(actually the Cv16)does not have the small semicircular process described in the original text;the ventral surface of the first dorsal vertebra(actually the 18 th cervical vertebra)described in the original text has no ventral keel,and the ventral portion of the confirmed first dorsal vertebra is missing;the first and second sacral vertebrae are not connected by the second sacral rib at the same time;the first sacral rib contributes to the sacricostal yoke.The material of Xinjiangtitan shanshanensis includes 18 almost complete cervical vertebrae;12 almost complete dorsal vertebrae excluding the ventral portions of centra of D1-9;5 articulated sacral vertebrae;39 caudal vertebrae;partial cervical and dorsal ribs;18 chevrons;left ilum;left and right pubes;left and right ischia;left femur;left tibia;left fibula;left astragalus;partial left pes.A number of autapomorphies of Xinjiangtitan shanshanensis were revised: the ventral surface of axis centra is without medial ridge;the ventral keel extends almost along the middle-posterior portion of the ventral surface in Cv 3–12 but is confined to the anterior concavity in Cv 13–17;the longest cervical is Cv 12;the ratio of the length of Cv 3: Cv 2 is 1.58;the horizontal ridge presents dorsal to the lateral pneumatic fossa of the last two cervical centra;only the last cervical neural spine is bifurcated with the median tubercle;the ratio of the length of the last two cervical centrum and the length of the femur and tibia is 0.54;the first chevron is bifurcated and sled-like;the pubis bears a prominent round ambiens process;the diameter of the femur midshaft is subequal to transverse diameter(the ratio of wide to length is less than 1.25).Three phylogenetic analyses were performed in order to investigate the phylogenetic position of Xinjiangtitan shanshanensis and the interrelationships of Mamenchisauridae using the dataset of Rauhut et al.2015),Xing et al.(2015b)and Xu et al.(2018).All the strict consensus trees resolve Xinjiangtitan shanshanensis as one of Mamenchisauridae and is positioned a more advanced status in Mamenchisauridae.Both the second and the third strict consensus trees further indicate Xinjiangtitan shanshanensis has close relationship with Mamenchisaurus and Qjianglong.The morphological comparation between Xinjiangtitan shanshanensis and other relevant sauropods suggest the same result as the result of phylogenetic analyses.However,Xingjiangtitan shanshanensis bears some unique features which similar to the more derived non-mamenchisaurid sauropods,such as the first chevron bifurcated and sled-like,pubis with a prominent round ambiens process,lateral bulge developed on one third of the proximal lateral margin of femur,which indicaties a more complicated evolution of Mamenchisauridae than we thought.