Anatomy And Phylogeny Of Early Cretaceous Ornithuromorpha From Changma, Gansu Province, China

Gansus yumenensis was reported in1984based on isolated partial left pelvic limb, and represents the first Mesozoic bird known in China. It was discovered from the Lower Cretaceous Xiagou Formation in Changma of Yumen City, Gansus Province, northwestern China. Numerous ornithuromorph and enantiornithine specimens have been recovered from Changma since then, making it an important site to study the evolution of birds, especially the oringin and early evolution of ornithuromorphs.Here detailed descriptions and comparative studies of13ornithuromorph specimens from Changma are given for the first time. Among them,10(five previously reported and five new specimens) belong to Gansus yumenensis, and the other three represent three new basal ornithuromorph taxa respectively: Yumenornis huangi, Changmaornis houi, and Jiuquanornis niui, which were reported in2013.Revised diagnosis of Gansus yumenensis includes: a U-shaped furcula without a hypocleidium; a sternum with a tall keel, two curved lateral trabeculae and two caudal fenestrae; lateral process of the coracoid hooked; a tibiotarsus with two fused projected cnemial crests, and the length ratio of tibiotarsus to femur is2.1; the length ratio of wing to hindlimb is0.9; trochlea of metatarsal Ⅱ highest; pedal digit IV longest; pedal unguals with remarkably pointed flexor tubercles. Remarkably, three new specimens preserve gastroliths.Yumenornis huangi has sternum with angular rostral margin, lateral processes, and robust, distally expanded lateral trabeculae. A deep distal fossa is present on the radius. The ratio of length of manus relative to humerus is approximately1.1. The morphology of sternum and forelimb of Yumenornis huangi is similar to those of Yixianornis grabaui.Changmaornis houi possesses a synsacrum composed of at least11sacral vertebrae with elongate distal transverse processes. Its metatarsal Ⅰ is J-shaped. Pedal digit Ⅲ is the longest in foot, and the ratio of pedal digit Ⅲ to tibiotarsus is about0.82. The large number of fused vertebrae in synsacrum of Changmaornis houi suggests that it is a derived basal ornithuromorph taxon.Jiuquanornis niui has a U-shaped furcula without a hypocleidium and a sternum with small lateral processes, distally expanded lateral trabeculae, and elongate intermediate trabeculae. The sternal morphology of Jiuquanornis niui strongly resembles that of Archaeorhynchus spathula, but can be clearly differentiated from that of other Early Cretaceous basal ornithuromorphs.A matrix of245characters and61taxa (including four ornithuromorphs from Changma) was analysed in TNT1.1. The strict consensus tree shows that all four Changma ornithuromorphs are more derived than ornithuromorphs from Jehol Group in western Liaoning, although the interrelationships of these four taxa remain unresolved. The result of cladistic analysis further supports that Changma avian fauna is different from Jehol avian fauna. Chemostratigraphy study in Changma constrains the age of the Changma avian fauna within early Aptian Stage. In light of chronostratigraphic studies on Jehol Fauna in western Liaoning, the age of Changma avian fauna could be slightly younger than120Ma, while the age of Jehol avian fauna was older than120Ma. Therefore the age of the base of the Aptian stage is more reasonable to be121Ma than125Ma.