Bolong Yixianesis Wu Et Al. 2010 From The Yixian Formation In Western Liaoning, China And The Phylogeny Of Basal Iguanodontia

Bolong yixianensis Wu, Godefroit and Hu, 2010 is a basal iguanodontian dinosaur, which was recently discovered from the Yixian Formation of western Liaoning, China. The holotype of Bolong yixianensis is virtually complete. It is the most complete known basal iguanodontian specimen found in China.The synthetic biological age of the Yixianian Formation is determined as the Late Jurassic to the Early Cretaceous. And the Jurassic-Cretaceous boundary should be between the Jianshangou Bed and the Dakangpu Bed. Most of the isotopic ages of the volcanic rocks which have been obtained in recent years are different from the synthetic biological age of the Yixian Formation. However this problem could not be resolved in a short time.The first detailed description of the holotype specimen of Bolong yixianensis is presented. And four autapomorphies of Bolong yixianensis are newly recognized from the postcranial skeleton. The revised diagnosis of Bolong yixianensis includes the following characters: depressed area at the junction between maxilla and lachrymal, corresponding to the emplacement of the antorbital fenestra in Iguanodon, Mantellisaurus and Ouranosaurus; caudal ramus of prefrontal forming a rostrocaudally depressed area above the orbital margin; rostrodorsal articular surface for the predentary occupying less than two-thirds of the height of the dentary and rostral tip of dentary therefore situated above the ventral third of the bone; primary ridge deflected distally on maxillary crowns; ulna and radius proportionally short and robust: ratio'craniocaudal height of proximal part/length'= 0.28 for ulna and 0.33 for radius; proximal and distal ends of radius nearly symmetrically enlarged craniocaudally and triangular in medial and lateral views; postacetabular process of ilium dorsoventrally narrow (ratio'length/maximal height'= 2); metatarsals are very short, ratio'metatarsal III length/tibia length'= 0.18.A phylogenetic analysis was carried out. One hundred and twenty six characters of 26 taxa of Iguanodontia are employed in the phylogenetic analysis. This analysis shows that Bolong is the most basal Hadrosauroidea; Two synapomorphies supporting the Hadrosauroidea: basipterygoid process extending nearly horizontally; ischium shaft nearly straight. Jinzhousaurus, also from the Yixian Formation of western Liaoning Province, is more advanced than Bolong. Contrary to other recent phylogenetic analyses here retain a monophyletic Iguanodontidae family, including Iguanodon, Mantellisaurus and Ouranosaurus. This clade is characterized by the following autapomorphies: the ossified epaxial tendons extend cranially from cervical 10; and the pollex is spike-like and conical. Within Iguanodontidae, Mantellisaurus and Ouranosaurus are united by the presence of elongated neural spines on the posterior dorsals and sacrals, more than 2.5 times centrum height. Iguanodontidae is the sister group of Hadrosauroidea.The complete closure of the sutures between the neural arches and centra throughout the vertebral column, the body length, the ratio between the forelimb length and hindlimb length, the ratio between the tibia length and femur length, the carpals were not coossified, the unguals on manus digits II and III are claw-shaped, and the other characters of Bolong suggest that the holotype specimen of Bolong is an adult individual when it died, its body mass is about 0.2 tonne, and Bolong spent considerable periods of time walking or running bipedally. However, the metatarsals and the distal portion of the forelimbs are particularly short in Bolong, like other vertebrates using their forelimbs for weight supporting. So, Bolong might have been a facultive quadruped, as in many other iguanodontians, and it probably has an inferior running ability compared to other iguanodonitans. Additionally, the forelimbs of Bolong are probably more agile.The earliest basal Iguanodontia is known from the Middle Jurassic in England. Iguanodontia are absent from Late Jurassic localities in Asia. Some vertebrates which were discovered from Late Jurassic Solnhofen Bed in Germany also have their phylogenetic affinites in Yixian Formation. This indecates that Asian and European continents were connected in the Late Juarassic, and the primitive iguanodontians (Rhabdodon and Zalmoxes) lived in Europe during the Late Cretaceous, perhaps just caused by the vicariant event which separated Asian and European continents in the Early Cretaceous. The Hadrosauroidea was origined from Asia, and they immigrated to North America during the Late Early Cretaceous.There's a lot of debate on the age of Yixian Formation. The reason is different taxa of Yixian Biota displays different ages. Although based on the current phylogenetic analysis, the geological age of Bolong is presumbly between Valanginian and Aptian. Here the geological age of Bolong is determined as the middle Early Cretaceous or later, based on the paleobiogeographical distribution of basal Iguanodontia and the synthetic biological age of the Dakangup Bed of Yixian Formation.In sum, the discovery and study of Bolong have great value on the study of the origin, evolution, phylogeny and paleobiogeography of basal Iguanodontia and basal Hadrosauroidea.