A second hadrosauroid dinosaur from the early Late Cretaceous of Zuoyun, Shanxi Province, China

A second basal hadrosauroid dinosaur, <i>Zuoyunlong huangi</i> gen. et sp. nov., is reported from the early Late Cretaceous Zhumapu Formation in Zuoyun County, Shanxi Province, northern China. <i>Zuoyunlong</i> preserves a partial right ilium and ischium and is unique in having a very short postacetabular process 50% as long as the iliac central plate. Our cladistic analysis recovers <i>Zuoyunlong</i> as the most basal Late Cretaceous hadrosauroid, with a sister taxon relationship with <i>Probactrosaurus</i> from the late Early Cretaceous of Inner Mongolia. Including <i>Zuoyunlong</i>, four Cenomanian basal hadrosauroids have been recorded, and the two taxa in North America (<i>Eolambia</i> and <i>Protohadros</i>) represent the earliest known hadrosauroids outside of Asia. In the light of the proposed phylogenetic topology and biogeographic data, the discovery of <i>Zuoyunlong</i> indicates that the first dispersal of hadrosauroids from Asia to North America probably happened around the boundary between the Early and Late Cretaceous.

本研究报道了产自中国华北地区山西省左云县晚白垩世早期助马铺组（Zhumapu Formation）的第二件基干鸭嘴龙形类（basal hadrosauroid）恐龙新属新种——左云龙（*Zuoyunlong huangi*，gen. et sp. nov.）。该属种保存有部分右侧髂骨与坐骨，其独有的形态特征为极短的髋臼后突（postacetabular process），该突起长度仅为髂骨中板（iliac central plate）的50%。本次支序分类分析结果显示，左云龙是目前已知最基干的晚白垩世鸭嘴龙形类，与产自内蒙古早白垩世晚期的原巴克龙（Probactrosaurus）构成姊妹群关系。连同左云龙在内，目前已记录有4种森诺曼阶（Cenomanian）基干鸭嘴龙形类；其中分布于北美的2个分类群——始鸭嘴龙（Eolambia）与原鸭嘴龙（Protohadros）——代表了亚洲之外已知最古老的鸭嘴龙形类。结合当前提出的系统发育拓扑结构与生物地理数据，左云龙的发现表明，鸭嘴龙形类从亚洲向北美首次扩散事件大概率发生在早白垩世与晚白垩世的交界时期。