摘要：根据发现于辽宁西部建昌早白垩世九佛堂组一件保存较为完好的标本，描述了原始今鸟类一新属种，李氏叉尾鸟(Schizooura lii gen. et sp. nov.)。系统发育分析表明，它较建昌鸟和古喙鸟进步，但又较热河生物群中其他已知的今鸟类原始。这一新种叉骨呈V型，且具有一短的叉骨突，这是这一特征首次在早白垩世今鸟中发现。其胸骨加长，后缘无窗孔，且无很深的凹口。肱骨三角肌脊膨大，近乎达到肱骨长的一半。新鸟的头骨保存较好，显示其具喙的特征，前颌骨直接与额骨相关节。值得关注的是，这件标本首次展示了在中生代鸟类中，除扇形尾羽之外的第二种尾羽类型。新标本具叉形尾羽，中间分隔明显。对现生鸟类的研究表明，叉尾型的空气动力学效率比扇尾型要低，但在性选择方面具有优势。这一新的发现说明，在热河生物群生活的森林环境中，今鸟类的基干种类在尾羽性状上或许已经存在着不同的选择策略。
A new basal beaked ornithurine bird from the Lower Cretaceous of western Liaoning, China
ZHOU Shuang, ZHOU Zhong-He, Jingmai O’CONNOR
Abstract We report a new species of basal ornithurine bird, Schizooura lii gen. et sp. nov., based on a well-preserved specimen from the Lower Cretaceous Jiufotang Formation in Jianchang, western Liaoning, China. Phylogenetic analysis indicates that it is more derived than Jianchangornis and Archaeorhynchus, but more basal than all other known Jehol ornithurines. The new specimen preserves a unique suite of characters that differentiate it from other known taxa, including several features previously unreported in Early Cretaceous ornithurines, including a V-shaped furcula with a short hypocleidium, a rostrocaudally elongate unperforated sternum with deep caudal notches absent, and a deltopectoral crest that extends for half the length of the humerus. The specimen preserves a nearly perfectly articulated skull that indicates the species was beaked and preserves details of the skull anatomy such as the premaxilla-frontal articulation. The most notable feature of this new specimen is the preservation of a feathered tail morphology previously unknown among Mesozoic birds. It is the second tail morphology known among early ornithurines, which have until now only preserved fan-shaped tails. The new specimen preserves a forked tail composed of elongate rectrices medially separated by a deep notch. This tail feather morphology in modern birds decreases aerodynamic efficiency relative to the fan-shaped tail, but increases chance of sexual reproduction. This discovery suggests that this tradeoff convergently evolved in basal members of Ornithurae, and is consistent with the wooded environment inferred for the Jehol.
Key words Jianchang, Liaoning, China; Early Cretaceous; Jiufotang Formation; ornithurine; beak; rectrices; forked tail