中国科学院古脊椎动物与古人类研究所徐星研究员和沈阳师范大学辽宁古生物博物馆胡东宇教授领导的科研小组报道了具备今鸟类飞行形态特征的反鸟类一个新属种——神秘翔鸟(Xiangornis shenmi)。这一发现不仅进一步缩小了反鸟类与今鸟类间的形态差异,而且对反鸟类以及整个早期鸟类的飞行演化是一个全新的认识。2012年5月出版的美国《古脊椎动物杂志》Journal of Vertebrate Paleontology刊发了该项研究成果。
反鸟类和今鸟类是鸟类进化的两个主要的谱系。反鸟类繁盛于白垩纪,灭绝于白垩纪末期,被认为是鸟类起源之后第一支成功地实现全球性辐射的谱系。今鸟类是鸟类向现生鸟类演化的谱系,包括所有现生鸟类及其祖先类群。两者组成了中生代鸟类中进步的、有主动扇动翅膀飞行能力的两大姐妹类群。
鸟类飞行能力的提高是与其前肢、特别是手部的结构功能演化分不开的。在已发现的中生代鸟类中,反鸟类在种类和数量上均占优势,显现出形态和生态上的高度分化,但是与飞行有关的形态特征却表现出较今鸟类原始落后。
新发现的神秘翔鸟标本采自辽宁西部朝阳大平房早白垩世九佛堂组地层,距今约1.2亿年。系统发育分析表明,该化石鸟类属于热河生物群反鸟类的一个新属种,但由于化石仅保存了部分头后骨额,因此其整体形态仍是一个谜,同时由于它表现出了极其进步的飞行特征,所以取名为神秘翔鸟。
研究人员发现,翔鸟已发育有一个与现代飞行鸟类几乎完全相同的腕掌骨。其近端和远端完全愈合,小翼掌骨短,与大掌骨完全融合,掌骨间孔向近端未达到小翼掌骨远端,特别是用于附着飞行运动肌肉的小翼掌骨伸突较同时代的古鸟类都发育,意味“翔鸟”已具备更强、更复杂的拍打飞行能力。
翔鸟这一进步的腕掌骨此前仅出现在晚白垩世今鸟类和现生鸟类中,因此它的发现进一步缩小了反鸟类与今鸟类间的形态差异,表明腕掌骨的飞行形态特征在反鸟类和今鸟类中可能是独立演化的,并且在反鸟类中可能更早地出现。
该项研究获得了国家自然科学基金和辽宁省自然科学基金的资助。(生物谷Bioon.com)
doi:10.1080/02724634.2012.652321
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A new enantiornithine bird from the Lower Cretaceous of Western Liaoning, China, and its implications for early avian evolution
Dongyu Hua, Xing Xuab, Lianhai Houab & Corwin Sullivanb
Recent studies have blurred the distinctness of two major avian groups: the Enantiornithes, a major radiation of early birds in the Cretaceous, and the Ornithuromorpha, the clade including extant birds. Here we describe a new enantiornithine bird from the Lower Cretaceous Jiufotang Formation of western Liaoning, China, Xiangornis shenmi, gen. et sp. nov., which further reduces the morphological gap between the two groups. Xiangornis shenmi has several enantiornithine features, including a furcula with a significantly elongated hypocleidium, a coracoid with a convex lateral margin, and a minor metacarpal that extends further distally than the major metacarpal. However, it also possesses some derived ornithurine features, such as a short alular metacarpal (about one-sixth as long as the major metacarpal) that is completely fused to the major metacarpal, a large extensor process on the alular metacarpal, proximal and distal fusion between the minor and major metacarpals, and an intermetacarpal space positioned significantly distal to the alular metacarpal. This new find indicates that a carpometacarpal morphology similar to that seen in modern birds probably evolved independently in enantiornithines and appeared earlier than in Ornithuromorpha, and demonstrates that character evolution in early birds was more complex than previously believed.