A New Species of Pengornithidae (Aves: Enantiornithes) from the Lower Cretaceous of China Suggests a Specialized Scansorial Habitat Previously Unknown in Early Birds

We describe a new enantiornithine bird, Parapengornis eurycaudatus gen. et sp. nov. from the Lower Cretaceous Jiufotang Formation of Liaoning, China. Although morphologically similar to previously described pengornithids Pengornis houi, Pengornis IVPP V18632, and Eopengornis martini, morphological differences indicate it represents a new taxon of the Pengornithidae. Based on new information from this specimen we reassign IVPP V18632 to Parapengornis sp. The well preserved pygostyle of the new specimen elucidates the morphology of this element for the clade, which is unique in pengornithids among Mesozoic birds. Similarities with modern scansores such as woodpeckers may indicate a specialized vertical climbing and clinging behavior that has not previously been inferred for early birds. The new specimen preserves a pair of fully pennaceous rachis-dominated feathers like those in the holotype of Eopengornis martini; together with the unique morphology of the pygostyle, this discovery lends evidence to early hypotheses that rachis-dominated feathers may have had a functional significance. This discovery adds to the diversity of ecological niches occupied by enantiornithines and if correct reveals are remarkable amount of locomotive differentiation among Enantiornithes.我们描述了一件发现于中国辽宁下白垩统九佛堂组的反鸟类（Enantiornithes）新属新种——长尾拟鹏鸟（Parapengornis eurycaudatus gen. et sp. nov.）。尽管其形态与此前已记述的鹏鸟科（Pengornithidae）物种侯氏鹏鸟（Pengornis houi）、IVPP V18632号鹏鸟标本以及始鹏鸟（Eopengornis martini）相似，但形态学差异表明它代表了鹏鸟科的一个新类群。基于该标本的新研究信息，我们将IVPP V18632标本归入拟鹏鸟属（Parapengornis）未定种。该新标本保存完好的尾综骨（pygostyle）阐明了该演化支（clade）的该骨骼形态，这一特征在中生代鸟类（Mesozoic birds）的鹏鸟科中独一无二。其与现代攀禽（scansores，如啄木鸟）的相似特征，可能表明它具备特化的垂直攀爬与抓附行为，这一行为此前未在早期鸟类中被推断存在。该新标本保存有一对完全发育的以羽轴为主的正羽（rachis-dominated pennaceous feathers），与始鹏鸟正模标本（holotype）中的羽毛一致；结合其独特的尾综骨形态，该发现为早期假说提供了实证支持：以羽轴为主的羽毛可能具备功能意义。这一发现丰富了反鸟类所占据的生态位多样性；若该结论成立，则揭示了反鸟亚纲中存在极为显著的运动机能分化。