First species of Enantiornithes from Sihedang elucidates skeletal development in Early Cretaceous enantiornithines

The Sihedang locality of the Lower Cretaceous Yixian Formation is the only recognized ornithuromorph-dominated locality in the Jehol Group of north-eastern China. Here we report on the first enantiornithine from this locality and erect a new taxon <i>Monoenantiornis sihedangia</i> gen. et sp. nov. The holotype and only specimen preserves a rare ontogenetic stage in which the intermedium is ossified but free from the other proximal tarsals and the tibia, consistent with the pattern of ossification that occurs in neornithines. In order to explore the pattern of skeletal development in the enantiornithines, we select several published specimens inferred to represent different ontogenetic stages and document their degree of fusion in five key compound bones and the sterna. This provides a preliminary hypothesis for the sequence in which compound bones form in Early Cretaceous enantiornithines. From this hypothesis, most known specimens can be categorized into four major ontogenetic stages: all the compound bones unfused; synsacrum and pygostyle fused; tarsals fused; tibiotarsus and carpometacarpus formed. http://zoobank.org/urn:lsid:zoobank.org:pub:347485CA-87E6-4834-BEB5-687B6798644E

中国东北热河群下白垩统义县组的四合当化石点，是目前热河群中唯一被确认的以今鸟型类（ornithuromorph）为主的化石产出点。本研究报道该化石点的首个反鸟类（enantiornithine）化石，并建立新属新种*单形反鸟（Monoenantiornis sihedangia）* gen. et sp. nov.。正模标本（亦是该物种唯一已知标本）保存了一种罕见的个体发育阶段：其中间骨（intermedium）已骨化，但仍与其余近端跗骨（proximal tarsals）及胫骨（tibia）未发生愈合，该特征与今鸟类（neornithines）的骨化模式一致。为探究反鸟类的骨骼发育模式，我们选取若干已发表的、被推断代表不同个体发育阶段的标本，记录其5处关键复合骨与胸骨（sterna）的愈合程度。据此，我们初步提出早白垩世反鸟类复合骨的形成时序假说。基于该假说，绝大多数已知标本可划分为4个主要个体发育阶段：所有复合骨均未愈合；综荐骨（synsacrum）与尾综骨（pygostyle）愈合；跗骨完成愈合；形成胫跗骨（tibiotarsus）与腕掌骨（carpometacarpus）。http://zoobank.org/urn:lsid:zoobank.org:pub:347485CA-87E6-4834-BEB5-687B6798644E