The oldest Asian hesperornithiform from the Upper Cretaceous of Japan, and the phylogenetic reassessment of Hesperornithiformes

Asian hesperornithiforms are extremely rare in contrast to the much more abundant record from North America. In Asia, these fossil birds are only known from fragmentary materials from Mongolia. Here we describe the skeletal remains of a new hesperornithiform Chupkaornis keraorum gen. et sp. nov. from the Upper Cretaceous Kashima Formation (Coniacian to Santonian) of the Yezo Group in Mikasa City, Hokkaido, Japan. This is the best-preserved hesperornithiform material from Asia and it is the first report of hesperornithiforms from the eastern margin of the Eurasian continent. Chupkaornis has a unique combination of characters: finger-like projected tibiofibular crest of femur, deep, emarginated lateral excavation with a sharply defined edge of the ventral margin of the thoracic vertebrae, and the heterocoelous articular surface of the thoracic vertebrae. Our new phylogenetic analysis revises the phylogenetic relationships of Hesperornithiformes. In contrast to previous studies, Enaliornis is assigned as the most basal taxon and Baptornis is positioned as more derived than Brodavis. Chupkaornis is a sister taxon to the clade of Brodavis and higher taxa. Parahesperornis and Hesperornis are positioned within Hesperornithidae, the derived Hesperornithiformes. Many of the skeletal character changes are concentrated at the base of Hesperornithidae (Parahesperornis and more derived taxa), and involve the modification of the pelvic girdle and hind limb morphology (e.g. dorsal directed antitrochanter of pelvis, short and sprawled femur, including probable lobe-toed feet suggested by the specialized distal articular surface of first digit of fourth toe, and predominantly robust digit IV phalanges). These skeletal modifications are likely adaptations for foot-propelled diving behaviour. http://zoobank.org/urn:lsid:zoobank.org:pub:FB783237-E565-4B74-9386-EADF8E12DFD4

亚洲的燕鹅类化石相对极为罕见，与北美地区丰富的记录形成鲜明对比。在亚洲，此类化石鸟类仅限于蒙古的零散材料。本研究描述了一种新的燕鹅类化石——Chupkaornis keraorum gen. et sp. nov.，该化石出土于日本北海道函馆市耶佐群上白垩统的加须玛组（康尼亚阶至桑托尼亚阶）。这是亚洲保存最完整的燕鹅类化石，也是首次报道出自欧亚大陆东部边缘的燕鹅类。Chupkaornis 具有独特的特征组合：股骨上的指状髁突、胸椎腹侧边缘深凹且边缘锐利、胸椎关节面异凹型。我们的新系统发育分析修订了燕鹅类的系统发育关系。与以往研究不同，Enaliornis 被归为最基的类群，而 Baptornis 被定位为比 Brodavis 更为进化的物种。Chupkaornis 是 Brodavis 类群及其更高类群的姐妹类群。Parahesperornis 和 Hesperornis 被置于燕鹅科，即衍生出的燕鹅类。许多骨骼特征的改变集中于燕鹅科的基础部分（Parahesperornis 及其更进化的物种），涉及骨盆带和后肢形态的改良（例如，骨盆背侧指向的副坐骨、短而伸展的股骨，以及由第四趾第一指节特殊远端关节面所暗示的可能蹠趾足，以及以坚实为主的第四指骨）。这些骨骼的改良可能是为了适应脚部推进的潜水行为。http://zoobank.org/urn:lsid:zoobank.org:pub:FB783237-E565-4B74-9386-EADF8E12DFD4