New fossil penguins (Aves, Sphenisciformes) from the Oligocene of New Zealand reveal the skeletal plan of stem penguins

Three skeletons collected from the late Oligocene Kokoamu Greensand of New Zealand are among the most complete Paleogene penguins known. These specimens, described here as Kairuku waitaki gen. et sp. nov. and Kairuku grebneffi sp. nov., reveal for the first time the unique proportions of a giant fossil penguin and the morphology of many key elements of the stem penguin skeleton associated with underwater flight, including the first reasonably complete sternum, one of only two complete forelimbs and the first described pygostyle. Relative proportions of the trunk, flippers and hindlimbs can now be determined from a single individual, offering insight into the body plan of stem penguins and improved constraints on size estimates for 'giant' taxa. Kairuku is characterized by an elongate, narrow sternum, a short and flared coracoid, an elongate narrow flipper and a robust hindlimb. The pygostyle of Kairuku lacks the derived triangular cross-section seen in extant Spheniscidae, suggesting the rectrices attached in a more typical avian pattern and the tail may have lacked the propping function utilized by living penguins. New materials described here, along with restudy of previously described specimens, resolves several long-standing phylogenetic, biogeographic and taxonomic issues stemming from the inadequate comparative material of several of the first-named fossil penguin species. An array of partial associated skeletons from the Eocene-Oligocene of New Zealand historically referred to Palaeeudyptes antarcticus or Palaeeudyptes sp. are recognized as at least five distinct species: Palaeeudyptes antarcticus, Palaeeudyptes marplesi, Kairuku waitaki, Kairuku grebneffi and an unnamed Burnside Formation species

三件采自新西兰渐新世晚期科科阿穆绿砂岩组（Kokoamu Greensand）的骨骼标本，是目前已知保存最为完整的古近纪（Paleogene）企鹅化石类群之一。本次研究将这批标本命名为新属新种怀塔基凯鲁库企鹅（Kairuku waitaki gen. et sp. nov.）与新种格雷布尼夫凯鲁库企鹅（Kairuku grebneffi sp. nov.），首次揭示了巨型化石企鹅独特的躯体比例，以及与水下飞行相关的基干企鹅（stem penguin）骨骼诸多关键部位的形态特征，包括首个较为完整的胸骨、仅存的两具完整前肢之一，以及首次被正式描述的尾综骨（pygostyle）。如今可通过单个个体确定躯干、鳍肢与后肢的相对比例，这为解析基干企鹅的躯体构型提供了关键依据，也为“巨型”企鹅类群的体型估算提供了更严格的约束条件。凯鲁库企鹅的鉴别特征包括修长狭窄的胸骨、短缩且外展的喙骨（coracoid）、修长窄长的鳍肢以及强健粗壮的后肢。其尾综骨并不具备现生企鹅科（Spheniscidae）所特有的三角形横截面，这表明尾羽（rectrices）的附着模式更贴近典型鸟类，且尾部可能不具备现生企鹅用以支撑身体的功能。本次研究描述的新材料，结合对此前已发表标本的重新研究，解决了数个长期悬而未决的系统发育、生物地理及分类学难题——这些难题的根源在于早期命名的数种企鹅化石类群缺乏足够的对比标本。此前被归入南极古冠企鹅（Palaeeudyptes antarcticus）或古冠企鹅未定种（Palaeeudyptes sp.）的、产自新西兰始新世-渐新世的多件部分关联骨骼标本，现被确认至少包含5个独立物种：南极古冠企鹅、马普尔斯古冠企鹅（Palaeeudyptes marplesi）、怀塔基凯鲁库企鹅、格雷布尼夫凯鲁库企鹅，以及1个未命名的伯恩赛德组（Burnside Formation）物种。