Modern whales and dolphins are superbly adapted for marine life, with tail flukes being a key innovation shared by all extant species. Some dolphins can exceed speeds of 50 km/h, a feat accomplished by thrusting the flukes while adjusting attack angle with their flippers [1]. These movements are driven by robust axial musculature anchored to a relatively rigid torso consisting of numerous short vertebrae, and controlled by hydrofoil-like flippers [2, 3, 4, 5, 6, 7]. Eocene skeletons of whales illustrate the transition from semiaquatic to aquatic locomotion, including development of a fusiform body and reduction of hindlimbs [8, 9, 10, 11], but the rarity of Oligocene whale skeletons [12, 13] has hampered efforts to understand the evolution of fluke-powered, but forelimb-controlled, locomotion. We report a nearly complete skeleton of the extinct large dolphin Ankylorhiza tiedemani comb. n. from the Oligocene of South Carolina, previously known only from a partial rostrum. Its forelimb is intermediate in morphology between stem cetaceans and extant taxa, whereas its axial skeleton displays incipient rigidity at the base of the tail with a flexible lumbar region. The position of Ankylorhiza near the base of the odontocete radiation implies that several postcranial specializations of extant cetaceans, including a shortened humerus, narrow peduncle, and loss of radial tuberosity, evolved convergently in odontocetes and mysticetes. Craniodental morphology, tooth wear, torso vertebral morphology, and body size all suggest that Ankylorhiza was a macrophagous predator that could swim relatively fast, indicating that it was one of the few extinct cetaceans to occupy a niche similar to that of killer whales.

现代鲸鱼和海豚非常适合海洋生物，尾吸虫是所有现存物种共有的一项关键创新。一些海豚的速度可以超过 50 公里/小时，这一壮举是通过推动尾鳍，同时用鳍调整攻击角来实现的 [1]。这些运动由坚固的轴向肌肉组织驱动，该肌肉组织固定在由许多短椎骨组成的相对刚性躯干上，并由类似水翼的鳍状肢控制 [2, 3, 4, 5, 6, 7]。始新世鲸鱼骨骼说明了从半水生运动到水生运动的转变，包括梭形身体的发育和后肢的减少 [8, 9, 10, 11]，但渐新世鲸鱼骨骼的稀有性 [12, 13] 阻碍了了解的努力由吸虫驱动但由前肢控制的运动的进化。Ankylorhiza Tiedemani梳子。n. 来自南卡罗来纳渐新世，以前只知道来自部分讲台。它的前肢在形态上介于干鲸类和现存分类群之间，而其轴向骨骼在尾部基部显示出刚开始的刚性，并具有灵活的腰部区域。Ankylorhiza位于齿鲸辐射基部附近的位置意味着现存鲸类的几种颅后特化，包括缩短的肱骨、狭窄的花梗和桡骨结节的丧失，在齿鲸和神秘鲸中会聚进化。颅牙形态、牙齿磨损、躯干椎体形态和体型都表明Ankylorhiza 是一种游泳速度相对较快的巨食性捕食者，这表明它是少数已灭绝的鲸类动物之一，占据了与虎鲸相似的生态位。