Department of Geology and Environmental Geosciences, College of Charleston, Charleston, SC 29424, USA; University of California Museum of Paleontology, University of California, Berkeley, CA 94720, USA.
Department of Biology, University of Wisconsin-Oshkosh, Oshkosh, WI 54901, USA.
Curr Biol. 2020 Aug 17;30(16):3267-3273.e2. doi: 10.1016/j.cub.2020.06.012. Epub 2020 Jul 9.
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 [2-7]. Eocene skeletons of whales illustrate the transition from semiaquatic to aquatic locomotion, including development of a fusiform body and reduction of hindlimbs [8-11], 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应海洋生活，所有现存物种都共享尾部尾叶这一关键创新。一些海豚的速度可以超过 50 公里/小时，这一壮举是通过在调整鳍状肢的攻击角度的同时推动尾叶来完成的[1]。这些运动是由强壮的轴肌驱动的，轴肌固定在相对刚性的躯干上，躯干由许多短的脊椎骨组成，并由类似水翼的鳍状肢控制[2-7]。始新世鲸鱼的骨骼化石说明了从半水生到水生运动的转变，包括梭形身体的发育和后肢的减少[8-11]，但始新世鲸鱼骨骼化石的稀有性[12,13]阻碍了对尾叶驱动但前肢控制的运动进化的理解。我们报告了一种已灭绝的大型海豚 Ankylorhiza tiedemani comb. n. 的近乎完整骨架，来自南卡罗来纳州的渐新世，以前仅从部分口鼻部了解到。它的前肢在形态上介于原始鲸类和现存分类群之间，而其轴骨骼在尾部基部显示出初步的刚性，腰部具有柔韧性。Ankylorhiza 位于齿鲸辐射的基部附近，这意味着现存鲸类的几个后躯特化，包括缩短的肱骨、狭窄的柄部和桡骨结节的丧失，在齿鲸类和须鲸类中都经历了趋同进化。颅齿形态、牙齿磨损、躯干脊椎形态和体型都表明 Ankylorhiza 是一种以大型猎物为食的掠食者，能够相对快速地游泳，这表明它是少数几种灭绝的鲸类之一，占据了类似于逆戟鲸的生态位。
