Abstract:  The transition in the evolution of cetaceans from terrestrial life to a fully aquatic existence is one of the most enduring evolutionary mysteries. Stem cetaceans are quite diverse and well documented in the fossil record. Five family level clades form the paraphyletic stem cetaceans (‘archaeocetes’): Pakicetidae, Ambulocetidae, Remingtonocetidae, Protocetidae, and Basilosauridae. The most basal group, the pakicetids probably had a semi-aquatic life, living near the freshwater environment. They likely represent the initial step in the transformation of a terrestrial artiodactyl to an aquatic cetacean. The more derived ambulocetids exhibit more characters likely linked to increasing aquatic adaptation, and they probably were more adapted to the marine realm than to a freshwater environment. Remingtonocetids show evolution of balance organs and the sound transmission mechanism in the direction of modern cetaceans. Stable oxygen isotope analyses suggest that remingtonocetids were probably exclusively marine. Protocetids are very diverse, and they are the first cetacean group that acquired a global distribution. Protocetids retain well-developed hind limbs, but their pelvic-vertebra articulation became loose or completely lost. Basilosaurids are the extinct sister group to the crown cetaceans. The phylogenetic relationships between cetaceans and other mammals have long been debated. Molecular, paleontological, and morphological data and analyses all support a close evolutionary link between cetaceans and artiodactyls. However, the prevalent hippopotamus-cetacean hypothesis does not receive solid support from paleontological and morphological data. Detailed review and more extensive phylogenetic analyses on anthracotheriids and entelodontids will aid the clarification of uncertainties related to the hippopotamus-cetacean phylogenetic hypothesis. If the phylogenetic positions of extant cetaceans and artiodactyls were not constrained by molecular data, the traditional mesonychid-cetacean relationship would still have support from the analyses based solely on paleontological and morphological data.

摘要： 在鲸类的演化历史中，由陆生动物转化成完全的水生动物的过程是一个由来已久的演化谜题。基干鲸类的多样性很高，化石记录也很完整。5个科一级的基干鲸类演化支系组成一个并系类群，包括：巴基鲸科(Pakicetidae)、游走鲸科(Ambulocetidae)、雷明顿鲸科(Remingtonocetidae)、原鲸科(Protocetidae)和龙王鲸科(Basilosauridae)。最基干的鲸类巴基鲸科动物可能是一种半水生动物，生活在接近淡水的环境中，代表了陆生偶蹄类向水生鲸类演化的初始一步。更为进步的游走鲸类具有更多适应于水生生活的特征，而且可能更加适应于海水环境。雷明顿鲸类的平衡觉器官和声音传导机制已经表现出向现代鲸类方向演化的趋势。基于稳定氧同位素分析的研究表明，雷明顿鲸类可能完全是海生的。原鲸类的多样性非常高，是鲸类中最先实现全球分布的类群。原鲸保留有发育良好的后肢，但是它们的髂骶关节很松甚至消失。龙王鲸类是鲸类冠类群的绝灭姊妹群。鲸类与其他哺乳动物的系统关系一直存在争议，分子生物学、古生物学和形态学证据都支持鲸类与偶蹄类的亲缘关系较近，但是流行的河马－鲸类亲缘假说尚缺乏坚实的古生物学和形态学数据支持。对石炭兽类和狶类开展详细的系统分析和研究，将有助于厘清河马－鲸类亲缘假说中的不确定关系。如果不使用分子数据来限定现代鲸类和偶蹄类的系统位置，仅使用古生物学和形态学数据的分析仍然支持传统的中兽－鲸类亲缘假说。