The earliest fully aquatic cetaceans arose during the Middle Eocene; however, the earliest stage of their divergence is obscure. Here, we provide a detailed redescription of an unusual early cetacean, “Platyosphys einori”, from the Late Eocene of Ukraine (37.8–35.8 million years ago), with new data on its body size, skeletal microanatomy and suggestions on phylogenetic relationships. The procedure of regression analysis has been developed for the body size estimation from the size of the longest vertebra and the width of a cervical vertebra. “Platyosphys einori”, identified here as Basilosauridae incertae sedis, is a fully aquatic cetacean with a pronounced neck, poorly developed or low frequency underwater hearing, and it has a mix of plesiomorphic and derived traits. It is unusually large for a cetacean of that time, approximately 10 m long, and it is possibly very young and fast-growing for its body size. There is no pachyosteosclerotic cortex in vertebrae, and there is a large amount of spongy tissue in the rib. Therefore, “Platyosphys einori”, even if juvenile, is at the intermediate stage between early cetaceans with osteosclerotic bones and modern whales with osteoporotic skeleton. Thus, “Platyosphys einori” shows adaptations to life in water typical for modern whales but unique for the Eocene cetaceans.

最早的完全水生鲸类动物始于中始新世。但是，它们分歧的最早阶段是模糊的。在这里，我们详细描述了来自乌克兰始新世（37.8–3580万年前）的不寻常的早期鲸类“ Platyosphys einori ”，并提供了其体型，骨骼显微解剖学和系统发育关系建议的新数据。已经开发了用于从最长椎骨的大小和宫颈椎骨的宽度进行人体大小估计的回归分析程序。“ Platyosphys einori ”，在这里确定为龙王鲸科地位未定，是完全水生的鲸类动物，具有明显的脖子，发育不良或低频水下听觉，并且混合了类状和衍生特征。对于当时的鲸类而言，它异常大，大约10 m长，并且它可能非常年轻，并且由于其体型而迅速增长。椎骨中没有厚皮硬皮层皮质，肋骨中有大量海绵状组织。因此，即使幼年，“鸭嘴兽”仍处于具有骨硬化性骨骼的早期鲸类与具有骨质疏松性骨架的现代鲸鱼之间的中间阶段。因此，“ Platyosphys einori ”显示了对鲸鱼在水中的适应性，这是现代鲸鱼的典型特征，而始新世的鲸类独有。