Cetacean fossils have been recorded from middle and late Eocene deposits on Seymour Island since the beginning of the twentieth century and include fully aquatic Basilosauridae and stem Neoceti. Here, we report a small cetacean vertebra tentatively referred to as Neoceti from the late Eocene of Seymour Island. It shows a mosaic of traits, some of which are characteristic of early Neoceti (anteroposteriorly long transverse processes; a ventral keel on the ventral side of the centrum; thin pedicles of the neural arch), whereas others are shared with Basilosauridae (low-placed bases of the transverse processes). However, some traits are unique and may be autapomorphic: presence of separate prezygapophyses on the vertebra at the thoracic/lumbar boundary and a proportionally short centrum. Both traits imply a fast swimming style, which is characteristic of modern dolphins rather than Eocene cetaceans. Thus, this specimen can be identified as Neoceti indet., with some hypothetical odontocete affinities. Along with a few other Eocene whale taxa, it seems to be among the earliest known members of Neoceti on Earth. The finding of small and fast-swimming Neoceti in Antarctica also demonstrates early diversification of cetaceans and ecological niche partitioning by them dating back as early as the late Eocene.

中文翻译：

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一只小鲸鱼揭示了南极洲始新世鲸类动物群的多样性

自 20 世纪初以来，在西摩岛的始新世中晚期沉积物中记录了鲸类化石，包括完全水生的巴西龙科和新鲸类。在这里，我们报告了西摩岛始新世晚期的一种小型鲸类脊椎动物，暂时称为新鲸类。它显示出一系列特征，其中一些特征是早期新鲸的特征（前后长横突；中心腹侧的腹侧龙骨；神经弓的薄蒂），而其他特征与龙王科共有（低位横突的基部）。然而，一些特征是独特的并且可能是自生的：在胸椎/腰椎边界处的椎骨上存在单独的前关节突和一个成比例的短椎体。这两个特征都意味着快速的游泳风格，这是现代海豚而非始新世鲸类的特征。因此，这个标本可以被确定为新鲸目，具有一些假设的齿鲸亲缘关系。与其他一些始新世鲸类群一起，它似乎是地球上已知最早的新鲸类成员之一。在南极洲发现的小型和快速游泳的新鲸也表明了早在始新世晚期的鲸类动物的早期多样化和生态位划分。