Unravelling the identity of the platanistoid Notocetus vanbenedeni Moreno, 1892 (Cetacea, Odontoceti) from the early Miocene of Patagonia (Argentina)

Platanistoidea remains one of the most evolutionarily intriguing lineages of toothed whales (Odontoceti). The clade comprises mostly extinct species from the late Oligocene–early Miocene onward and a single extant riverine genus (Platanista). There is an ongoing debate as to the membership of Platanistoidea and the causes of their near extinction. In Patagonia (Argentina), the most abundant platanistoid recorded in the lower Miocene Gaiman Formation is Notocetus vanbenedeni, first described by Moreno in 1892 based on two individuals. The goal of the present contribution is to conduct an updated anatomical, palaeobiological and phylogenetic analyses of Notocetus vanbenedeni and hence contribute to an understanding of the evolutionary history of the Platanistoidea. Our analyses, including at least 26 individuals (12 undescribed), show that Notocetus vanbenedeni is a valid platanistoid taxon, recovered as part of a new clade. Among its most outstanding features, this taxon has an elevated dorsal tubercular supraorbital crest formed mainly by the frontal, the precursor of the pneumatized crest of the extant Platanista. Notocetus vanbenedeni also shows initial stages of the plesiomorphic bony connection between the earbones and skull as in Platanista, although the functional implications for hearing remain elusive. The nasal sac system, pterygoid sinus system and morphology of the earbones suggest that this species was able to hear high-frequency sounds and echolocate underwater, similar to extant odontocetes. Thus, Notocetus vanbenedeni presents a mosaic of features that suggest an intermediate platanistoid morphotype. Anatomical differences and phylogenetic analyses suggest that Peruvian specimens could not be referred to this species. The feeding apparatus of Notocetus vanbenedeni makes it the only combination suction-feeder recorded in the early Miocene of Patagonia and among the smallest odontocetes. Finally, the abundant records of Notocetus vanbenedeni in an inner shelf environment with freshwater influence suggest a possible early preference for such protected habitats.恒河豚总科（Platanistoidea）仍是齿鲸（Odontoceti）中演化上最引人关注的支系之一。该演化支主要包含渐新世晚期至中新世早期以来的绝大多数灭绝物种，仅存1个现生淡水生属——恒河豚属（Platanista）。目前学界围绕恒河豚总科的类群组成及其近乎灭绝的成因仍存在持续争论。在阿根廷巴塔哥尼亚地区，下中新统盖曼组（Gaiman Formation）中产出的丰度最高的恒河豚总科物种为诺氏突吻河豚（Notocetus vanbenedeni），该物种最早由Moreno于1892年基于2件标本正式命名描述。本研究旨在对诺氏突吻河豚开展最新的解剖学、古生物学与系统发育分析，以期为阐明恒河豚总科的演化历史提供新的科学认知。本次分析共纳入至少26件标本（其中12件为尚未正式描述的标本），结果显示诺氏突吻河豚是一个有效的恒河豚总科分类单元，并被归入一个全新的演化支。该类群最显著的特征包括：由额骨主导形成的隆起背侧结节状眶上嵴，这一结构是现生恒河豚属气生化嵴的前体形态；同时其也展现出与现生恒河豚类似的、耳骨与颅骨之间近祖状骨性连接的初始阶段，尽管该结构对应的听觉功能意义仍有待明确。其鼻囊系统、翼状窦系统以及耳骨形态均表明，该物种具备与现生齿鲸相似的高频听觉能力与水下回声定位能力。综上，诺氏突吻河豚呈现出镶嵌演化的特征组合，代表了一种过渡型的恒河豚总科形态型。解剖学差异与系统发育分析结果显示，此前发现的秘鲁标本无法归入该物种。诺氏突吻河豚的取食器官使其成为巴塔哥尼亚中新世早期唯一被记录到的吮吸摄食者，同时也是体型最小的齿鲸类群之一。最后，该物种在受淡水影响的内陆陆架环境中拥有大量化石记录，这暗示其可能在演化早期就偏好这类受保护的栖息生境。