Data from: A new specimen of Agorophius pygmaeus (Agorophiidae, Odontoceti, Cetacea) from the Early Oligocene Ashley Formation of South Carolina, USA

The holotype partial skull of Agorophius pygmaeus (the monotypic form for both the genus Agorophius and the Family Agorophiidae) has been missing for approximately 140 years. Since the discovery of Agorophius pygmaeus, many additional taxa and specimens have been placed in the Family Agorophiidae, only to be reclassified and removed later. This has created confusion as to what is and what is not an agorophiid and a lack of clarity as to what characteristics delimit the Agorophiidae. A newly discovered skull of an agorophiid recently collected from an underwater cliff face of the Ashley River, South Carolina, USA, is assigned to Agorophius pygmaeus. It derives from the base of the Ashley Formation (early Oligocene). The new specimen consists of most of the skull and periotics, which are well preserved and described for the first time in an agorophiid. The new specimen provides an opportunity to diagnose the Agorophiidae and place the genus and species within the phylogenetic context of the early odontocete radiation in the Oligocene, along with other taxa such as the Ashleycetidae, Mirocetidae, Patriocetidae, Simocetidae, Waipatiidae, and Xenorophidae. Based on this new understanding, Agorophiidae are known with certainty only from the early Oligocene of South Carolina, with other undescribed, potential agorophiid specimens from the Oligocene of the North Pacific region (Japan, Mexico, and Washington State).

侏儒始喙鲸（Agorophius pygmaeus）是始喙鲸属（Agorophius）与始喙鲸科（Agorophiidae）的单型类群，其正模部分头骨标本已失踪约140年。自侏儒始喙鲸被发现以来，先后有诸多类群与标本被归入始喙鲸科，但后续均被重新分类并移出该科，这不仅造成了学界对始喙鲸类界定标准的混淆，也使得始喙鲸科的鉴别特征始终缺乏明确定义。近日，一件从美国南卡罗来纳州阿什利河的水下崖壁中采集到的始喙鲸科新头骨标本被归为侏儒始喙鲸，该标本产自阿什利组（Ashley Formation）底部，地质时代为渐新世早期。新标本保存了大部分头骨与完好的听泡骨（periotics），这也是始喙鲸科类群中首次被详细描述的听泡骨材料。该新标本为我们界定始喙鲸科的分类特征提供了重要契机，同时也使得我们能够将该属与该种置于渐新世早期齿鲸类（odontocete）辐射演化的系统发育框架之中，并与阿什利鲸科（Ashleycetidae）、迷鲸科（Mirocetidae）、祖鲸科（Patriocetidae）、西蒙鲸科（Simocetidae）、怀帕蒂鲸科（Waipatiidae）及异豚科（Xenorophidae）等类群一同开展相关研究。基于这一新的研究认知，目前可确定始喙鲸科仅确切发现于美国南卡罗来纳州的渐新世早期地层；而北太平洋区域（日本、墨西哥及华盛顿州）产出的其他未正式描述的疑似始喙鲸科标本，其分类地位仍需进一步验证。