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

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, U.S.A., 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, as yet undescribed, potential agorophiid specimens from the Oligocene of the North Pacific.

侏儒始喙鲸（Agorophius pygmaeus）的正模（holotype）部分头骨——该属仅含该单型种，同时也是始喙鲸科（Agorophiidae）的唯一单型代表——已失踪约140年。自侏儒始喙鲸被发现以来，已有诸多分类单元与标本被归入始喙鲸科，后续却均被重新分类并移出该科，这造成了学界对何为始喙鲸类的认知混乱，也使得界定始喙鲸科的形态特征始终模糊不清。近日，一件全新发现的始喙鲸类头骨标本从美国南卡罗来纳州阿什利河的水下崖壁中被采集到，该标本被归为侏儒始喙鲸，其产出层位为阿什利组（Ashley Formation）底部，时代为渐新世（Oligocene）早期。该新标本保存有大部分头骨及耳外骨（periotics），其保存状态完好，且是始喙鲸科类群中首次被详细描述的耳外骨材料。该新标本为我们诊断始喙鲸科提供了契机，也使得我们能够将始喙鲸属与该物种置于渐新世早期齿鲸类（odontocete）辐射演化的系统发育框架中，同时可与阿什利鲸科（Ashleycetidae）、迷鲸科（Mirocetidae）、帕特里奥鲸科（Patriocetidae）、西蒙鲸科（Simocetidae）、怀帕蒂鲸科（Waipatiidae）以及异海豚科（Xenorophidae）等其他类群进行对比分析。基于这一新的研究认知，始喙鲸科目前可确定仅分布于美国南卡罗来纳州的渐新世早期地层，而北太平洋渐新世地层中尚有部分未被正式描述的疑似始喙鲸类标本。