Data from: A new basal chaeomysticete (Mammalia: Cetacea) from the late Oligocene Pysht Formation of Washington, USA

Modern baleen-bearing mysticetes are descendant from archaic, toothed mysticetes known from the Late Eocene and Oligocene of the Pacific Ocean. These toothed mysticetes, such as aetiocetids and mammalodontids, dominate the Oligocene fossil record. However, larger bodied eomysticetids have recently been described from the Oligocene of the Pacific and western Atlantic. These eomysticetids lack functional teeth as adults and are thus considered to be the earliest members of Chaeomysticeti. A new fossil from the Olympic Peninsula, Washington State, including a partial skull, tympanic bullae, mandibles and postcrania, is here described as Sitsqwayk cornishorum gen. et sp. nov. S. cornishorum is a Late Oligocene mysticete lacking functional teeth and is the most phylogenetically basal chaeomysticete. Accordingly, S. cornishorum exhibits a mosaic of aetiocetid and eomysticetid characteristics. The presence of S. cornishorum in the North Pacific, and its phylogenetic position relative to the eomysticetids from the Southern Ocean, suggests a possible a North Pacific point of origin for Chaeomysticeti.

现代具须板的须鲸亚目（Mysticeti）类群起源于已知分布于太平洋始新世晚期与渐新世地层的古老齿状须鲸类。这类齿状须鲸类以埃俄鲸科（Aetiocetidae）和乳齿鲸科（Mammalodontidae）为代表，主导了渐新世的化石记录。不过近年在太平洋及西大西洋的渐新世地层中，研究者报道了体型更大的始古鲸科（Eomysticetidae）类群。该类群成体不具备功能性牙齿，因此被视为真须鲸小目（Chaeomysticeti）的最早成员。
本次研究描述了一件产自美国华盛顿州奥林匹克半岛的新化石标本，包含部分头骨、中耳鼓室泡、下颌骨及颅后骨骼，将其命名为新属新种Sitsqwayk cornishorum gen. et sp. nov.。该物种为渐新世晚期的无功能性牙齿须鲸类，同时也是系统发育位置最基干的真须鲸小目类群。因此，Sitsqwayk cornishorum兼具埃俄鲸科与始古鲸科的镶嵌演化特征。该物种发现于北太平洋，且其相对于南大洋始古鲸科类群的系统发育位置，暗示真须鲸小目可能起源于北太平洋。