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.现代具须须鲸类（baleen-bearing mysticetes）起源于古老的齿状须鲸类，这类齿状须鲸类的化石发现于太平洋的始新世晚期与渐新世地层。其中，阿氏鲸科（aetiocetids）与乳齿鲸科（mammalodontids）是渐新世化石记录中的主导类群。近年来，研究人员在太平洋及西大西洋的渐新世地层中报道了体型更大的始新鲸科（eomysticetids）类群，该类群的成体不具备功能性牙齿，因此被视为冠须鲸小目（Chaeomysticeti）的最早成员。本文记述了一件发现于美国华盛顿州奥林匹克半岛的新化石标本，该标本包含部分头骨、鼓泡（tympanic bullae）、下颌骨及颅后骨骼，正式命名为Sitsqwayk cornishorum gen. et sp. nov.。该物种为渐新世晚期的须鲸类，成体无功能性牙齿，且是系统发育位置最基干的冠须鲸小目类群，兼具阿氏鲸科与始新鲸科的镶嵌演化特征。该物种在北太平洋的发现，以及其相对于南大洋始新鲸科类群的系统发育位置，表明冠须鲸小目可能起源于北太平洋区域。