Yaquinacetus meadi, a new latest Oligocene–early Miocene dolphin (Cetacea, Odontoceti, Squaloziphiidae, fam. nov.) from the Nye Mudstone (Oregon, U.S.A.)

Represented by a nearly complete cranium with associated mandible, teeth, and vertebrae, Yaquinacetus meadi is a new genus and species of archaic homodont odontocete from the latest Oligocene–early Miocene (24–19.2 Ma) of Oregon, U.S.A. The new species is characterized by a moderately elongated rostrum bearing approximately 51 alveoli per tooth row and a knob-like, rectangular vertex. Together with Squaloziphius emlongi from the early Miocene of Washington State, Y. meadi constitutes a new odontocete family, Squaloziphiidae, fam. nov., diagnosed by a unique combination of characters, including transversely wide dorsal opening of the mesorostral groove at base of rostrum, followed posteriorly by an abrupt narrowing; thickened lateral margin of the maxilla in the antorbital region making a long and laterally concave crest; and massive, anteroposteriorly and ventrally long postglenoid process of the squamosal. Although sharing with Ziphiidae the presence of transverse premaxillary crests on the vertex, Squaloziphiidae differs in the pterygoid sinus fossa being shorter anteriorly and ventrally; the tubercule of the malleus being less reduced; and lacking a pair of enlarged alveoli for mandibular tusks. Our phylogenetic analysis confirms the sister-group relationship between S. emlongi and Y. meadi, either as late diverging stem odontocetes or as early crown odontocetes, but distant from Ziphiidae. These results confirm the northeastern Pacific as a center of diversification for several groups of archaic homodont odontocetes during the late Oligocene–early Miocene.

以近乎完整的颅骨及关联下颌骨、牙齿与椎骨为化石代表的Yaquinacetus meadi，是产自美国俄勒冈州渐新世晚期-中新世早期（2400万至1920万年前）的古老同齿型齿鲸类新属新种。该新种的鉴别特征为：吻突适度延长，每列齿列约具51个齿槽，颅顶呈结节状矩形。结合产自华盛顿州中新世早期的Squaloziphius emlongi，Y. meadi共同构成了齿鲸类一新科——Squaloziphiidae fam. nov.（新科），该新科以一系列独特的性状组合为诊断依据：吻突基部的吻中沟背侧开口横向宽阔，其后骤然变窄；眶前区的上颌骨侧缘增厚，形成长而侧凹的嵴突；鳞骨的关节后突硕大，前后向与腹向均显著延长。尽管Squaloziphiidae与剑吻鲸科（Ziphiidae）同样在颅顶具前上颌骨横嵴，但二者存在显著差异：翼窦窝的前腹侧更短；锤骨结节的退化程度更弱；且不具一对用于下颌獠牙的扩大齿槽。我们的系统发育分析证实，S. emlongi与Y. meadi构成姊妹群关系，二者既可能属于分化较晚的基干齿鲸类，也可能属于早期冠群齿鲸类，但与剑吻鲸科的亲缘关系较远。上述结果确认，东北太平洋是渐新世晚期-中新世早期若干古老同齿型齿鲸类类群的分化中心。