Kawatani Ayako, Kohno Naoki
Graduate School of Life and Environmental Sciences, University of Tsukuba, 1-1-1, Tennoudai, Tsukuba, Ibaraki 305-8577, Japan.
National Museum of Nature and Science, 4-1-1, Amakubo, Tsukuba, Ibaraki 305-0005, Japan.
R Soc Open Sci. 2021 Mar 3;8(3):201152. doi: 10.1098/rsos.201152.
A new species of a beaked whale that belongs to the extant genus is described from the Middle to Late Miocene boundary age Tsurushi Formation ( 12.3-11.5 Ma) on the Sado Island, Niigata Prefecture, Japan. The new species, sp. nov. represents the oldest record of this genus and provides a minimum age for the emergence of this extant genus. sp. nov. has the following generic characters: the ratio between the width of the premaxillary crests and the width of the premaxillary sac fossae is 1.0-1.25, nodular frontals make isolated protuberance on the posterior part of the vertex. Among the species within the genus, sp. nov. shares a unique character with : the apices of the left and right hamular processes of the pterygoids contact medially, forming together a posteriorly directed medial point. In addition, sp. nov. displays a unique combination of the following characters: it is extremely small in size, and the nasals are short, the ratio between the length of the medial suture of nasals on the vertex and the maximum width of nasals is less than 0.4. sp. nov. fills the gap between the origin of the genus and later diversifications of the extant species. This discovery is also key to elucidate the process of the emergence and dispersal of the genus during the Middle to Late Miocene. Based on the distributional patterns of the fossil and extant species of the genus, the western North Pacific including the Sea of Japan may have been one of the areas for the evolution and radiation of this genus at the time before 11 Ma.
在日本新潟县佐渡岛的中新世中期至晚期边界年龄的鹤志组(1230 - 1150万年前)中,描述了一种属于现存属的喙鲸新物种。这个新物种,[新物种名称]代表了该属的最古老记录,并为这个现存属的出现提供了最小年龄。[新物种名称]具有以下属的特征:前颌嵴宽度与前颌囊窝宽度之比为1.0 - 1.25,结节状额骨在头顶后部形成孤立的突起。在该属的物种中,[新物种名称]与[另一物种名称]共有一个独特特征:翼骨左右钩状突的顶端在内侧接触,共同形成一个向后指向的内侧点。此外,[新物种名称]还表现出以下特征的独特组合:体型极小,鼻骨短,头顶鼻骨内侧缝长度与鼻骨最大宽度之比小于0.4。[新物种名称]填补了该属起源与现存物种后期多样化之间的空白。这一发现也是阐明该属在中新世中期至晚期出现和扩散过程的关键。基于该属化石和现存物种的分布模式,包括日本海在内的北太平洋西部可能在1100万年前是该属进化和辐射的区域之一。