Abstract
A new species of a beaked whale that belongs to the extant genus Berardius is described from the Middle to Late Miocene boundary age Tsurushi Formation (ca 12.3–11.5 Ma) on the Sado Island, Niigata Prefecture, Japan. The new species, Berardius kobayashii sp. nov. represents the oldest record of this genus and provides a minimum age for the emergence of this extant genus. Berardius kobayashii 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, B. kobayashii sp. nov. shares a unique character with B. minimus: the apices of the left and right hamular processes of the pterygoids contact medially, forming together a posteriorly directed medial point. In addition, B. kobayashii 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. Berardius kobayashii 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.
Highlights
The family Ziphiidae is a clade among Odontoceti (Cetartiodactyla, Cetacea) represented by 23 extant species [1,2] and almost the same number of extinct species [3,4]
We describe one such fossil whale, based on an incomplete skull with ear bones and a partial mandible, from the Middle to Upper Miocene boundary Tsurushi Formation on the Sado Island, which was initially reported by Takahashi et al [14] as an indeterminate species ‘for the time being’ in the genus Berardius, and we identify it to elucidate its phylogenetic relationships and the related phylogeographic diversifications of this genus
It confirms that the new species based on SCM 5530-6 as the holotype belongs in the genus Berardius, and it indicates that B. minimus is known only by the extant individuals in the northern North Pacific, the generic emergence could have been much earlier than the emergence and diversification of the other previously known two antitropical species; i.e. the northern North Pacific B. bairdii and the Antarctic B. arnuxii
Summary
The family Ziphiidae is a clade among Odontoceti (Cetartiodactyla, Cetacea) represented by 23 extant species [1,2] and almost the same number of extinct species [3,4]. In the case of the Sado Island in the western margin of the North Pacific (figure 1a), the deep-sea sediments are distributed on and around this island [6,7] as a result of relatively strong tectonic movements around this area after the Late Miocene [8], and it is easy to observe such sediments and obtain fossils of the deep sea and/or pelagic organisms from there. Based on foraminiferal, radiolarian and diatom fossils, the Lower to Upper Miocene formations distributed on the Sado Island are thought to be deposited just after the Sea of Japan expanded and became a deep sea [7,9,10]. Many whale fossils including several ziphiids [12,13,14] have been found from the Middle to Upper Miocene Tsurushi Formation distributed on the Kosado mountain range of the Sado Island (figure 1b). Studies of fossil whales from the Sado Island have been stagnating despite the accumulation of fossils that followed the preliminary studies by Sado Research Group for Marine Mammalian Fossils [11] and Takahashi et al [14], and only a few additional studies have been published on the marine mammal fossils from the Tsurushi Formation until now [15]
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