Niche Partitioning in Oligocene Toothed Mysticetes (Mysticeti: Aetiocetidae)


Niche partitioning has long been considered to be a fundamental part of speciation and difference in body size is a straightforward proxy for ecology and niche partitioning. Here we report a new aetiocetid specimen from the upper part of Morawan Formation (Oligocene; late Chattian, ~26–24 Ma), Ashoro, Hokkaido, Japan. This specimen comprises an isolated left squamosal and an isolated left periotic; the morphology of the squamosal is distinguishable from all aetiocetids, but shows a close match with the aetiocetid Morawanocetus yabukii in that: the dorsal margin of the zygomatic process of the squamosal is twisted dorsomedially; the zygomatic process expands dorsoventrally; the zygomatic process is transversely thin; and the anterior tip of the zygomatic process is acutely tapered. The estimated body size of this Morawanocetus-like specimen is ca. 8 m, almost twice the size of all known aetiocetids (3–4 m). Moreover, the preserved periotic reveals structures that are otherwise poorly known in aetiocetids. This large Morawanocetus-like aetiocetid sheds new light on niche partitioning of Oligocene toothed mysticetes and evolution of body size in Mysticeti.

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We thank Katsuie Yabuki for collecting and donating AMP 9 to the Ashoro Museum of Paleontology; Tatsuya Shinmura for life reconstruction of AMP 9; John R. Wible (Editor-in-chief), Erich Fitzgerald, and an anonymous reviewer for constructive comments; Robert Boessenecker for pre-submission review. For access to collections and allowing photography during CHT and/or TA’s visits, we thank Nicholas Pyenson and David Bohaska (National Museum of Natural History, Smithsonian Institution, USA); Patricia Holroyd (University of California Museum of Paleontology); Lawrence Barnes and Samuel McLeod (Natural History Museum of Los Angeles County, USA). CHT thanks the Ashoro Museum of Paleontology (Hiroshi Sawamura, TA, and Tatsuya Shinmura) for hosting research visits. James Mead (Washington, DC) and Robert Boessenecker (California) kindly accommodated Tsai during various visits. CHT is supported by a University of Otago Doctoral Scholarship.

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Correspondence to Tatsuro Ando.

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Tsai, CH., Ando, T. Niche Partitioning in Oligocene Toothed Mysticetes (Mysticeti: Aetiocetidae). J Mammal Evol 23, 33–41 (2016).

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  • Body size
  • Feeding
  • Niche differentiation
  • Mysticete
  • Whale