A new desmostylian mammal from Unalaska (USA) and the robust Sanjussen jaw from Hokkaido (Japan), with comments on feeding in derived desmostylids

Abstract

Derived members of the enigmatic mammalian order Desmostylia have molars comprising appressed columns whose morphology does not render their function in feeding simple to discern. Here we describe a new genus and species, Ounalashkastylus tomidai, more derived than Cornwallius but less derived than Desmostylus and Vanderhoofius, which develop a hypertrophied medial eminence on the dentary ontogenetically. Tooth morphology, vaulted palate and the medial eminence, which can rise to the level of the occlusal surface of M₂, suggest that derived desmostylids clenched their teeth strongly while employing suction during feeding, most likely on marine and coastal plants.

Keywords::

• marine mammal
• phylogeny
• paleoecology
• North Pacific

Acknowledgements

It is our great pleasure to honour our friend, colleague, fellow student and mentor, Dr. Yukimitsu Tomida, with this contribution to the understanding of desmostylids, a group of fossil mammals important in Japan and the United States, the two countries that have produced most of their fossils. We thank the Museum of the Aleutians in Unalaska for facilitating the loan of Alaskan material for our study. We thank Dr. David Norton for his participation in the field component in Alaska as well as his role in facilitating our introductions to the Unalaska community. Preparation of the fossil material from the Museum of the Aleutians was performed at the Perot Museum of Nature and Science directed by Dr. Ronald Tykoski and at SMU by Kent Newman. We thank the crew (Atsushi Kurosawa, Takuma Nishi, Daisuke Aihara) from the lab of Dr. Nachio Minoura, former professor in Hokkaido University Museum, who excavated the Sanjussen specimen. We thank Dr. Makoto Manabe for permission to use the micro-CT scanner at the National Museum of Nature and Science, Tokyo. We thank Dr. Ryoko Matsumoto and Chisako Sakata for operation of the micro-CT scanner and members of Dr. David Evans' lab (University of Toronto) for discussion and Dr. Hiroshi Sawamura for his hospitality and helpfulness at the Ashoro Museum, Japan. We thank David Bohaska (USNM) for his help. Kurt Ferguson of SMU determined stable isotope values from Unalaska. LLJ benefitted from Dr. Anthony Fiorillo who facilitated his inclusion in the Alaska field programme and a Visiting Professorship at Hokkaido University Museum, providing affiliation in Japan with Dr. Yoshitsugu Kobayashi and his students (KC, KT). Time spent with them provided a broader opportunity, helped define this study and facilitated an important intellectual friendship with Dr. Naoki Kohno. LLJ's recent students (Y Kimura and YN) are important contributors to this paper. Portions of this research were addressed in an MS thesis by YN directed by LLJ at SMU and funded by the Institute for the Study of Earth and Man. We thank Drs. Nicholas Pyenson and David Bohaska for access to specimens at the USNM and Dr. Pat Holroyd at the UCMP. We are grateful for improvements to the manuscript facilitated by an anonymous reviewer.

Disclosure statement

No potential conflict of interest was reported by the authors.

Supplemental data

Supplemental data for this article can be accessed here: 10.1080/08912963.2015.104671810.1080/08912963.2015.1046718

Additional information

Funding

Fieldwork was supported by grants from the Alaska Region, US National Park Service, to ARF, and the Institute for the Study of Earth and Man to LLJ.