New records of the fur seal Callorhinus (Carnivora: Otariidae) from the Plio-Pleistocene Rio Dell Formation of Northern California and comments on otariid dental evolution

ABSTRACT

New fossils representing two species of the fur seal Callorhinus are reported from the uppermost Pliocene to lower Pleistocene Rio Dell Formation of northern California. The finds include latest Pliocene–earliest Pleistocene dentaries and postcrania of Callorhinus gilmorei, and a partial dentary of early Pleistocene age identified as Callorhinus sp. The aforementioned material is ascribed to C. gilmorei due to the incipient single-rooted condition of the p1–2, retention of double-rooted p3–m1, and overall small size. The dentary identified as Callorhinus sp. exhibits a more derived pattern of tooth morphology, including single-rooted p1–p4 (and double-rooted m1), larger size than C. gilmorei, and in the size range of extant Callorhinus ursinus (which typically exhibit fused roots on all postcanine teeth). Fusion of postcanine roots began with the p2 and continued posteriorly, and is likely an adaptation to accommodate crowded teeth anteriorly in the jaws. Callorhinus gilmorei has previously been reported from the upper Pliocene of southern California and Japan, and this new record extends the range of this taxon further north in the Northeast Pacific. Callorhinus sp. is the most complete pinniped fossil to be described from the early Pleistocene of the Northeast Pacific. The wide biogeographic range of Callorhinus during the Pliocene and Pleistocene documents the persistence of this taxon, potentially as a Pliocene-Holocene anagenetic lineage. This highlights the antiquity of the Callorhinus lineage, which has persisted in the Northeast Pacific since the Pliocene, establishing it as the oldest and earliest diverging crown otariid.

ACKNOWLEDGMENTS

I am indebted to R. J. Bushell (formerly of Eureka, California), who discovered, expertly prepared, and kindly donated the fur seal fossils described herein. I thank the editor (J. H. Geisler) and three anonymous reviewers for their careful and detailed comments, which significantly improved the manuscript. I would like to thank C. Carr and M. Churchill for their helpful comments on an earlier version of the manuscript. I have also benefited from numerous discussions with P. J. Adam, L. G. Barnes, A. Berta, M. Churchill, T. A. Deméré, D. Fowler, N. Kohno, J. Scannella, and A. Poust. Thanks to J. Horner (MOR) for use of the preparation laboratory and equipment. I would like to thank C. Conroy (University of California Museum of Vertebrate Zoology), G. Bromm (Sierra College), T. A. Deméré (SDNHM), M. Flannery (California Academy of Sciences), R. Hilton (Sierra College), P. Holroyd (UCMP), and K. Randall (SDNHM), who allowed me to examine specimens under their care. Extra thanks go to G. Bromm and R. Hilton (Sierra College) for their assistance in loaning material for this study.

Handling editor: Jonathan Geisler.