Tusked walruses (Carnivora: Odobenidae) from the Miocene–Pliocene Purisima Formation of Santa Cruz, California (U.S.A.): a new species of the toothless walrus Valenictus and the oldest records of Odobeninae and Odobenini

ABSTRACT

Currently limited to cold climates near the Arctic circle, living walruses are the sole survivors of a previously much more diverse clade that occupied coastal waters throughout the northern hemisphere during the Mio–Pliocene. Though pinniped faunas have the highest diversity of walruses in the Miocene, the Purisima Formation of California records a moderately diverse assemblage of four walrus species. We report new specimens of tusked walruses (Odobeninae) including the oldest known members of Odobeninae, and Odobenini, and fossils of the specialized toothless odobenine walrus Valenictus Mitchell, 1961. Among these is the new species Valenictus sheperdi sp. nov., represented by a complete skull and referred post-crania from lower Pliocene strata within the Purisima Formation (5.33–4.89 Ma). Additionally, we report a geochronologically younger skull of Valenictus chulavistensis Deméré, 1994 from further up section (4.89–3.59 Ma). Expanded phylogenetic analysis recovers Odobeninae including Ontocetus Leidy, 1859 as the earliest diverging lineage in the Odobenini, and places a monophyletic Valenictus as the sister taxon to Pliopedia, Kellogg, 1921 which is included in a phylogeny for the first time; Odobenus is sister to the Valenictus + Pliopedia clade. Discovery of an isolated metacarpal near the base of the formation provides the oldest known well-dated evidence of odobenines. A diverse assemblage of molluskivores characterized the Neogene eastern North Pacific and their extinction around the Pliocene–Pleistocene boundary coincided with tectonically driven paleogeographic changes on the Pacific coast. The loss of temperate walruses may have provided opportunities for both new molluskivores and the otariid and phocid pinnipeds that make up present North Pacific pinniped communities.

ACKNOWLEDGMENTS

First and foremost, we wish to thank F. Sheperd and family for discovering and donating the holotype of Valenictus sheperdi, along with the donors of other specimens reported in this study including E. Burman, D. Landes, and F.A. Perry. Thanks to J. Goedert for funding acid preparation at CCNHM. J. Velez-Juarbe graciously provided photographs of UCMP 137426 for this study. This study would not have been possible without extensive discussions on the Purisima Formation and mentorship from F.A. Perry and C.L. Powell, Jr. Thanks to C.L. Powell, Jr. for discussions on Pliocene mollusks. Discussions of walrus evolution and morphology with L.G. Barnes, A. Berta, T.A. Deméré, and N. Kohno helped us over the course of this study. We thank Gabriel Vogeli for photography at the SDNHM. We also thank the following curators and collections managers who facilitated access to specimens under their care: P.A. Holroyd (UCMP); L.G. Barnes, S. McLeod, and V. Rhue (LACM); T.A. Deméré, K. Randall (SDNHM); D.J. Bohaska, J. Ososki (USNM); F.A. Perry, K. Aston (SCMNH). Thanks to the editors (A. Friscia, P. Godoy) and two reviewers, T.A. Deméré and J. Velez-Juarbe, for comments that greatly improved the quality of this manuscript.

DISCLOSURE STATEMENT

No potential conflict of interest was reported by the author(s).

AUTHOR CONTRIBUTIONS

RWB designed the project and drafted the manuscript, all authors gathered the data, and RWB and MC analyzed the data. All authors edited the manuscript.

SUPPLEMENTARY FILE(S)

Supplementary File 1: Boessenecker et al. Valenictus matrix nexus.nex: cladistic matrix from this study, in nexus format.

Supplementary File 2: Boessenecker et al. Valenictus matrix TNT.tnt: cladistic matrix from this study, in tnt format.

Supplementary File 3: Trees all walruses no weighting 9_7.tre: tree file saved from search with all walrus OTUs.

Supplementary File 4: Trees no incomplete walruses no weighting 9_7.tre: tree file saved from more exclusive search.

Supplementary Information revised: phylogenetic character list from this study (Appendix S1), list of specimens examined from this study (Appendix S2), and additional reporting of phylogenetic analysis (Appendix S3).