Table S1.xlsx from New evidence for the antiquity of Desmostylus (Desmostylia) from the Skooner Gulch Formation of California

<i>Desmostylus</i> is an extinct marine mammal genus that belongs to Desmostylia, a clade of extinct herbivorous mammals. While desmostylian remains are widely reported from Paleogene and Neogene marine strata of the North Pacific Rim, occurrences of the genus <i>Desmostylus</i> are almost entirely limited to middle Miocene strata, with only a few early Miocene records from Japan. Here we report a <i>Desmostylus</i> tooth from the earliest Miocene (Aquitanian) Skooner Gulch Formation in northern California, USA. This specimen exhibits cuspules around the crown, a primitive trait of the subfamily Desmostylidae, as seen in more basal branching desmostylid taxa such as <i>Cornwallius</i> and <i>Ounalashkastylus</i>, but with a high tooth crown and thickened enamel. The specimen is also diagnostically different from all other desmostylid genera, such as <i>Cornwallius,</i> and <i>Ounalashklastylus</i>. The Aquitanian age of the Skooner Gulch Formation implies that the distinctive tooth morphology of <i>Desmostylus</i> has persisted, largely unchanged, for more than 15 million years and that desmostylids possibly originated in western North America.