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The newly erupted upper premolar (P4) of Dipodomys shows an arrangement of cusps that is distinctive in five species studied. The cusps swiftly wear away to leave the tooth as a column of radial enamel and dentine. The pulp cavity of P4 is elongated and vestigial roots may form in some species. The Miocene ancestor of the Dipodomyinae may have been a species of Cupidinimus. The direct ancestor of Dipodomys was a Blancan species of Prodipodomys. Nine extinct species of Prodi- podomys have been named but only two extinct Dipodomys. Two cusp patterns occur in the P4 of Prodipodomys: the 8-pattern and C-pattern. Dipodomys elator is probably descendent from a species with the 8-pattern and D. ordii, D. compactus, D. spectabilis and D. merriami from a species with the C-pattern. Dipodomys is thus a diphyletic genus. D. ordii and D. compactus have distinct hypocones on P4, and well developed postentostyle valleys. In D. spectabilis and D. merriami, the hypocone is folded back against the posterior cingulum, and the postentostyle valley is obsolete or absent. |
