Gomphochelys nanus, new genus and species, is described from the earliest Wasatchian (biohorizon Wa 0; ∼55.8 Ma) of the southeastern Bighorn Basin, Washakie County, Wyoming. The new taxon represents the only known dermatemydid from the Paleocene—Eocene Thermal Maximum (PETM) interval and extends the lineage back from previous records by approximately 2 million years. Gomphochelys nanus has a thick tricarinate carapace and differs from other dermatemydids in attaining a smaller adult body size, having reduced plastral features, a posteriorly situated gular—humeral sulcus, an acarinate pygal, and thick shortened peripherals. Reexamination of previously described fossil dermatemydids suggests that the taxa Baptemys tricarinata and Kallistira costilata are junior synonyms of the middle—late Wasatchian Notomorpha garmanii, and Baptemys fluviatilis is likely a junior synonym of Baptemys wyomingensis. Gomphochelys nanus is a stem dermatemydid that is similar to N. garmanii but differs in possessing symplesiomorphies with the Late Cretaceous—Paleocene genera Agomphus and Hoplochelys. Aspects of shell morphology suggest that G. nanus was a commensurate swimmer and bottom-walker like extant Dermatemys and Staurotypus. The presence of a dermatemydid (a tropically distributed clade) in the southeastern Bighorn Basin during the PETM (when global temperatures increased by 5°C–10°C over a period of ∼60 ky) further supports the hypothesis that climate was megathermal in the region during this interval and is consistent with previously documented geographic range changes in both plants and animals. Dermatemydids disappear from the fossil record at the end of the PETM and don't reemerge until the next warming event, Eocene Thermal Maximum 2.

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