Two new distinctive families, Crassimarginatidae and Scoliostomatidae, each characterized by unusual gerontic apertural morphotypes, are established on the basis of study of the richly diverse Lower Devonian gastropod fauna of the Road River Formation in the Royal Creek area, Yukon Territory. The Early Devonian genus CrassimarginataJhaveri, 1969, is transferred from the family Palaeotrochidae and placed, together with the new genus Yukonoconcha, into the new family Crassimarginatidae, which is characterized by a pupiform shell with an explanate outer apertural lip in the gerontic growth stage. The new family Scoliostomatidae unites Devonian gastropod genera (ScoliostomaBraun, 1838; BrilonellaKayser, 1873; AnarconchaHorný, 1964; Eoscoliostoma new genus; MitchelliaKoninck, 1877; and Pseudomitchellia new genus), which are characterized by a distinctive, free, twisted (both outwards and backwards) gerontic final half whorl. More detailed morphological comparison of members of this family has allowed their division into two new subfamilies: the Scoliostomatinae and Mitchelliinae. New genera include Yukonoconcha, Eoscoliostoma, and Pseudomitchellia. The former two genera are represented each by a single new species from Yukon Territory, Yukonoconcha pedderi and Eoscoliostoma norrisi. The latter genus is represented by two species, the type Pseudomitchellia bohemica (Perner) from the Koněprusy Limestone (Pragian) of the Czech Republic and Pseudomitchellia macqueeni new species from Yukon Territory.