Mimotonids have recently been recognized as the likely ancestors to Lagomorpha (rabbits, hares, and pikas). Here a new species of mimotonid, Gomphos ellae, from Tsagaan Khutel locality, Valley of Lakes, Mongolia is described. This new material shows typical mimotonid features while also exhibiting important derived lagomorph features and helps to bridge the morphological and temporal gap between mimotonids and lagomorphs. The fossils comprise a right dentary, including cheek teeth and incisor, and a partial maxilla with two teeth. The dental features of G. ellae are similar to the mimotonid G. elkema, but G. ellae is distinguished from G. elkema and all other mimotonids by its long diastema. The diastema length of G. ellae is intermediate between all other mimotonids and all living and extinct lagomorphs. This feature distinguishes mimotonids from lagomorphs and the intermediate diastemal length of G. ellae further supports a strong pattern of lengthening of the diastema throughout Duplicidentata. Geologic and paleontological evidence suggest that the specimen is of Middle Eocene age, consistent with the notion that this taxon is an advanced mimotonid that likely lies near the ancestry of the lagomorphs.