The Middle Eocene North American Myomorph Rodent Elymys, Her Asian Sister Aksyiromys, and Other Eocene Myomorphs

Abstract

Abstract The early Bridgerian (early middle Eocene) myomorph rodent Elymys complexus, from the Sheep Pass Formation near Ely, Nevada, is more variable in its dental morphology than originally described. Its similarity to Aksyiromys and other middle Eocene myomorphs from China and Central Asia suggests close relationships, which requires intercontinental interchange of these small rodents during the middle Eocene. At present, the biogeographic scenario for the origin and early evolution of Myomorpha that best fits the known record of occurrences is: evolution of Myomorpha from Sciuravidae in North America at least as early as the beginning of the middle Eocene, emigration of an Elymys-like myomorph from North America to Asia in the early middle Eocene, diversification of dipodoids and muroids in the later Eocene of Asia, emigration of Eucricetodon-like cricetids from Asia to North America (eumyine cricetids) in the latest Eocene, followed later by dipodids in the Miocene.