| Abstract: | Simimys is a late Eocene and earliest Oligocene genus that shares dipodoid (zygoma) and muroid (dental) characters. The Oligocene record of dipodoid rodents includes Plesiosminthus from middle Oligocene deposits of Asia and late Oligocene deposits of Europe. The Oligocene record of muroid rodents includes at least two genera (Eucricetodon and (Cricetops)) from Asia, six genera (Eucricetodon, Pseudocricetodon, Melissiodon, Paracricetodon, Heterocricetodon and Adelomyorion)) from Europe, and three genera (Eumys, Scottimus and Nonomys)) from North America. The known record, as given above, suggests that Siminys is the earliest and most primitive genus with muroid affinities; it also implies that muroid rodents were derived from unknown Eocene dipodoid rodents.The Oligocene cricetid rodents display progressive expansion and inclination of the anterior plate of the zygoma. These changes in the zygoma probably reflect evolutionary stages in the development of a myomorphous zygoma from an hystricomorphous zygoma. Changes in the zygoma apparently took place at different rates and times in Asia. Europe, and North America; they probably reflect differenciation of Cricetodontinae in Palaearctica, and Eumyinae in Nearctica during the Oligocene. |
