INTRODUCTION

The earliest record of cylindrodontid rodents is Dawsonomys woodi from the Wasatchian of Wyoming (Korth, 1984). The family reached its greatest diversity in the Chadronian, where four genera and 13 species have been recognized, but before the end of Chadronian time the family became extinct in North America. Two subfamilies, the Cylindrodontinae and Jaywilsonomyinae, are recognized on the basis of dental characters (Wood, 1974), and can be traced back to the Bridgerian and Uintan, respectively; the subfamilial allocation of the Wasatchian Dawsonomys is uncertain, but is probably to Cylindrodontinae.

Outside North America, cylindrodontids are rare. The genus Ardynomys (originally referred to the Ischyromyidae) was first described from the latest Eocene or earliest Oligocene Ardyn Obo Formation of Mongolia by Matthew and Granger (1925). From the same deposits, Shrevyreva (1972) later named Morosomys silentiumis, which she classified as a cylindrodont; we consider Morosomys a synonym of Tsaganomys, and not a cylindrodont.