Abstract
In response to environmental changes in the Northern hemisphere, several lines of brachyodont-bunodont cricetid rodents evolved during the Late Miocene as "microtoid cricetids." Major evolutionary trends include increase in the height of cheek tooth crowns and development of prismatic molars. Derived from a possible Megacricetodon or Democricetodon ancestry, highly specialised microtoid cricetids first appeared with Microtocricetus in the Early Vallesian (MN 9) of Eurasia.
Because of the morphological diversity and degree of parallelism, phylogenetic relationships are difficult to detect. The Trilophomyinae, a more aberrant cricetid side branch, apparently became extinct without descendants.
Two branches of microtoid cricetids can be recognized that evolved into "true" arvicolids: (1) Pannonicola (= Ischymomys) from the Late Vallesian (MN 10) to Middle Turolian (MN 12) of Eurasia most probably gave rise to the ondatrine lineage (Dolomys and Propliomys) and possibly to Dicrostonyx, whereas (2) Microtodon known from the Late Turolian (MN 13) and Early Ruscinian (MN 14) of Eurasia and possibly parts of North America evolved through Promimomys and Mimomys eventually to Microtus, Arvicola and other genera. The Ruscinian genus Tobienia is presumably the root of Lemmini.
Under this hypothesis, in contrast to earlier views, two evolutionary sources of arvicolids would be taken into consideration. The ancestors of Pannonicola and Microtodon remain unknown, but the forerunner of Microtodon must have had a brachyodont-lophodont tooth crown pattern similar to that of Rotundomys bressanus from the Late Vallesian (MN 10) of Western Europe.
Possibly, Pannonicola and Microtodon share a common ancestor. The fossil record suggests that an important center of origin for arvicolids was located in northeast Asia.
From this region arvicolids could have dispersed to Europe and North America and vice versa during the late Cenozoic at various times.