CONCLUSIONS

New material from the Zuurland borehole (The Netherlands) indicated that Mimomys hordijki from Early Pleistocene (Gelasian) level of the Zuurland faunal sequence represents an early stage of the peculiar Cromeromys group known from Early Pleistocene (Gelasian to 'Calabrian' equivalents) faunas of Beringia and from Irvingtonian faunas of the conterminous United States. Cromeromys share characteristic molar morphology of the m1 with a strongly developed Mimomys-ridge and without an enamel islet, and of the M3 with a deep LRA3 that is not reduced by insulation (formation of an enamel islet).

This study of the molars of Mimomys hordijki indicated a number of similarities between M. hordijki and the genus Lemmiscus. The general assumption is that the Nearctic sagebrush vole Lemmiscus is closely related to the Palearctic lagurines. However, our observations support karyological data that suggest a close relation of this genus to the tribe Arvicolini being a sister taxon of the genus Microtus. The morphological similarities between Lemmiscus and the lagurines most likely reflect parallel evolution in similar environmental conditions of open landscapes.

During the early Early Pleistocene, Mimomys (Cromeromys) voles occurred in northern Europe as well as western and eastern Beringia and dispersed to more southern areas of North America in the late Early Pleistocene. Morphological evidence suggests a phyletic transformation of this group of rhizodont voles into rootless Lemmiscus took place at the end of the Early Pleistocene. More studies are required to support or reject the proposed hypothesis.