Article: Discontinuity in the Plio-Pleistocene Eurasian water vole lineage

A supposed lineage extending from Mimomys occitanus - M. ostramosensis to Arvicola terrestru through at least seven species has been widely accepted. The early part of the lineage, from M. occitanus to M ostramosensis, has been shown mathematically to be a good example of phyletic gradualism. But the transition from M. ostramosensis to M. savini is somewhat hypothetical. A new morphological analysis of the type population of M. savini (West Runton, England) has demonstrated that this species cannot be derived from M. ostramosensis, but must stem from another parent species, possibly M. coelodus or Cromeromys irtyshensis. There are, in fact, two distinct lineages characterized by different tinea sinuosa and occlusal morphologies, the first corresponding to M. occitanus - M. ostramosensis, and the second to Cromeromys savini - A. terrestris. Phyletic gradualism is demonstrated by a new graphic presentation of both lineages. This changed phyletic relationship has no biostratigraphical repercussions.