We investigated the variation in complexity of the lower first molar M₁ of the common pine vole Microtus subterraneus. Eight different morphotypes were found. The M₁ with additional structures was significantly longer than the ‘typica’ morph. This structural gain is not caused by alternating dentin triangles, as known from Microtus arvalis or M. agrestis, but by a second ‘pitymys’ field in combination with a complete anterior lobus. This morphotype is named ‘pitymys +’. Two possible routes towards increased complexity are described and depicted schematically. It depends on whether, initially, the anterior lobus narrows to the ‘maskii’ morph, and the new and complete anterior lobe will be constricted in the following step out of the remaining loop. Alternatively, additional anticlines are first formed at the anterior lobe, and the constriction of the second ‘pitymys’ rhombus takes place in the next step.