Variation and complexity of the enamel pattern in the fi rst lower molar of the Field vole, Microtus agrestis (L., 1761) (Mammalia: Rodentia: Arvicolinae)

The grinding surfaces of the persistently growing molars of microtine rodents (Arvicolinae) show a peculiar pattern of salient angles and re-entrant folds along the lingual and labial borders of the teeth. Number, form, and relative size of transversal loops and alternating dental triangles, even though a distinctive feature for the discrimination of genera and species, are subjected to considerable intra-specifi c variation, in particular in the fi rst lower molar, M1 (ANGERMANN, 1974; SCHIMMELPFENNIG, 1991, KRAFT & KAPISCHKE, 2008; 2009; KAPISCHKE & KRAFT, in press). In the Field vole, Microtus agrestis, variability of molar pattern had been described e.g. by RÖRIG & BÖRNER (1905), ZIMMERMANN (1956), REICHSTEIN & REISE (1965), REICHSTEIN (1966), KAPISCHKE (1992), LOCATELLI & PAOLUCCI (1995), and JENTZSCH (2006). Aberrations from the general enamel pattern of the lower molars comprise the development of additional transversal loops or an increase in the number of constricted segments. Additional data for populations from different German counties are presented.


Introduction
The grinding surfaces of the persistently growing molars of microtine rodents (Arvicolinae) show a peculiar pattern of salient angles and re-entrant folds along the lingual and labial borders of the teeth.Number, form, and relative size of transversal loops and alternating dental triangles, even though a distinctive feature for the discrimination of genera and species, are subjected to considerable intra-specifi c variation, in particular in the fi rst lower molar, M 1 (ANGERMANN, 1974;SCHIM-MELPFENNIG, 1991, KRAFT & KAPISCHKE, 2008;2009;KAPISCHKE & KRAFT, in press).In the Field vole, Microtus agrestis, variability of molar pattern had been described e.g. by RÖRIG & BÖRNER (1905), ZIMMERMANN (1956), REICHSTEIN & REISE (1965), REICHSTEIN (1966), KAPISCHKE (1992), LOCATELLI &PAOLUCCI (1995), andJENTZSCH (2006).Aberrations from the general enamel pattern of the lower molars comprise the development of additional transversal loops or an increase in the number of constricted segments.Additional data for populations from different German counties are presented.

Material and methods
In the course of mapping the small mammal fauna of their home counties, the authors were able to sight large series of the Field vole from Bavaria (R. Kraft, M. Hiermeier), Saxony, Brandenburg, and Mecklenburg-Vorpommern (H.-J.Kapischke), and Saxony-Anhalt (M.Jentzsch, H.-J. Kapischke).The bulk of the material was extracted from owl pellets, more than 800 specimens were trapped.For details on localities, collectors, and storage of the samples see KAPISCHKE (1992, in prep.),JENTZSCH (2006), and KRAFT (2008, p. 8 ff).
Variation and complexity of the enamel pattern in the fi rst lower molar of the Field vole, Microtus agrestis (L., 1761) (Mammalia: Rodentia: Arvicolinae) From about 2500 specimens, individuals with aberrant tooth pattern were sort out and classifi ed according to the criteria proposed by ANGERMANN (1974), JENTZSCH (2006), and KRAFT & KAPISCHKE (2008).

Results
In general, the fi rst lower molar of the Field vole has fi ve closed triangles (T1 to T5), a crescent-shaped posterior loop and a more complex anterior one.The latter displays a labial and lingual transversal loop, T6 and T7, respectively (fi g. 1 a).

Occurrence of an additional salient angle on anterior loop: the oeconomus + -morph
The apical knob of the anterior loop, that is round in most specimens (fi g. 1 a), can display a pronounced edge on the lingual side, which represents an extension of the peripheral enamel sheet (fi g. 1 b).With an anterior loop, that resembles that of a knight (chess piece), this molar is very similar to that of the Root vole, Microtus oeconomus (PALLAS, 1776), but has fi ve instead of four closed triangles between anterior and posterior loop.Therefore, JENTZSCH (2006) proposed the term "oeconomus+-morph" for this molar shape.The plus sign stands for the presence of the additional closed dental triangle.Beside the Field vole, the oeconomus+-morph had also been reported for the Common vole, Microtus arvalis (PALLAS, 1779), by KRAFT & KAPISCHKE (2008).
After "normal" this is the most common molar form in the Field vole populations.Approximately 25 % of the specimens examined show this additional salient angle on M 1 (Tab.1).In most cases, this variant is symmetrically present on both sides and just occasionally on one side only.

Constriction of additional dental triangles
Another development towards increased molar complexity is by dividing the anterior loop of M 1 into two or three closed dental fi elds.This may happen by constriction of the apical knob of the anterior loop, resulting in the maskii-morph (fi g. 1 c).The resulting rhombus can be partially or completely divided by longitudinal displacement of the angles T6 and T7 in the opposite direction, resulting in a molar with seven alternating dental triangles and a constricted apical knob.This was found in the right M 1 of a specimen from Brandenburg (fi g. 1 e, KAPISCHKE, in prep.) and in the left M 1 of another specimen from Bavaria.In the corresponding opposite molars, however, only T6 is forming a closed dental triangle, whereas T7 is still confl uent with the apical part of the loop (fi g. 1 d) To emphasize the existence of seven closed alternating triangles in an arvicolid M 1 , we propose the term "arvalis/agrestis++" for this morph, suspecting that this variation eventually will be located in Microtus arvalis.
In the specimen from Brandenburg, a small lingual edge or angle appears in outlines on the lingual side of the constricted apical knob (fi g. 1 d, e), combining characters of the arvalis/agrestis++ morph with that of the oeconomus++ variant.
Starting either from the standard form or from the oeconomus+-morph, also the labial edge (instead of the apical knob) of the anterior loop can be separated by indentation of the re-entrant fold between T5 and T7, thus forming a sixth closed triangle, T6 (fi g. 1 f).Also from this stage, the development of the arvalis/ agrestis++ morph, is conceivable by successive constriction of T7.In the maskii-M 1 as well as in other variants, the apical knob of the anterior loop can display a lingual salient angle T8 as in the oeconomus+ morph (fi gs. 1 d, e, g).For variants with six closed dental triangles and a pronounced T8 on the anterior loop, the term oeconomus++ (KRAFT & KAPISCHKE, 2008) is appropriate.
While in Microtus agrestis a single specimen with seven closed triangles in the M 1 has been already mentioned by KAPISCHKE (1992), only up to six closed triangles are hitherto reported for the Common vole Microtus arvalis (e.g.KRAFT & KAPISCHKE, 2008, Abb. 3 e, f).
These different routes from the standard form to the most complex (arvalis/agrestis++-) morph are depicted schematically in fi g. 2.

Discussion
Gradual development of supernumerary loops and constriction of alternating dental triangles on the M 1 via several intermediate morphs (e.g.maskii, oeco no-mus+, oeconomus++), nearly always has its origin in the anterior loop.Hence changes towards increased complexity in the lower molars always start from the mesial part of the tooth.It is noteworthy to mention that the same is reported for several phylogenetic lines within microtine rodents, e.g. in Dicrostonychidae and for the line from Allophaiomys to Microtus (v.KOENIGS WALD, 1982).When changes do occur in a more distal region of the molar, these are mostly reductions in complexity.Amongst many thousands of individuals of Microtus arvalis from the German counties Bavaria and Saxony, KAPISCHKE & KRAFT (in press, fi g. 1 j, k) were able to fi nd only two specimens that show a supernumerary loop on a more posterior part of the M 1 .In the upper molars, however, it is always the posterior end that is subjected to analogous processes (v. KOENIGSWALD, 1980).
It is emphasized by several authors (e.g.GUTHRIE, 1971), that an increase of enamel loops or triangles enhances masticatory effi ciency and reduces abrasion of the grinding surface in microtine molars.Individuals with a more complex M 1 should be favoured within a population due to better exploitation of food.Therefore, such variants can demonstrate changes in the evolution of arvicolids from forms with a simple molar tooth pattern to those with a more complex one (v.KOENIGSWALD, 1982).

Fig. 1 .
Fig. 1.Variants of the fi rst lower molar of the Field vole, Microtus agrestis.a b c d e f g

Tab. 1 .
Occurrence of the described morphs of the fi rst lower molar of Field voles in populations from Bavaria and Saxony-Anhalt nos. of closed dental trinagles