Dikkomys, the earliest known genus of the tribe Dikkomyini, can be taken as a starting point of evolution for the subfamily Geomyinae. The Pliocene genus Pliosaccomys is the only other known geomyine having primitive features closely resembling those of Dikkomys. The relatively close but previously unrecognized relationship between Dikkomys and Pliosaccomys can be understood when patterns of wear on the occlusal surfaces of the cheek teeth are taken into account. It appears that Pliosaccomys descended from Dikkomys-like stock, if not Dikkomys itself. Although Dikkomys is towards the beginning of this phyletic sequence and Pliosaccomys towards the end of the sequence, the primitive features shared by the two provide a generalized morphotype for the subfamily Geomyinae.
In the molariform dentition, an almost complete series of stages of wear in Pliosaccomys has been preserved, and those of Dikkomys can be reconstructed with reasonable accuracy from those that are known (see [Fig. 4]):
(1) In the initial stage of wear in Dikkomys the anterior and posterior columns are separated by an intervening valley ([Fig. 4A]), and the occlusal surface of each column bears a loph of dentine surrounded by a ring of enamel: protoloph on the anterior column and metaloph on the posterior column of the upper teeth (protolophid and hypolophid in corresponding positions in the lower teeth). Actually this stage is not preserved in the known material of Dikkomys, but does occur in both geomyines and entoptychines in all stages of evolution, and it must have also occurred in Dikkomys in order for the next two stages, which are preserved, to have developed.
(2) The occlusal surfaces are ground down to a level where the enamel loops of the two columns join at their mid-points, thus forming an H-shaped pattern ([Fig. 4B]), or more exactly a pattern resembling a figure 8. Probably this was the primitive pattern in the final stage of wear in the geomyid ancestor of the Oligocene.
Fig. 3. Diagram depicting geologic range and probable phyletic relationships of the family Geomyidae. Dashed lines represent parts of lineages that are not represented by fossil records, and solid lines represent parts of lineages verified by actual specimens. Question marks indicate uncertainty of suggested ancestry of known taxa. The relationships within the subfamily Entoptychinae are modified after Wood (1936), and the temporal range of the Miocene geomyids have been adjusted to agree with current stratigraphic correlations. Hence, Pleurolicus, Gregorymys and Dikkomys are illustrated as ranging into the Hemingfordian, rather than being confined to the Arikareean (see MacDonald, 1963, and Black, 1961).
(3) In the pre-final stage of wear, the anterior and posterior lophs of the first and second molars unite secondarily at the edge of their protomeres (labial side in the lower and lingual in the upper), thus enclosing an isolated enamel fossette ([Fig. 4C]). Lateral union occurs in the lower teeth because the vertical depth of the labial re-entrant angle is less than the depth of the lingual re-entrant fold. In the upper teeth the reverse is true. The re-entrant angle on one side of the premolar is as deep vertically as the angle on the other side of that tooth, and both reach the base of the crown; therefore, they do not disappear at any stage of attrition. The same pertains in the third lower molar.