Upper Incisor Grooves in the Geomyinae

Abstract

Currently used systems of classification and phylogeny of geomyines rely largely on the number and position of upper incisor grooves. They state that, of the living genera, Geomys and Zygogeomys are bisulcate with both lingual and labial grooves, Thomomys is asulcate with a rare lingual groove, and Pappogeomys and Orthogeomys are unisulcate with only a medial groove. Reevaluation of groove patterns shows that Geomys and Zygogeomys are actually bisulcate but, of 908 Thomomys , 99.3 per cent have a lingual groove and 29.7 per cent have a faint medial groove. Also, 75.3 per cent of 85 Pappogeomys and 34.2 per cent of 38 Orthogeomys have a lingual groove in addition to the medial. The medial groove probably functions primarily to strengthen the tooth, whereas the lingual groove probably reinforces the lingual edges of the upper incisors where they contact one another. The apparent phylogenetic sequence of groove development from a primitive smooth incisor is: formation of a lingual groove; addition of a medial groove; and, lastly, reduction of the lingual groove in some large taxa. Because similar grooving patterns may have developed in different lineages, the use of groove patterns to interpret phylogenetic relationships may be subject to error. Bisulcate fossil geomyines may be related to Pappogeomys or Orthogeomys as well as to Geomys or Zygogeomys .