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Abstract—In the San Miguel de Allende basin, Guanajuato State, Mexico, two mandibles of gomphotheriids were collected, and by their diagnostic characters have been assigned to Stegomastodon primitivus. The jaws correspond to different ontogenetic and stratigraphic ages. In the Rancho El Ocote fauna, in the upper part of the Hemphillian stratigraphic sequence, the mandible of a young individual and isolated upper and lower molars were collected together with the mandible from the Blanco layer, which has been assigned a latest Hemphillian (Hh4) age. The molars have characters considered more primitive than those described for Stegomastodon primitivus (= Stegomastodon rexroadensis), characteristic of an early Blancan age in North American faunas. The jaw from the Blanco layer has a very short anterior symphysis, straight and ending in a narrow structure that forms the lingual canal. It has no evidence of tusks. The isolated molars present the most primitive characters referred to a Stegomastodon individual: The trefoil and entotrefoil cusps are simple without folds (ptychodonty) or accessory enamel tubercles (choerodont). The isolated M3/m3 has with four lophs/lophids and two large cusps posterior to the fourth lophs/lophids. The m2 has three lophids and two small conids behind the tritolophid, that, in advanced states of wear, form a posterior half lophid more evident in molars with greater wear. There are no stratigraphic indexes of a late Hemphillian or early Blancan age in the Blanco Layer. Throughout the Blanco Layer only Dinohippus mexicanus is present. Zircons separated from ash in the same layer where the Stegomastodon primitivus mandible was collected yielded a 4.85+0.17 Ma U-Pb age, which corresponds to the latest Hemphillian (Hh4). This Rancho El Ocote record is the oldest known among North American faunas. This result assumed that the possible diversification of gomphotheriids in faunas of central Mexico happened before that expected by Savage (1955) in his probable phyletic dispersal pattern of the North American gomphotheriids. A mandible of an old adult gomphothere was collected in the Arroyo Earth Watch, in Los Galvanes area. The fossil was found in sediments assigned to the early Blancan. The jaw is complete without distortion, and it only retains the m3 in an advanced state of wear. This tooth only has five lophids that differentiate it from the m3s of Cuchillo Negro Creek and Elephant Butte Lake Stegomastodon of the early Blancan of New Mexico, that have 5+ to six lophids. The zircons analyzed by the U / Pb method gave an age of 2.9±0.07 Ma for the Stegomastodon jaw from Los Galvanes, consistent with the early Blancan. This record correlates with specimens from New Mexico’s Cuchillo Negro Creek and Elephant Butte Lake faunas, that have a radiometric age of 3.1±0.3 Ma, early Blancan. The similarity of these radiometric ages suggests that Stegomastodon primitivus had a wide geographic distribution in the early Blancan.