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Abstract—This work describes and compares the skull and lower jaw of two species of Rhynchotherium discovered in early Blancan deposits in central Mexico. This comparison establishes the differences between the species Rhynchotherium falconeri and R. browni and is the first record of R. browni outside of San José de Pimas, Sonora. The description of the skull and jaw of the same individual, referred to the gomphotheriid proboscidean Rhynchotherium falconeri, collected in deposits of early Blancan age (3.6 Ma), from the Rancho La Goleta locality, state of Michoacán, Mexico, corresponds to an adult individual that shows these diagnostic characters: deflection of the symphysis down and forward with an angle of 61°, the anterior edge of the ascending ramus at 90°, and the ventral inclination of the horizontal ramus at 25°, which gives it greater depth characteristics in the anterior region under the protolophid of m2. These are characters that support referral to Rhynchotherium falconeri, however, M3/m3 are simple tetraloph/tetralophid, with low cusps and simple trefoils, without accessory cusps or a heel. The anteroposterior length of M3 is among the smallest of Rhynchotherium falconeri known in faunas of North America. These measurements are within the range considered part of sexual dimorphism, so the specimen is assumed to correspond to a female. The skull is domed, the occipital almost straight in relation to the occlusal surface of the molars, the face is more elongate, and the premaxillae are narrow and open, so they are separated in the anterior medial part, the alveoli are complete with apparent divergent direction, and two long fragments of tusks are separated from the alveoli with small evidence of enamel. In the San Miguel de Allende basin, in deposits of early Blancan age (3 Ma) in the Los Galvanes area, the first known record of Rhynchotherium browni was collected outside of San José de Pimas, Sonora. The jaw is larger, and the structures are more robust. The symphysis, although reconstructed, shows a probable inclination of ~77°, and the anterior edge of the ascending rami has an angle of 107°. The mandible presents the alveoli of m1’s, and in front of them an extension of the lingual canal of approximately 70 mm. The horizontal ramus is longer and straighter, and shallower but wider transversely at the base of the ascending rami compared to Rhynchotherium falconeri. The M2 has three lophs, and the metaloph has a remarkable double trefoil. In m2 the tritolophid has a small cusp with signs of wear in the posterior medial part. The M3/m3 are tetraloph with simple trefoils and higher cusps, and have a heel represented by small cusps. The right i2 is complete, with no evidence of enamel.
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Stegomastodon primitivus (Proboscidea: Gomphotheriidae) records (latest Hemphillian-early Blancan) from the San Miguel Allende Basin, central Mexico
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.
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- Award ID(s):
- 1949742
- PAR ID:
- 10339546
- Date Published:
- Journal Name:
- New Mexico Museum of Natural History and Science Bulletin
- Volume:
- 88
- Page Range / eLocation ID:
- 77-95
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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