Rodents from the Hagerman local fauna, Upper Pliocene of Idaho [Les rongeurs de la faune locale d'Hagerman, pliocène supérieur de l'Idaho].

Résumé

The Hagerman local fauna is known from deposits of the Glenns Ferry Formation exposed west of the town of Hagerman in Twin Falls County, Idaho. Approximately 300 localities are known over an area of six square miles through 500 feet of beds (elevation 2900 to 3400 feet above sea level). Rodents are represented in the fauna by a t least 14 genera and 19 species. These include five species from the family Sciuridae, two from the Geomyidae, three from the Heteromyidae, two from the Castoridae, and seven from the Cricetidae. Five of the species are described as new: Pliogeomys parvus, Perognathus magnus, P. maldei, Dipoides intermedius, and Baiomys aquilonius. The beaver Castor and the microtines Cosomys and Pliopotamys are the most abundant mammalian forms in the fauna. This fact suggests a marsh-meadow habitat near the depositional area. Upland forms, though rare in the fauna, become relatively more abundant above 3100 feet. Changes in the population structure of the microtines are demonstrated as one goes higher in the section. These changes include a slight increase in size for Cosomys and Pliopotamys and an increase in the number of triangles on the MI in Cosomys and Ophiomys. There is also a gradual replacement of Cosomys by Pliopotamys and Ophiomys, which apparently becomes complete above the elevation of 3265 feet. On the basis of the fauna and a potassium-argon date of 3.48 t 0.27 X 10' years on a basalt which has been correlated with the basalt approximately 150 feet below the Horse Quarry, the Hagerman local fauna is considered to be Upper Pliocene in age. Comparison with other Upper Pliocene local faunas in regard to the stage of evolution of the rodents suggests a post-Rexroad age for the Hagerman local fauna.