A PLIOCENE RECORD OF CAPROMERYX (MAMMALIA: ANTILOCAPRIDAE) IN MÉXICO

Journal of Paleontology, Nov 2004 by Jim�nez-Hidalgo, E, Carranza-Casta�eda, O, Montellano-Ballesteros, M

ABSTRACT-

The Antilocupridae was a diverse artiodactyl family present in some late Tertiary faunas of North America. In M�xico, its Tertiary fossil record is poorly known. The antilocaprid material described in this paper was collected from the early Blancan fluvial deposits of the San Miguel de Allende Area, state of Guanajuato, Mexico. It includes isolated upper and lower prcmolars and molars, dental series, and some rami fragments. The material is assigned to Capromeryx tauntonensis. The presence of this species in the early Pliocene of central Mexico represents the oldest record in North America and extends its known geographic distribution from the northwestern United States to central Mexico.

INTRODUCTION

THE ANTILOCAPRIDAE is a North American endemic family of ruminant artiodactyls that appeared in the fossil record in the early Hemingfordian (early Miocene); it showed its greatest diversity during the middle and late Miocene, and its diversity declined during the Pliocene (Blancan) and Pleistocene (Janis and Manning, 1998). The only surviving species of the family is Antilocapra americana (Ord, 1815), the pronghorn antelope, which lives in western North America. The group includes two subfamilies, the paraphyletic "Merycodontinae," which has a biochronologic range from early Hemingfordian (early Miocene) to late Clarendonian (late Miocene), and the Antilocaprinae, whose record extends from late Barstovian (middle Miocene) to Recent (Janis and Manning, 1998).

In M�xico, the fossil record of Antilocapridae spans from the middle Miocene to the late Pleistocene. Known Mexican records include the merycodont Merycodus Leidy, 1854 from the Barstovian of Oaxaca, southeastern Mexico (Jim�nez et al., 1999; Jim�nez-Hidalgo, 2000); Hexobelomeryx Furlong, 1941, from the Hemphillian of Chihuahua, Jalisco, Guanajuato, and Hidalgo (Furlong, 1941 ; Carranza-Castaneda and Ferrusquia-Villafranca, 1978; Lindsay, 1984; Carranza-Castaneda, 1989, 1994; Millerand Carranza-Castaneda, 1998, 2001; Castillo-Ceron, 2000) and the Blancan of Guanajuato (Carranza-Castaneda and Miller, 2000); Capromeryx Matthew, 1902, from the Rancholabrean of Aguascalientes, Hidalgo, Estado de Mexico, and Puebla (Furlong, 1925; Mooser, 1958; Mooser and Dalquest, 1975; Miller and CarranzaCastaneda, 1984; Cabral-Perdomo, 2000); Stockoceros Prick, 1937, from the Irvingtonian of Sonora and the Rancholabrean of Nuevo Leon, Zacatecas, and Aguascalientes (Mooser and Dalquest, 1975; Lindsay, 1984; Arroyo-Cabrales and Johnson, 1998; Guzm�n and Polaco, 2000); and Tetrameryx Lull, 1921, from the Rancholabrean of Aguascalientes, Jalisco, and Puebla (Downs, 1958; Mooser, 1958; Dalquest, 1974; Mooser and Dalquest, 1975; Miller and Carranza-Castaneda, 1984).

The small-sized pronghorn Capromeryx is represented in the United States by four species: C. tauntonensis Morgan and Morgan, 1995 from the early Blancan of Washington; C. arizonensis Skinner, 1942 from the early Blancan of Nebraska, late Blancan of Arizona, Florida, and Nebraska, as well as from several early Irvingtonian localities; C. furcifer Matthew, 1902 from diverse late Irvingtonian and early Rancholabrean localities; and C. minor Taylor, 1911, collected in numerous Rancholabrean localities (Skinner, 1942; Webb, 1974; Kurten and Anderson, 1980; Morgan and Morgan, 1995; Janis and Manning, 1998). In Mexico the only recognized species, C. mexicanus Furlong, 1925, has been recovered from sediments of late Pleistocene age in central and southern M�xico.

As a result of the paleontological work carried out in the late Tertiary continental sediments of central M�xico, especially in the San Miguel de Allende Graben, a large collection of mammals has been acquired. Its importance is based on the strict stratigraphic control exercised during the collecting.

Recent examination of the artiodactyl material from the San Miguel de Allende Area revealed the presence of an unidentified antilocaprid. Detailed study of this antilocaprid material supports its referral to the genus Capromeryx. The purpose of this paper is to describe the referred Capromeryx fossil material recovered from several Blancan localities of the subject area.

GEOLOGIC SETTING

Location.-The San Miguel de Allende Area is located in the northeastern part of the state of Guanajuato, central Mexico, within the northern limit of the Transmexican Volcanic Belt. The landscape of the study area is characterized by low hills and flat lands within the San Miguel de Allende Graben. The main access to the area is by Highway 51, the road that connects the cities of San Miguel de Allende and Dolores Hidalgo (Fig. 1).

Lithostratigraphy.-Fluvial sediments of late Miocene to late Pliocene age, informally named Rancho Viejo beds by CarranzaCasta�eda et al. (1994), bear an important mammalian fauna. The Capromeryx material comes from yellow brown sandy and silty clays with some lenses of volcanic ash and sand, which has been interpreted as floodplain deposits (Fig. 2). The ash deposits were dated by fission-track and ^sup 40^Ar/^sup 39^Ar, yielding ages of 3.60 and 3.36 Ma (Kowallis et al., 1986, 1998).