Pleistocene vertebrates from Rio Arriba and Taos counties, northernmost New Mexico

—Nine Pleistocene vertebrate fossil sites are known from Rio Arriba and Taos counties in northernmost New Mexico: Gobernador, Youngsville, Abiquiu, Santa Cruz, Ojo Caliente, and San Antonio Mountain (SAM) Cave in Rio Arriba County and Mesa Vibora, Picuris, and Vadito in Taos County. The Gobernador site in northwestern Rio Arriba County is in the Colorado Plateau physiographic province, and the Santa Cruz site in southeastern Rio Arriba County is in the Basin and Range province; the other seven sites are located in the Southern Rocky Mountains province. All nine Pleistocene sites from these two counties occur above 6000 ft (1830 m) in elevation, and SAM Cave is the highest Pleistocene site in New Mexico at 8980 ft (2737 m). Six of the nine sites from Rio Arriba and Taos counties contain a single species of large mammal: Gobernador (Equus niobrarensis), Youngsville (Mammuthus columbi), Ojo Caliente (proboscidean, probably mammoth), Mesa Vibora (Bison sp.), Picuris (Mammuthus sp.), and Vadito (Mammuthus columbi). The Abiquiu site has three large mammals (Euceratherium collinum, Bison antiquus, and Mammuthus sp.), and Santa Cruz has five large mammals (Canis dirus, Equus sp., Camelops hesternus, Bison sp., and Mammuthus sp.). SAM Cave has the most diverse Pleistocene vertebrate fauna in this region (41 species), almost all of which are small species. SAM Cave is the oldest Pleistocene vertebrate fauna in these two counties, dating to the late early Pleistocene (medial Irvingtonian, ~0.74-0.99 Ma). The Youngsville, Abiquiu, Santa Cruz, Mesa Vibora, and Vadito sites are medial or late Pleistocene (Rancholabrean) in age based on the presence of Bison or Mammuthus columbi, both of which are typical of Rancholabrean faunas. The only Pleistocene site from this region with an absolute date is Mesa Vibora; an AMS radiocarbon date of 24,740 ± 140 years Before Present, on bone collagen from a Bison humerus. FIGURE 1. Map of New Mexico showing the location of Pleistocene vertebrate sites in Rio Arriba and Taos counties. Numbers refer to the following sites described in the text: 1. Gobernador; 2. Youngsville; 3. Abiquiu; 4. Santa Cruz; 5. Ojo Caliente; 6. San Antonio Mountain (SAM) Cave; 7. Mesa Vibora; 8. Picuris; 9. Vadito. 417 PLEISTOCENE VERTEBRATES FROM RIO ARRIBA AND TAOS COUNTIES the earliest mammoth records in North America (~1.6 Ma) are from early Irvingtonian sites in New Mexico—Tijeras Arroyo in Bernalillo County and Mesilla Basin fauna C in Doña Ana County—although neither of these sites occurs in the region covered in this paper. The only Irvingtonian vertebrate fauna in this area is the medial Irvingtonian SAM Cave fauna. Late medial and late Pleistocene (~10-300 ka) vertebrate faunas are placed in the Rancholabrean NALMA, defined by the first appearance in North America of the genus Bison, also an immigrant from Eurasia. Three of the faunas in this region, Abiquiu, Santa Cruz, and Mesa Vibora, contain Bison, and are thus Rancholabrean in age. Two other sites, Youngsville and Vadito, lack Bison but have the advanced mammoth Mammuthus columbi, which is also typical of Rancholabrean faunas. Abbreviations used in this paper are: NALMA (North American land-mammal “age”); yrBP (radiocarbon years Before Present); AMNH (American Museum of Natural History); F:AM (Frick American Mammals, AMNH); MCZ (Museum of Comparative Zoology, Harvard University); NMMNH (New Mexico Museum of Natural History); RHMP (Ruth Hall Museum of Paleontology, Ghost Ranch, New Mexico); UTEP (University of Texas, El Paso). PLEISTOCENE VERTEBRATE SITES Gobernador—The Gobernador site is located about 1 km southeast of Gobernador, northwestern Rio Arriba County, New Mexico (Fig. 1, site 1). No further data are available on the exact location or stratigraphic occurrence of this site. The vertebrate paleontology collection at the American Museum of Natural History has three associated lower cheek teeth (AMNH 32662) of the large horse Equus niobrarensis from the Gobernador site. Youngsville—Lucas et al. (this volume, road log) discuss several mammoth fossils from a site near Youngsville in Rio Arriba County (NMMNH site L-2639; Fig. 1, site 2). A partial mammoth tooth and tusk were collected from a cobble bed associated with the channel of the Rito Encino, just north of NM State Route 96, about 1 km west of Youngsville (36o11’N, 106o34’W). In the NMMNH locality records, this site is referred to as the “Youngsville Mammoth.” The fossils were discovered in 1992 during road construction on New Mexico State Route 96. Peter Reser and an NMMNH field crew excavated the Youngsville mammoth on 3-5 June 1992. The fossils from Youngsville consist of two tooth fragments and a partial tusk of the Columbian mammoth Mammuthus columbi (NMMNH 7177; Figs. 2A-C). Morgan and Lucas (2005) first mentioned the presence of M. columbi from Youngsville. The tooth fragments are identified as M. columbi on the basis of their thin, complicated enamel (enamel thickness <3.1 mm) and thin plates (plate thickness 8-10 mm). The presence of M. columbi suggests a late Pleistocene (Rancholabrean) age for the Youngsville site. Abiquiu—The Abiquiu site (Fig. 1, site 3) was discovered in the early 1960s during large-scale valley-floor excavations for the Abiquiu Dam on the Rio Chama, about 10 km northwest of Abiquiu, Rio Arriba County, New Mexico (36o14’N, 106o26’W). The exact provenance, stratigraphic occurrence, and faunal associations of the fossils from the Abiquiu site are unknown. Simpson (1963, p. 583) noted that “...a few remains of comparatively large animals were noticed [from the Abiquiu Dam site] and have been saved through the interest of William H. Carr of the Ghost Ranch Musem...” Carr donated two horn cores of the shrub ox Euceratherium from the Abiquiu site to the Museum of Comparative Zoology (MCZ) at Harvard University. The remainder of the fossils collected by Carr from Abiquiu Dam, including two partial skulls with horn cores of Bison and several postcranial elements of mammoth, were originally housed in the Ghost Ranch Living Museum, but have since been transferred to the Ruth Hall Museum of Paleontology (RHMP) at Ghost Ranch, New Mexico. FIGURE 2. Partial teeth of mammoths from Rio Arriba and Taos counties, New Mexico. A, B, occlusal views and C, side view, two tooth fragments of Mammuthus columbi from Youngsville, Rio Arriba County, NMMNH 7177; D, F, occlusal views and E, G, side views, partial tooth of M. columbi from Vadito, Taos County, NMMNH 45013. 418 MORGAN AND LUCAS The vertebrate fauna from the Abiquiu site consists of three large mammals: the shrub ox Euceratherium collinum, the extinct bison Bison antiquus, and the mammoth Mammuthus sp. Simpson (1963) identified two horn cores from the Abiquiu Dam site as either Euceratherium or Preptoceras. Kurtén and Anderson (1980) referred these horn cores to Euceratherium collinum, as did Harris (1993). The two Euceratherium horn cores from Abiquiu (MCZ 8437 and 8438) are housed in the vertebrate paleontology collection at the Museum of Comparative Zoology (see Figures 3A-C for illustrations of MCZ 8438). MCZ 8437 is the larger of the two individuals, consisting of most of a horn core, broken at the tip and base. MCZ 8438 is from a smaller individual but is more complete, including the horn core with a broken tip, the base of the horn core, and part of the cranium. Although they differ in size, the two horn cores are similar in morphology and clearly belong to the same species. As is typical for the genus Euceratherium, the horn cores of the Abiquiu specimens are compressed at their base and rise at a rather steep angle dorsally from the frontals. Simpson (1963) compared the Abiquiu horn cores to two similar horn cores from Burnet Cave, New Mexico. Although Schultz and Howard (1935) originally referred the Burnet Cave horn cores to two separate species in two different genera (Euceratherium collinum and Preptoceras sinclairi), it seems fairly certain that the Burnet Cave and Abiquiu specimens belong to the same species, now referred to Euceratherium collinum. E. collinum is known from three other late Pleistocene sites in New Mexico: Burnet Cave (Schultz and Howard, 1935) and Musk Ox Cave (Logan, 1981), both in the Guadalupe Mountains in Eddy County, southeastern New Mexico, and U-Bar Cave in Hidalgo County, southwestern New Mexico (Harris, 1993). The Ruth Hall Museum of Paleontology at Ghost Ranch near Abiquiu has two partial skulls with horn cores of Bison antiquus from the Abiquiu site. RHMP 1058 is a partial cranium, nearly complete posterior to the orbits with both right and left horn cores (Figs. 4A, B). The tip of the left horn core is damaged. RHMP 1059 is a partial cranium possessing the left horn core and the cranium posterior to the orbits on the left side (Figs. 4C, D). Both skulls have an intact occipital region. The measurements of these two partial skulls (Table 1) are within the observed range of a large series of measurements of B. antiquus skulls in McDonald (1981, table 21). Furthermore, the large size of the Abiquiu Bison horn cores, particularly RHMP 1059, which is near the maximum size for B. antiquus in several horn core measurements, strongly suggests that both skulls are males. RHMP 1058 and 1059 closely match the diagnosis and description of horn core characters of B. antiquus in McDonald (1981), including: the length of the horn core along the upper curve is less than the circumference of the horn core at its base (RHMP 1058, upper curve 284 mm, circumference at base 335 mm; RHMP 1059, upper curve 305 mm, circumference at base 401 mm); the horn cores project subhorizontally from the frontals, are triangular in cross section at their base, taper gradually toward their tip, and are not strongly curved. Both Abiquiu skulls agree in size and morphological characters with the extinct late Pleistocene B. antiquus, and are confidently referred to that species. Other fossils from Abiquiu include a sac