San Antonio Mountain (SAM) Cave:

San Antonio Mountain (SAM) Cave is located in the San Luis basin about 10 km south of the Colorado border and 7 km northwest of San Antonio Mountain in Rio Arriba County in northernmost New Mexico (fig. 12.1, site 1). Several factors distinguish SAM Cave from all other Blancan and Irvingtonian localities in New Mexico, in particular, its mode of occurrence and high elevation (Rogers et al., 2000). SAM Cave is a lava tube formed in the Pliocene Servilleta Basalt, and the fossils occur in locally derived cave sediments. All other sites discussed in this paper are derived from alluvial, fluvial, or lacustrine sediments associated with the ancestral Rio Grande or Gila Rivers. SAM Cave is also considerably higher in elevation (2737 m) than any other Blancan or Irvingtonian site in New Mexico. Eleven localized sites within SAM Cave have produced vertebrate fossils, 10 of which are medial Irvingtonian in age (NMMNH sites L-4385–4390, 4392–4395) and 1 of which is Rancholabrean (NMMNH site L-4381). Among the medial Irvingtonian sites, there appears to be a range of ages from about 1.0 Ma to just after the Matuyama/Brunhes boundary at 0.78 Ma (Rogers et al., 2000). In table 12.2 and in the following discussion, we have arbitrarily combined the nine medial Irvingtonian sites in Rogers et al. (2000, table 2) that date to the late Matuyama Chron, after the Jaramillo Subchron and before the Matuyama/Brunhes boundary (between about 1.0 and 0.78 Ma). A 10th fauna (NMMNH site L-4385) occurs in normally magnetized sediments of the Brunhes Chron, but is not included here because it contains no species of mammals not found in the older sites.

The combined vertebrate assemblage from the nine medial Irvingtonian sites in SAM Cave includes a minimum of 41 species (faunal list from Rogers et al., 2000, complete list of mammals in table 2): one species of trout; 2 species of amphibians, tiger salamander (Ambystoma tigrinum), and chorus frog (Pseudacris triseriata); 3 reptiles, horned lizard (Phrynosoma douglassii), rattlesnake (Crotalus viridis), and garter snake (Thamnophis elegans); 9 birds, short-eared owl (Asio cf. A. flammeus), least grebe (Tachybaptus cf. T. dominicus), vireo (Vireo sp.), chickadee (Parus sp.), junco (Junco sp.), savannah sparrow (Passerculus cf. P. sandwichensis), sparrow (Ammodramus), and 2 wood warblers (Family Parulidae); and 26 species of mammals, including a shrew, a bat, 5 carnivores, 2 lagomorphs, and 17 rodents. The SAM Cave vertebrate assemblage is dominated by small species, and thus it is very difficult to make comparisons with other New Mexico Irvingtonian faunas, which are composed primarily of large mammals. Therefore, comparisons are mostly with Irvingtonian small mammal faunas outside of New Mexico, in particular, Hansen Bluff, located about 40 km northeast of SAM Cave in southern Colorado (Rogers et al., 1992). The only larger mammals in SAM Cave are the badger Taxidea taxus and the wolf Canis rufus. However, the identification of C. rufus from Sam Cave is questionable, and it is more likely to be the Irvingtonian wolf C. armbrusteri (see Berta, 1995).

The age of the SAM Cave vertebrate assemblage is determined primarily by the biochronology of its extensive microtine (= arvicoline) rodent fauna (Rogers et al., 2000). The presence of the microtines Mictomys kansasenis, Allophaiomys, Lemmiscus curtatus, and Microtus cf. M. californicus helps date the oldest sites in SAM Cave. The first appearance of M. kansasenis is in the late early Irvingtonian Sappa Fauna of Nebraska (1.3 Ma; Martin and Schultz, 1985). The presence of an advanced unnamed species of Allophaiomys indicates an age greater than 0.85 Ma, based on comparisons with the nearby Hansen Bluff fauna (Repenning, 1992; Rogers et al., 2000). SAM Cave apparently documents the evolution of the sagebrush vole Lemmiscus from Allophaiomys between about 1.0 and 0.85 Ma (Rogers et al., 2000). The occurrence of Microtus cf. M. californicus indicates that the fauna is younger than the beginning of the Jaramillo Subchron (1.07 Ma; Repenning, 1992). The red-backed vole Clethrionomys first appears in two SAM Cave sites that date from 0.85 Ma to just above the Matuyama/Brunhes boundary. The occurrences of Clethrionomys and Lemmiscus in SAM Cave represent the oldest records of these genera in North America (Rogers et al., 2000).

The presence of Mictomys kansasenis, Allophaiomys, Lemmiscus curtatus, and Microtus cf. M. californicus in SAM Cave indicates an age range between about 1.0 Ma and 0.85 Ma (Rogers et al., 2000), which places this fauna in the early part of the medial Irvingtonian. A slightly younger medial Irvingtonian site in SAM Cave contains Clethrionomys, and is between 0.85 and 0.78 Ma in age. SAM Cave is younger than other Irvingtonian faunas known from New Mexico, including Tijeras Arroyo, Tortugas Mountain, and Mesilla Basin Fauna C, all of which are early Irvingtonian.

Ancha Sites:

Blancan mammals are known from two sites in the southernmost Española basin southwest of Santa Fe in Santa Fe County (fig. 1, site 3). These two sites represent the northernmost occurrences of Blancan mammals in New Mexico. A volcaniclastic unit correlative with the Ancha Formation, exposed in an abandoned cinder quarry near the Santa Fe Airport (NMMNH site L-3116), preserves a trackway (NMMNH 25583) of a large camelid, either Camelops or one of the giant Blancan camels (e.g., Gigantocamelus). The camel tracks occur in a fine-grained volcaniclastic deposit that is part of the Cerros del Rio volcanic field, dated to between 2.8 and 2.3 Ma (Koning et al., 2001). In 1999, at a second site about 10 km farther south and about 2 km west of Turquoise Hill (NMMNH site L-4321), Dan Koning collected a mandible with p4–m3 (NMMNH 30493) of the prairie dog Cynomys in a sandy unit about 6 m below the local top of the Ancha Formation. In a review of the fossil history of prairie dogs, Goodwin (1995) noted that the earliest fossil record of the genus Cynomys is late Blancan. The mandible from the Ancha Formation most closely resembles the extinct species C. hibbardi from the late Blancan White Rock LF in Kansas (Eshelman, 1975).

Santo Domingo:

The Santo Domingo LF (fig. 12.1, site 4) was derived from axial river gravels of the Sierra Ladrones Formation (Smith and Kuhle, 1998), east of the Rio Grande near the Santo Domingo Pueblo in Sandoval County (Tedford, 1981). Tedford identified the horses Equus calobatus and E. scotti from the Santo Domingo fauna, both of which are known from the late Blancan and early Irvingtonian. Recent examination of fossils from Santo Domingo in the Frick Collection revealed two additional taxa, the small three-toed horse Nannippus peninsulatus and the large peccary Platygonus cf. P. bicalcaratus, both of which are indicative of Blancan faunas. The presence of Nannippus suggests an age of 2.2 Ma or older. Tedford noted that these deposits are interbedded with the Santa Ana Mesa basalts, dated at 2.67 Ma and 2.41 Ma (Smith and Kuhle, 1998), and are overlain by the lower Bandelier Tuff dated at 1.61 Ma (Izett and Obradovich, 1994). The biostratigraphic and geochronologic constraints indicate an age between 2.7 and 2.2 Ma for the Santo Domingo LF.

Western Mobile:

Two species of mammals are known from the Western Mobile gravel pit (NMMNH Site L-1546; fig. 12.1, site 5), located about 3 km northeast of Bernalillo in Sandoval County (Lucas et al., 1993; Morgan and Lucas, 2000a). These fossils, including glyptodont scutes and a proboscidean tusk fragment, were collected as float and thus lack precise stratigraphic provenance, although they were almost certainly derived from the Sierra Ladrones Formation. The most diagnostic fossils from the Western Mobile site are five well-preserved interior carapacial osteoderms of the glyptodont Glyptotherium arizonae. These osteoderms are identified as G. arizonae on the basis of their large size, comparatively great thickness, and small, flat central figure (Gillette and Ray, 1981). G. arizonae also occurs in the early Irvingtonian Tijeras Arroyo fauna, located about 30 km south (see below). Most other published records of G. arizonae from the southwestern United States are early Irvingtonian in age, including Gilliland and Rock Creek in Texas, Holloman in Oklahoma (Gillette and Ray, 1981), and Mesilla Basin Fauna C from the Mesilla basin in southernmost New Mexico (Vanderhill, 1986). This glyptodont is also known from two latest Blancan sites in New Mexico, Mesilla Basin Fauna B and the Virden LF in the Duncan basin in the southwestern part of the state (Morgan and Lucas, 2000b). The Western Mobile glyptodont scutes are also significant because the location of the gravel pit at about 35°20′ North latitude represents one of the northernmost records for the genus Glyptotherium in North America (Gillette and Ray, 1981).

Loma Colorado de Abajo:

Loma Colorado de Abajo is a prominent hill within the city limits of Rio Rancho in Sandoval County, about 20 km northwest of Albuquerque in the northern Albuquerque basin (fig. 12.1, site 6). Between 1990 and 1996, Paul Knight collected several skulls of small rodents from indurated, fine-grained reddish sandstones near the base of the exposed section on the south-facing escarpment of Loma Colorado de Abajo (NMMNH Site L-1462). The fossiliferous level is in the Loma Barbon Member in the upper part of the Arroyo Ojito Formation of Connell et al. (1999), about 8 m below the base of the overlying Ceja Member of the same formation. The Loma Colorado de Abajo LF consists of just three taxa (Morgan and Lucas, 1999, 2000a), a small land tortoise and two genera of rodents, Spermophilus and Geomys. The same stratum from which the rodent fossils were collected also contains numerous ichnofossils that appear to be rodent burrows. The Loma Colorado de Abajo LF is unique among New Mexico Blancan faunas in consisting entirely of small burrowing vertebrates.

A ground squirrel of the genus Spermophilus is represented in the Loma Colorado de Abajo LF by a partial skull with P4 from a small species in the size range of living S. tridecemlineatus. It is considerably smaller than Spermophilus cf. S. bensoni from the Blancan of southeastern Arizona (Tomida, 1987), a species tentatively identified from the early Blancan Buckhorn LF in southwestern New Mexico (Morgan et al., 1997). The Loma Colorado Spermophilus skull is also smaller than S. pattersoni and S. matachicensis from the late Hemphillian Yepómera Fauna in northern Mexico (Wilson, 1949; Lindsay and Jacobs, 1985). Three specimens from Loma Colorado de Abajo are provisionally referred to the primitive pocket gopher, Geomys (Nerterogeomys) minor, including a nearly complete skull, a rostrum with a complete dentition, and an edentulous left mandible. The two skulls are identified as Geomys on the basis of their bisulcate upper incisors, unrooted cheek teeth, and absence of enamel on the posterior surface of P4. Earlier pre-Blancan geomyids such as Pliogeomys have rooted cheek teeth. The fragmentary mandible lacks cheek teeth, but can be identified as a member of the extinct subgenus Geomys (Nerterogeomys) by the placement of the mental foramen ventral to the masseteric crest (Tomida, 1987). Geomys (Nerterogeomys) first appears in the early Blancan and becomes extinct in the early Irvingtonian. The Loma Colorado pocket gopher skulls are smaller than most described skulls of Geomys (Nerterogeomys), and compare most closely to the small species, G. minor, known from the early Blancan Rexroad Fauna in Kansas and Verde LF in Arizona, and the medial Blancan Beck Ranch LF in Texas and Benson Fauna in Arizona (Hibbard, 1967; Dalquest, 1978; Czaplewski, 1990). Repenning and May (1986) reported G. minor from the early Blancan Truth or Consequences LF from the Palomas Formation in Sierra County in central New Mexico. The Loma Colorado mandible is smaller than pocket gopher mandibles from the Pajarito and Belen faunas in the Albuquerque basin referred to G. (Nerterogeomys) paenebursarius (see Morgan and Lucas, 2000a). The smaller species of G. (Nerterogeomys) that are most similar in size to the Loma Colorado Geomys (e.g., G. minor) are restricted to the Blancan, whereas the species that survive into the Irvingtonian (e.g., G. anzensis, G. garbanii, and G. persimilis) are larger.

The age of the Loma Colorado de Abajo LF is probably early or medial Blancan. Small species of Geomys (Nerterogeomys), such as G. minor, are typical of faunas of this age. Also, the medial to late Blancan (older than 2.2 Ma) Mountainview LF is known from the Ceja Member of the Arroyo Ojito Formation in Tijeras Arroyo, a unit that overlies the Loma Barbon Member. The Loma Colorado de Abajo LF is stratigraphically below and thus older than the Blancan fauna from Tijeras Arroyo. However, these two faunas have no taxa in common, so more detailed biostratigraphic comparisons are not possible.

Mountainview:

Most of the vertebrate fossils from Tijeras Arroyo, located just south of the Albuquerque International Airport in Bernalillo County, are derived from the Sierra Ladrones Formation and are early Irvingtonian in age (Lucas et al., 1993; see below). However, one locality (NMMNH Site L-1458) at the base of the exposed stratigraphic section in Tijeras Arroyo (fig. 12.1, site 7) has produced two species that are indicative of a Blancan age. This site is here named the Mountainview Local Fauna to eliminate any confusion with the younger Tijeras Arroyo Irvingtonian sites. The fossils from the Mountainview site were derived from a sandstone comprising unit 1 in the stratigraphic section of Lucas et al. (1993). The lowermost part of the section in Tijeras Arroyo, including unit 1, was referred to the Ceja Member of the Arroyo Ojito Formation by Connell et al. (1999).

Lucas et al. (1993) referred a partial mandible with p3 from Mountainview to the rabbit Hypolagus cf. H. gidleyi. This rabbit is found in late Hemphillian and Blancan faunas, but is unknown from the Irvingtonian (White, 1987). The horse Equus cf. E. cumminsii was identified from this site based on a partial skull with a nearly complete dentition and several associated postcranial elements (Lucas et al., 1993). E. cumminsii occurs in several medial and late Blancan sites in Texas, but is unknown from the Irvingtonian (Kurtén and Anderson, 1980). There are two other Blancan records of E. cumminsii from New Mexico, the medial Blancan Arroyo de la Parida LF in Socorro County (Tedford, 1981; Lucas and Morgan, 1996) and the late Blancan Pearson Mesa LF in Hidalgo County (Morgan and Lucas, 2000b).

Both mammals from the Mountainview site in the lower portion of the Tijeras Arroyo section, Hypolagus cf. H. gidleyi and Equus cf. E. cumminsii, are typical of Blancan faunas, and do not occur in the Irvingtonian. The extinction in the late Pliocene of several characteristic Blancan genera (the Nannippus extinction event of Lindsay et al. 1984), including Hypolagus, indicates that the Mountainview site is older than 2.2 Ma. Equus cf. E. cumminsii is absent from early Blancan faunas (Kurtén and Anderson, 1980). These two species suggest that Mountainview is medial to late Blancan in age (between 3.6 and 2.2 Ma). The absence of Guaje Pumice from the Ceja Member of the Arroyo Ojito Formation in the lower part of the Tijeras Arroyo section indicates that these beds predate deposition of the Bandelier Tuff at 1.6 Ma (Lucas et al., 1993).

Tijeras Arroyo:

Ten localities in Tijeras Arroyo (fig. 12.1, site 8) have produced a significant vertebrate fauna (table 12.2) of early Irvingtonian age (Lucas et al., 1993; Morgan and Lucas, 2000a). More than 75 m of the Sierra Ladrones Formation is exposed in Tijeras Arroyo, consisting of sandstones, pumiceous sandstones, and gravels, with minor amounts of mudstone and diatomite. These sediments represent axial river deposits of an ancestral Rio Grande. The most important lithological marker in these beds is the presence of reworked Guaje Pumice derived from the Bandelier Tuff, Ar/Ar dated at 1.61 Ma (Izett and Obradovich, 1994), in the units associated with an Irvingtonian fauna (units 6–8 of Lucas et al., 1993). An extensive flora of leaves and pollen from a localized volcanic ash bed was collected in the Tijeras Arroyo section (NMMNH Site L-1445). The Tijeras Arroyo flora indicates that the cottonwood forest or bosque currently found along the banks of the Rio Grande in New Mexico dates back to the early Pleistocene (Knight et al., 1996).

The land tortoise Hesperotestudo and five species of mammals, including Glyptotherium cf. G. arizonae, Equus scotti, Equus sp., Camelops sp., and Mammuthus meridionalis occur together in the Tijeras Arroyo section above the Mountainview Blancan site discussed above (Lucas et al., 1993; Morgan and Lucas, 2000a). These species constitute a fairly typical fauna of early Irvingtonian age. Three additional species of mammals, a small species of Equus, the llama Hemiauchenia macrocephala and the mammoth Mammuthus imperator, occur somewhat higher in the Tijeras Arroyo section than the remainder of the fauna, but probably are Irvingtonian age as well.

A caudal osteoderm of a glyptodont from Tijeras Arroyo (Lucas et al., 1993) probably is not diagnostic at the species level, although this specimen almost certainly represents Glyptotherium arizonae. Tentative referral of this osteoderm to G. arizonae is reasonable as its association with Mammuthus rules out a Blancan age, and the Rancholabrean G. floridanum is restricted to the Atlantic and Gulf coastal plains (Gillette and Ray, 1981). The large horse Equus scotti is the most common mammal in the Tijeras Arroyo Irvingtonian fauna, represented by mandibles, isolated teeth, and postcranials (Lucas et al., 1993; Morgan and Lucas, 2000a). E. scotti is the typical large horse in late Blancan and early Irvingtonian faunas in the southwestern United States (Hibbard and Dalquest, 1966), and occurs in medial Blancan through early Irvingtonian faunas in New Mexico (Tedford, 1981; Morgan et al., 1998). A complete equid metacarpal from Tijeras Arroyo is more slender than metacarpals of E. scotti, and represents a second, smaller species of Equus (Hibbard and Dalquest, 1966; Harris and Porter, 1980). A partial skull of a small Equus occurs higher in the Tijeras Arroyo section.

Lucas and Effinger (1991) and Lucas et al. (1993) referred a mandible with left and right m3 from Tijeras Arroyo to the primitive mammoth Mammuthus meridionalis on the basis of its low plate count and extremely thick enamel. This is one of only two records of mammoths from New Mexico referred to M. meridionalis, indicating that this fauna is almost certainly early Irvingtonian. The other record consists of several partial teeth, tentatively referred to M. meridionalis, from an early Irvingtonian fauna in the Mesilla basin (Vanderhill, 1986). Lucas et al. (1993) referred a left M3 in a maxillary fragment from Tijeras Arroyo to the mammoth Mammuthus imperator. The teeth of M. imperator are more advanced than M. meridionalis in having a higher plate count, higher lamellar frequency, and thinner enamel. The M. imperator specimen was found about 12 m higher in the section than the remainder of the Tijeras Arroyo fauna, and thus is somewhat younger, although an Irvingtonian age is still likely (Lucas et al., 1993).

The presence of mammoths in unit 6 and above clearly establishes an Irvingtonian age for the upper part of the Tijeras Arroyo section, as Mammuthus is one of the defining genera for the Irvingtonian NALMA. The first appearance of Mammuthus in the New World occurred sometime in the early Pleistocene (early Irvingtonian) between about 1.8 and 1.6 Ma. The mammoth jaws from Tijeras Arroyo represent one of the oldest well-documented records of Mammuthus from North America, based on an Ar/Ar date of 1.61 Ma on Guaje Pumice from the Sierra Ladrones Formation in Tijeras Arroyo (Lucas et al., 1993; Izett and Obradovich, 1994; Lucas, 1995, 1996). Although the pumice date provides a maximum age for this site, evidence from other pumice deposits of exactly the same age farther south in the Rio Grande Valley (Mack et al., 1996, 1998) indicates that the pumice is very close in age to the fossils. The association of M. meridionalis with Glyptotherium arizonae and Equus scotti is indicative of an early Irvingtonian age for the Tijeras Arroyo fauna. Correlative early Irvingtonian faunas include the Tortugas Mountain LF (Lucas et al., 1999, 2000) and Mesilla Basin Fauna C (Vanderhill, 1986) from the Mesilla basin in southern New Mexico, Gilliland in Texas (Hibbard and Dalquest, 1966), and Holloman in Oklahoma (Dalquest, 1977).

Pajarito:

The Pajarito LF is located along the eastern escarpment of the Rio Puerco in southern Bernalillo County, extending from the Pajarito land grant to the northern end of the Isleta Reservation (fig. 12.1, site 9). This site was discovered in 1946 by Ted Galusha, Frick Laboratory, who collected a small sample of vertebrate fossils from just south of the northern boundary of the Isleta Reservation. According to Galusha's field notes (copy kindly provided by Richard Tedford), he found fossils in two distinct units, a lower unit with “potato-shaped” concretions and a pinkish sandy unit about 5 m higher in the section. We recently collected additional fossils from these same two units on the Pajarito land grant just north of the Isleta Reservation boundary. Maldonado and Atencio (1998) placed both the concretionary bed and the pink sand in their “silt, sand, and clay lithofacies of the Isleta Reservation.” A pumice that occurs between the two fossil-bearing units has been Ar/Ar dated at 3.12 ± 0.10 Ma (Maldonado et al., 1999). Tedford (1981) called this the Laguna site and attributed it to the Ceja Member, which was recently placed in the Arroyo Ojito Formation (Connell et al., 1999). Morgan and Lucas (2000a) renamed the Laguna site the Pajarito LF to eliminate any confusion regarding its location.

The Pajarito LF consists of eight species of vertebrates: a turtle, a bird, and six species of mammals, including three rodents, two camels, and a deer (table 12.1). The most diagnostic fossils are two pocket gopher mandibles referred to the extinct subgenus Geomys (Nerterogeomys). The Pajarito mandibles are similar in size and morphology to a Geomys mandible from the Belen Fauna (see below), all three of which are tentatively referred to Geomys (Nerterogeomys) paenebursarius, a species originally described from the late Blancan Hudspeth LF and Red Light LF in southwestern Texas (Strain, 1966; Akersten, 1972). A humerus of the ground squirrel Spermophilus and two rodent jaws with incisors but lacking cheek teeth are also known from the Pajarito LF. The rodent jaws represent a smaller species than Geomys or Spermophilus, but cannot be identified further pending the discovery of specimens with cheek teeth. The camelids from Pajarito represent two species, including an M3 referred to the common Blancan llama, Hemiauchenia cf. H. blancoensis, and a carpal from a larger camel, probably Camelops. An unidentified cervid is represented by a partial antler. Fossils occur in both the concretionary bed, which is just below the pumice dated at 3.12 Ma (Maldonado et al., 1999), and in the pink sand, which is just above the dated pumice. One of the two Geomys mandibles was collected from the concretionary bed and the other from the pink sand. These two specimens are indistinguishable, suggesting that these two units are similar in age. The radioisotopic date of 3.1 Ma indicates a medial Blancan age for the Pajarito LF. This age is in agreement with the evolutionary grade of the Geomys mandibles and with the absence of South American immigrant mammals, which first appear in North American faunas about 2.7 Ma. This site holds significant potential for new discoveries, as only a small amount of the exposed outcrop has been explored for fossils.

Isleta:

In 1999, Dave Love collected a nearly complete metacarpal of a small camelid on the east side of the Rio Grande, about 3 km northeast of the Isleta Pueblo (fig. 12.1, site 10), from strata that are probably correlative with the Arroyo Ojito Formation of Connell et al. (1999). The fossil was derived from a unit stratigraphically below the pumice-bearing sands of the Sierra Ladrones Formation that contain early Irvingtonian mammals in Tijeras Arroyo, located about 10 km to the north. This specimen is referable to the genus Hemiauchenia, but is considerably smaller than metacarpals of the typical Blancan species, H. blancoensis, known from several other Blancan faunas in New Mexico (e.g., a complete metacarpal from the medial Blancan Tonuco Mountain LF), as well as the type locality, the Blanco LF in Texas (Dalquest, 1975). A small and apparently undescribed species of Hemiauchenia, mostly represented by isolated postcranial elements, is found in New Mexico faunas ranging in age from early to late Blancan. Based on its stratigraphic position, the small Hemiauchenia metacarpal from Isleta is probably either medial or late Blancan in age.

Los Lunas:

The Los Lunas site (fig. 12.1, site 11) is located in strata of the Arroyo Ojito Formation that underlie the Los Lunas volcano, about 7 km west of Los Lunas in Valencia County (Tedford, 1981). A mandible of the giant marmot Paenemarmota, a genus found in Hemphillian and Blancan faunas in western North America (Kurtén and Anderson, 1980; Zakrzewski, 1998), was collected at this site. The large size of the Los Lunas mandible suggests referral to P. barbouri, a species that occurs from the early through the late Blancan (Morgan and Lucas, 2000a). A K/Ar date of 1.1–1.3 Ma on the andesite of the Los Lunas volcano (Bachman and Mehnert, 1978), which overlies the strata that yielded the Paenemarmota jaw, provides a minimum age for the Los Lunas site (Tedford, 1981).

Several postcranial bones of a large camel were collected from the Arroyo Ojito Formation a few kilometers northwest of the Paenemarmota site, about 1 km north of New Mexico Route 6 (NMMNH Site L-3738; Morgan and Lucas, 2000a). These specimens, including a partial distal radio-ulna and a magnum, probably represent a large Camelops. A large undescribed species of Camelops occurs in several medial and late Blancan faunas elsewhere in New Mexico (Vanderhill, 1986; Morgan et al., 1998).

Belen:

Beginning south of the Los Lunas volcano and extending south of Belen into northern Socorro County, badlands representing the Arroyo Ojito Formation of Connell et al. (1999) are well exposed in an east-facing escarpment just west of Interstate Highway 25 and several kilometers west of the Rio Grande. The NMMNH has two collections of Blancan vertebrates from southwest of Belen in Valencia County (fig. 12.1, site 12). In 1992, Bill Wood collected vertebrate fossils about 5 km southwest of Belen (NMMNH Site L-3778). Fossils from this site include lower jaws of the gomphotheriid proboscidean Stegomastodon mirificus and postcranial elements of Equus. Christopher Whittle and several students collected fossils from conglomeratic sandstone and slightly indurated sandstone about 2 km southwest of Belen (NMMNH Site L-3737), about 4 km north of site L-3778. Fossils from this site include a snake, the mole Scalopus, Geomys, Equus, and a small antilocaprid. Because of the close proximity of sites L-3737 and 3778 southwest of Belen and their occurrence in similar strata referred to the Arroyo Ojito Formation, the fossils from these two sites are combined as the Belen Fauna (Morgan and Lucas, 2000a).

The Belen Fauna (Morgan and Lucas, 2000a) is composed of five species of mammals, including Scalopus (Hesperoscalops) cf. S. blancoensis, Geomys (Neterogeomys) cf. G. paenebursarius, Equus cf. E. calobatus, a small antilocaprid, and Stegomastodon mirificus. A dentary with m1–m3 from the Belen Fauna is the first mole (family Talpidae) ever reported from New Mexico, recent or fossil (Morgan and Lucas, 1999, 2000a). This mole is referred to Scalopus (Hesperoscalops), an extinct subgenus of Scalopus restricted to the Blancan. Three species of S. (Hesperoscalops) have been described, S. sewardensis from the very early Blancan Saw Rock Canyon LF in Kansas, S. rexroadi from the early Blancan Rexroad and Fox Canyon faunas in Kansas and the medial Blancan Beck Ranch LF in Texas, and S. blancoensis from the late Blancan Blanco LF in Texas (Hibbard, 1953; Dalquest, 1975, 1978; Kurtén and Anderson, 1980). The Belen dentary is tentatively referred to S. blancoensis based on its similarity to that species in size and morphological features. A dentary with a complete dentition from Belen is identified as the extinct pocket gopher subgenus Geomys (Nerterogeomys). The morphology and size of this mandible are similar to the species G. (N.) paenebursarius, also identified from the Pajarito LF, and first described from the late Blancan Hudspeth and Red Light LFs of southwestern Texas (Strain, 1966; Akersten, 1972).

The most common fossils in the Belen Fauna are postcranial elements of horses of the genus Equus, most of which are not diagnostic at the species level. A nearly complete metatarsal is tentatively referred to the large, stilt-legged horse, E. calobatus, a species known from the late Blancan Santo Domingo LF (Tedford, 1981) and from late Blancan and early Irvingtonian faunas in the Mesilla basin (Vanderhill, 1986). A well-preserved pair of mandibles with right and left m2–m3 is referred to the gomphothere Stegomastodon mirificus. The presence of seven lophids on m3 separates this specimen from Rhynchotherium and Cuvieronius, and the highly complicated enamel with double trefoiling distinguishes the teeth from the more primitive species S. rexroadensis.

Four mammals in the Belen Fauna are age diagnostic. The extinct subgenus Scalopus (Hesperoscalops) is restricted to the Blancan and the species S. blancoensis occurs in the late Blancan. Geomys (Nerterogeomys) paenebursarius is known from two late Blancan faunas in southwestern Texas (Strain, 1966; Akersten, 1972), and the medial Blancan Pajarito LF in the northern Albuquerque basin (Tedford, 1981; Morgan and Lucas, 2000a). Stegomastodon mirificus is known from the medial Blancan through the early Irvingtonian, and Equus calobatus occurs in the late Blancan and Irvingtonian (Kurtén and Anderson, 1980). The age of the Belen Fauna is either medial or late Blancan. S. blancoensis and E. calobatus occur in late Blancan faunas, but are not known from the medial Blancan, whereas G. (N.) paenebursarius and S. mirificus first appear in the medial Blancan. The lack of South American immigrants in the Belen Fauna suggests a medial Blancan age, although their absence could be related to biogeographic factors. Neotropical mammals are unknown from Blancan faunas in northern New Mexico; however, Glyptotherium occurs in two early Irvingtonian faunas in the Albuquerque basin, Tijeras Arroyo and Western Mobile. We tentatively place the Belen Fauna in the medial Blancan (fig. 12.2) based on similarities with other medial Blancan faunas (e.g., Pajarito) from the Arroyo Ojito Formation in the Albuquerque basin.

Mesas Mojinas:

An astragalus of a small camelid (NMMNH 29936) was collected by Richard Lozinsky in 1986 from the Arroyo Ojito Formation, southeast of Mesas Mojinas in the Gabaldon badlands west of Belen in Valencia County (NMMNH Site L-4253; fig. 12.1, site 13). In their review of the early Hemphillian (late Miocene) Gabaldon Fauna from the Popotosa Formation, which underlies the Arroyo Ojito Formation, Lozinsky and Tedford (1991) referred this specimen to Hemiauchenia sp. and noted that it was the only fossil recovered from post-Hemphillian strata in the Gabaldon badlands. The Gabaldon specimen is very similar to two astragali of a small Hemiauchenia from the early Blancan Buckhorn LF in southwestern New Mexico (Morgan et al., 1997). This small species of Hemiauchenia is known from several other Blancan faunas in New Mexico.

Veguita:

A site in the Arroyo Ojito Formation about 1 km east of the Rio Puerco and 10 km northwest of Veguita in northernmost Socorro County (NMMNH Site L-2941; fig. 12.1, site 14) in the southern Albuquerque basin has produced a partial maxilla with left P2–M3 of the large horse Equus scotti (Morgan and Lucas, 2000a). These teeth are characterized by their large size, fairly complicated enamel pattern of the fossettes, elongated protocone with a lingual indentation, and presence of a pli caballin. E. scotti is the common large horse in New Mexico faunas ranging in age from medial Blancan through early Irvingtonian. A Blancan age is more likely considering that most sites from the Arroyo Ojito Formation in the southern portion of the Albuquerque basin produce Blancan mammals.

Sevilleta:

Denny (1940) found Blancan fossils in sands and gravels, probably derived from the Arroyo Ojito Formation, from bluffs west of the Rio Grande and just north of the Rio Salado in the southernmost portion of the Albuquerque basin in northern Socorro County (fig. 12.1, site 15). This site is located on land now included within the Sevilleta National Wildlife Refuge. The fauna reported by Denny consists of the gomphothere Stegomastodon mirificus and a tooth of Equus. We have not been able to relocate Denny's fossils, and thus the identifications are taken from his paper and must be considered tentative. We recently collected fossils from outcrops of the Arroyo Ojito Formation in this same general area, north of the Rio Salado on the La Joya Game Refuge. We found teeth and postcranial elements representing two species of Equus, E. simplicidens and a larger horse, tentatively identified as E. scotti. The Sevilleta fauna is similar to the medial Blancan Arroyo de la Parida LF, derived from the Palomas Formation about 15 km farther south in the northern Socorro basin.

Arroyo de la Parida:

Vertebrate fossils were first found in Arroyo de la Parida in 1935, about 6 km northeast of Socorro, Socorro County (fig. 12.1, site 16). Needham (1936) reported a complete pair of lower jaws of the gomphotheriid proboscidean Rhynchotherium and a lower molar of the horse Plesippus (now considered a subgenus of Equus) from an exposure of sands and gravels of the Santa Fe Group on the south side of Arroyo de la Parida, about 2 km east of its confluence with the Rio Grande. Additional vertebrate fossils were collected from this same exposure by students from the New Mexico Institute of Mining and Technology (DeBrine et al., 1963). Curt Teichert, a well-known German invertebrate paleontologist, collected a sample of vertebrate fossils from the vicinity of Arroyo de la Parida in 1953, and donated these fossils to the American Museum of Natural History. The only locality information associated with Teichert's sample was that the fossils were collected “about four miles north of Socorro, New Mexico.” Based on the general locality, preservation of the fossils, and the composition of the fauna, there is little doubt that Teichert's fossils are from the area that yields the Arroyo de la Parida LF. The fossils collected by Teichert were summarized by Tedford (1981), and include three species of horses, Equus simplicidens, E. cf. E. cumminsii, and E. cf. E. scotti, the small antilocaprid Capromeryx, and the gomphothere Stegomastodon. Lucas and Morgan (1996) described and illustrated the mandibles of Rhynchotherium first mentioned by Needham, and referred them to the species R. falconeri, originally described from the Blanco LF in Texas. Lucas and Morgan (1996) also summarized the biostratigraphy of the Arroyo de la Parida LF, including fossils collected in 1996 by two students from New Mexico Tech, Ed Frye and Mike O'Keeffe. We visited the Arroyo de la Parida area several times during 2000 and collected numerous additional fossils from 15 different sites (Morgan et al., 2000).

The Arroyo de la Parida LF is derived from a 70-m sequence of sands and gravels that constitute the axial river (ancestral Rio Grande) facies of the Palomas Formation. The strata in the vicinity of Arroyo de la Parida are located at the northern end of the Socorro basin, representing one of the northernmost occurrences of the Palomas Formation, which has its type area about 100 km farther south in Palomas Creek near Truth or Consequences in Sierra County (Lozinsky, 1986). The Arroyo de la Parida LF is composed of 10 vertebrates: the land tortoise Hesperotestudo; the ground sloth Megalonyx cf. M. leptostomus; three horses, Equus cf. E. cumminsii, E. scotti, and E. simplicidens; two camelids, a large Camelops and a small Hemiauchenia; the small antilocaprid Capromeryx; and two proboscideans, Rhynchotherium falconeri and Stegomastodon sp. This is a fairly typical faunal assemblage found in New Mexico Blancan sites, mostly consisting of large grazing ungulates and dominated by horses of the genus Equus.

Five mammals from the Arroyo de la Parida LF are restricted to the Blancan, including Megalonyx leptostomus, Equus cumminsii, E. simplicidens, the large Camelops, and Rhynchotherium falconeri. The most age-diagnostic of these taxa is Rhynchotherium, a gomphothere that became extinct in the late Pliocene at about 2.2 Ma together with several other characteristic genera of Blancan mammals. The lower jaws of R. falconeri from Arroyo de la Parida were collected near the top of the local section of the Palomas Formation, suggesting that the entire fauna, most of which occurs some 40 m lower in the section, is older than 2.2 Ma. An early Blancan age for the Arroyo de la Parida LF can be ruled out by the presence of E. scotti and Camelops, both of which first appear in New Mexico faunas during the medial Blancan. The absence of South American immigrants suggests an age greater than 2.7 Ma. Megalonyx is the only Blancan mammal of South American origin that was not a participant in the Great American Interchange. Megalonyx or its progenitor arrived from South America in the late Miocene about 9 Ma. M. leptostomus is fairly widespread in early through late Blancan faunas. The Arroyo de la Parida LF is interpreted to be medial Blancan in age (3.6–2.7 Ma), and is similar to the Cuchillo Negro Creek LF from the Palomas Formation in the Engle Basin near Truth or Consequences.

Fite Ranch:

Tedford (1981) reported postcranial bones of Equus and Camelops collected by George Pearce in 1953 from pumice-bearing sands on the Dean Fite Ranch near San Antonio in the Socorro basin, Socorro County (fig. 12.1, site 17). Specimens in the F:AM collections from Fite Ranch consist of an astragalus of Equus and a partial metatarsal of Camelops. Needham (1936) reported a partial humerus, radius, and ulna of the turkey Meleagris, apparently from this same locality (Tedford, 1981). Needham (p. 537) described this locality as “a bed of pumicite … about three and one half miles northeast of San Antonio, Socorro County, along the east bluff of the Rio Grande. It [the bed of pumicite] is underlain by some thirty feet of light-colored gravel and sand and buff silt, typical of the Santa Fe formation as developed east of the Rio Grande near Socorro” (i.e., Arroyo de la Parida). Tedford noted that the pumice-bearing sands on the Fite Ranch were attributed to the Bandelier eruptions, indicating an age of 1.6–1.2 Ma (Izett et al., 1981; Izett and Obradovich, 1994). This would suggest an early Irvingtonian age, although the fossil vertebrates from Fite Ranch are not age diagnostic.

Silver Canyon:

Gary Morgan, Mike O'Neill, and Brenda Wilkinson collected a small sample of fossils in 1998 from the Palomas Formation near Silver Canyon in the San Marcial basin at the very northern end of Elephant Butte Lake in southern Socorro County (NMMNH site L-3682; fig. 12.1, site 18). The Silver Canyon site consists of three taxa of mammals, a rodent tentatively identified as the wood rat Neotoma, the horse Equus, and an indeterminate gomphothere. The most common fossils are proboscidean tooth and tusk fragments and postcranials, identifiable only as Gomphotheriidae. Equus is represented in the fauna by postcranial remains. A mandible of a fairly large rodent appears to be referable to Neotoma, although the teeth are too worn for a species-level identification. The age of this fauna cannot be determined on the basis of the fossil material currently known, although a Blancan age is most likely considering that all other vertebrate faunas so far known from the Palomas Formation are Blancan.

Elephant Butte Lake:

Joseph Rak and Charles Falkenbach of the Frick Laboratory collected fossil vertebrates in the late 1920s from axial river gravels of the ancestral Rio Grande in the Engle and Palomas basins in the vicinity of Hot Springs (now called Truth or Consequences) in Sierra County. Fossils from the Engle basin on the western side of Elephant Butte Lake north of Truth or Consequences are called the Elephant Butte Lake Fauna (fig. 12.1, site 19). Tedford (1981) attributed these strata to the Sierra Ladrones Formation, but they have since been referred to the Palomas Formation (Lozinsky, 1986). The precise localities and stratigraphy of these fossils are not known; however, they were certainly derived from the Palomas Formation, and are similar in age to the nearby Cuchillo Negro Creek LF (see below). Mammals identified by Tedford from Elephant Butte Lake are the horse Equus simplicidens, the tapir Tapirus, and the giant camel Gigantocamelus. Since 1981, additional fossils referable to the Elephant Butte Lake Fauna have been collected in the vicinity of Rock Canyon on the western shore of Elephant Butte Lake about 5 km north of Truth or Consequences, and include the giant tortoise Hesperotestudo, a mandible of Gigantocamelus cf. G. spatula, and a skull of the gomphothere Stegomastodon cf. S. rexroadensis. E. simplicidens and Gigantocamelus are indicative of the Blancan, and S. rexroadensis suggests an early or medial Blancan age. Tedford considered these sites to be medial Blancan because of their close stratigraphic association with a basalt flow dated at 2.9 ± 0.3 Ma from Mitchell Point on the western side of Elephant Butte Lake (Bachman and Mehnert, 1978).

Cuchillo Negro Creek:

Vertebrate fossils from the Palomas Formation north of Cuchillo Negro Creek in the Engle Basin in Sierra County (fig. 12.1, site 20) were named the Cuchillo Negro Creek LF by Lucas and Oakes (1986). Fossils were collected from the Cuchillo Negro Creek area in 1983 and 1984 by University of New Mexico field crews and in 1998 and 1999 by NMMNH field crews. Lozinsky (1986) recognized two informal members of the Palomas Formation in this area, the axial facies consisting of about 30 m of sand with lenses of gravel and clay and the piedmont facies consisting of about 50 m of sandy silt and conglomerate. Most of the fossils were derived from the axial facies, which was deposited by an ancestral Rio Grande. A basalt flow dated at 2.9 Ma from Mitchell Point, about 15 km north of the Cuchillo Negro Creek sites, interfingers with the axial facies of the Palomas Formation (Lozinsky, 1986).

The Cuchillo Negro Creek LF is composed of 10 species (Lucas and Oakes, 1986; this report): 3 turtles, including a small land tortoise of the genus Hesperotestudo, the mud turtle Kinosternon, and an aquatic member of the family Emydidae, and 7 mammals, including the borophagine canid Borophagus hilli, the procyonid Bassariscus sp., the equid Equus cf. E. simplicidens, the camelids Camelops sp., Hemiauchenia blancoensis, and Hemiauchenia small sp., and the gomphotheriid proboscidean Stegomastodon rexroadensis.

A nearly complete dentary with i2–3, c, and p2–m2 identified as Borophagus diversidens by Lucas and Oakes (1986) was referred to B. hilli by Wang et al. (1999). Although most Borophagus fossils from the early Blancan were referred to B. diversidens by previous authors, these specimens are more similar in size and morphology to the late Hemphillian and early Blancan species B. hilli. The larger and more highly derived species B. diversidens appears in the late early Blancan and survives until the late Blancan (Wang et al., 1999). An edentulous mandible from Cuchillo Negro Creek represents the first New Mexico Pliocene record of the ringtail Bassariscus. The only Blancan species of Bassariscus, B. casei from the early Blancan Rexroad Fauna in Kansas, is distinguished from the extant B. astutus on the basis of dental characters that cannot be observed in the Cuchillo Negro Creek jaw.

Lucas and Oakes (1986) identified the palate of a horse from the Cuchillo Negro Creek LF as Equus simplicidens. Two associated upper molars found in 1998 match the original dental sample. Although these teeth have several features that differ somewhat from typical E. simplicidens (e.g., fairly complicated fossettes and slight lingual indentation of the protocone), this species is the only Blancan Equus that is similar in morphology and size to the teeth from Cuchillo Negro Creek. There are three taxa of camelids from Cuchillo Negro Creek. The typical large Blancan llama Hemiauchenia blancoensis was reported previously (Lucas and Oakes, 1986). A small species of Hemiauchenia is known from an adult distal tibia. This apparently undescribed species of llama occurs in six Blancan faunas in New Mexico. A large undescribed species of Camelops, represented by a large proximal phalanx from Cuchillo Negro Creek, occurs in several other Blancan faunas in New Mexico (Morgan et al., 1998).

A proboscidean is represented at Cuchillo Negro Creek by a pair of mandibles with heavily worn m3s (Lucas and Oakes, 1986) and by a recently collected partial m3 in medium wear. The mandibles have a short, straight symphysis lacking tusks. The m3s of these two specimens are similar in size and dental features, including the presence of six lophids and a simple trefoil pattern. The combination of six lophids on the m3 and the absence of lower tusks excludes all other Blancan genera of proboscideans except Stegomastodon. The smaller number of lophids on m3 and simple trefoiling distinguish the Cuchillo Negro Creek teeth from the more advanced species S. mirificus, and suggest referral to the early to medial Blancan S. rexroadensis.

Lucas and Oakes (1986) proposed a medial Blancan age (about 3.0 Ma) for the Cuchillo Negro Creek LF, based on biostratigraphy and correlation with the 2.9 Ma Mitchell Point basalt. They suggested that the Cuchillo Negro Creek LF is similar to the Rexroad Fauna in Kansas and the Benson Fauna in Arizona. The presence of Camelops, a genus that supposedly did not appear until after 3.7 Ma (Lindsay et al., 1984), is indicative of a medial Blancan age, and Stegomastodon rexroadensis occurs in both early and medial Blancan faunas. The absence of South American immigrant mammals indicates a pre-late Blancan age (older than 2.7 Ma). The presence of Borophagus hilli suggests an early Blancan age according to Wang et al. (1999), although this same species occurs in the medial Blancan Hagerman Fauna in Idaho. A more precise age determination for the Cuchillo Negro Creek LF must await the discovery of additional age-diagnostic taxa, or other data such as magnetostratigraphy. In the near future, we intend to determine the relative stratigraphic position of this fauna compared to the better known early Blancan Truth or Consequences LF, located about 5 km farther south.

Truth or Consequences:

The Truth or Consequences LF is derived from fine-grained sediments of the Palomas Formation in a roadcut on Interstate 25 about 2 km south of Truth or Consequences in Sierra County (Repenning and May, 1986; fig. 12.1, site 21). The site was discovered by Arthur Harris of the University of Texas at El Paso and then worked in the early 1980s by Charles Repenning and Steven May of the U.S. Geological Survey in Denver. Gary Morgan, Paul Sealey, and Spencer Lucas began work at this site in 1997 and have added numerous taxa to the fauna, including both large and small species (table 12.1). Repenning and May recovered most of their fossils by excavation, whereas the more recent NMMNH collections have been obtained primarily through screenwashing.

The stratigraphic section that includes the Truth or Consequences LF is composed of about 20 m of poorly consolidated sand and pebbly sand, with two interbedded mudstone layers, referred to the Palomas Formation. Most of the vertebrate fossils occur in greenish to reddish mudstones in the lower third of the roadcut. The preservation of fossils in these mudstones is excellent, and includes numerous mandibles, maxillae, and partial skulls of both small and large mammals. We recently recovered fossils of aquatic species, including a duck and two partial shells of emydid turtles, in fine sand at the base of the local section. Fine-grained units are uncommon and discontinuous within the axial river facies of the Palomas Formation, which is dominated by sands and gravels.

The Truth or Consequences LF, as originally described by Repenning and May (1986), consisted of 13 species: 4 reptiles, including the mud turtle Kinosternon, the box turtle Terrapene, the fence lizard Sceloporus, and the coachwhip snake Masticophis; and 9 mammals, including the rabbits Hypolagus vetus and Notolagus lepusculus, the pocket gopher Geomys (Nerterogeomys) minor, the cotton rat Sigmodon, the rice rat cf. Oryzomys, the wood rat Neotoma quadriplicata, the horse Equus (Plesippus), the deer Odocoileus brachyodontus, and the gomphothere Stegomastodon. Recent additions to the fauna include an aquatic emydid turtle, another snake, a duck and a galliform bird, and six mammals—a shrew, the mole Scalopus, a small sciurid, the large canid Borophagus cf. B. hilli, the peccary Platygonus cf. P. bicalcaratus, and the small antilocaprid Capromeryx. The Truth or Consequences LF is now composed of 23 species, including 15 mammals (table 12.1), making it one of the most diverse Blancan vertebrate faunas from New Mexico.

Many of the mammals from the Truth or Consequences LF are age diagnostic, several of which suggest an early Blancan age. Repenning and May (1986) referred the small rabbit to Notolagus lepusculus, a species named by Hibbard (1939) from the late early Blancan Rexroad 3 Fauna of Kansas (Blancan II of Repenning, 1987). The larger rabbit was identified as Hypolagus vetus, a species known from the Hemphillian through the medial Blancan (White, 1987). The pocket gopher was referred to the small species Geomys (Nerterogeomys) minor, described from Rexroad 3 (Hibbard, 1950, 1967). The primitive wood rat Neotoma (Paraneotoma) quadriplicata is intermediate in size between the oldest species of pack rat, N. (P.) sawrockensis from the very early Blancan Saw Rock Canyon LF in Kansas, and N. (P.) quadriplicata from Rexroad 3 (Hibbard, 1941, 1967). Repenning and May (1986) tentatively referred a small sigmodontine rodent to the extant genus Oryzomys. Czaplewski (1987) noted that the Oryzomys from Truth or Consequences was similar to his new genus Jacobsomys from the early Blancan (about 4 Ma) Verde LF in Arizona, but was smaller than the Verde species J. verdensis. The Sigmodon from Truth or Consequences, referred here to S. medius, may represent one of the earliest records of this genus. Czaplewski (1987) also recorded S. medius from the Verde LF.

The Truth or Consequences LF consists primarily of microvertebrates. Because of the rarity of large mammals, it is difficult to make biostratigraphic comparisons with other New Mexico Blancan faunas, most of which are composed of large vertebrates. The cervid Odocoileus brachyodontus, identified from Truth or Consequences by a maxillary fragment and antler, was first described from the early Blancan Fox Canyon Fauna of Kansas (Oelrich, 1953). Odocoileus appears in the early Blancan as an immigrant from the Old World. It is one of the genera whose first appearance helps to define the beginning of the Blancan (e.g., Woodburne and Swisher, 1995). The rostrum of a large peccary is similar to the large species Platygonus bicalcaratus. Although the genus Platygonus supposedly appeared at the beginning of the medial Blancan at about 3.6 Ma (Lindsay et al., 1984), an occurrence in the early Blancan would not be surprising. A p2 of Borophagus is similar in size and morphology to the p2 of B. hilli from Cuchillo Negro Creek, a species known elsewhere from the late Hemphillian and early Blancan (Wang et al., 1999). Fossils of Equus (Plesippus), Capromeryx, and Stegomastodon are too incomplete for further identification. Nannippus and camelids are both absent from the Truth or Consequences LF.

Repenning and May (1986) suggested an early Blancan age (between 4.2 and 4.0 Ma, early Blancan II of Repenning, 1987) for the Truth or Consequences LF based on both biostratigraphy and magnetostratigraphy. Repenning and May (1986) correlated the Truth or Consequences magnetostratigraphic section of 11 normally magnetized samples from three levels to the Nunivak Subchron of the Gilbert Chron, primarily on the basis of the similarity of its fauna with other faunas from the Nunivak or the next oldest normal subchron in the Gilbert (Sidufjall), and a more primitive aspect than faunas (e.g., Rexroad 3) derived from the next youngest normal subchron in the Gilbert (Cochiti). They gave an age range of 4.20–4.05 Ma for the Nunivak, but the age of this subchron has since been revised downward to 4.62–4.48 Ma (Berggren et al., 1995). The correlation of the Truth or Consequences magnetostratigraphic section to the Nunivak Subchron should be considered tentative since it was based on a few samples through a limited stratigraphic section (Mack et al., 1993). We intend to measure a longer stratigraphic section of the Palomas Formation through the Truth or Consequences LF, in hopes of determining the stratigraphic position of this fauna relative to three other nearby Blancan faunas from the Palomas Formation, Cuchillo Negro Creek and Elephant Butte Lake to the north and Palomas Creek to the south. We anticipate that a more detailed magnetostratigraphic section eventually will be taken through this section.

Palomas Creek:

Joseph Rak and Charles Falkenbach collected fossil vertebrates in 1927 and 1928 from exposures of the distal piedmont facies of the Palomas Formation southwest of Hot Springs (= Truth or Consequences) in Palomas Creek, Sierra County (Tedford, 1981; fig. 12.1, site 22). Palomas Creek is the type area of the Palomas gravel, now called the Palomas Formation (Lozinsky, 1986). The Palomas Creek LF is in the Palomas basin about 8–10 km southwest of the Truth or Consequences LF. Palomas Creek has a fairly diverse Blancan fauna composed of nine species (table 12.1, after Tedford, 1981): the tortoise Hesperotestudo and eight mammals, including the pocket gopher Geomys (Geomys); the cotton rat Sigmodon medius; the horses Nannippus peninsulatus, Equus simplicidens, and E. cf. E. cumminsii; a peccary; a large Camelops; and the mastodont Mammut raki. Nannippus, E. simplicidens, E. cumminsii, and the large Camelops are characteristic of New Mexico Blancan faunas. The absence of South American immigrants suggests a pre-late Blancan age. The type mandible of Mammut (= Mastodon) raki (Frick, 1933) from Palomas Creek is one of the few known Blancan Mammut, and the only one referred to a species other than M. americanum, the typical Pleistocene mastodon (Lucas and Morgan, 1999).

Magnetostratigraphy and radioisotopic dates provide additional geochronologic data pertaining to the age of Palomas Formation in the Palomas basin (Mack et al., 1993, 1998). K/Ar dates on basalts in the Palomas basin include a flow dated at 4.5 ± 0.1 Ma in the western part of the basin that is either below or within the lowermost Palomas Formation and a flow dated at 3.1 ± 0.1 Ma interbedded with alluvial fan conglomerates of the Palomas Formation in the eastern part of the basin (Seager et al., 1984; Mack et al., 1998). A magnetostratigraphic section in the Palomas Formation from Palomas Creek sampled mostly reversely magnetized strata referred to the Matuyama Chron, but included about 10 m of normally magnetized strata referred to the Gauss Chron at the bottom of the section (Mack et al., 1993). The precise stratigraphic position of the Palomas Creek fossils is unknown, but the paleomagnetic data strongly suggest the fauna was derived from low in the section. The combination of magnetostratigraphy and biostratigraphy indicates a medial Blancan (3.6–2.7 Ma) age for the Palomas Creek LF.

Hatch:

Blancan and early Irvingtonian fossils occur in the Camp Rice Formation in southern New Mexico, from Hatch and Rincon Arroyo in the Hatch-Rincon basin in northern Doña Ana County south through the Jornada and Mesilla basins and into Texas. Hawley (1978) mentioned the presence of Blancan fossils from the vicinity of Hatch, but did not list any taxa. Paul Sealey and NMMNH field crews collected vertebrate fossils in 1990 and 1998 from the Camp Rice Formation about 5 km west of Hatch (fig. 12.1, site 23). The Camp Rice Formation is unconformably underlain by Miocene red beds of the Rincon Valley Formation in the vicinity of Hatch. This is similar to a stratigraphic section about 30 km southeast of Hatch in the Jornada basin near Tonuco Mountain, where the Tonuco Mountain LF was collected (Morgan et al., 1998). The only fossil previously reported from Hatch is a partial skeleton of the long-nosed snake Rhinocheilus (Lucas et al., 1995).

On the basis of fossils in the NMMNH collection, the Hatch LF is composed of 12 species: the land tortoises Gopherus and Hesperotestudo, the mud turtle Kinosternon, Rhinocheilus, and eight species of mammals (table 12.1) including two unidentified rabbits, the pocket gopher Geomys (Nerterogeomys) cf. G. (N.) paenebursarius, the badger Taxidea, a small cat probably of the genus Lynx, the horse Equus, the camelid Hemiauchenia cf. H. blancoensis, and the small antilocaprid Capromeryx. Land tortoises are the most abundant members of the Hatch fauna. Specimens of mammals are less common, particularly ungulates. Equus is represented by fragmentary teeth, Hemiauchenia by postcranial remains, and Capromeryx by a single tooth. Two leporids are present based on two distal femora of very different size. Taxidea is identified from a distal radius. Badgers are known from two other New Mexico Blancan faunas, Buckhorn and Tonuco Mountain (Morgan et al., 1997, 1998). The small cat is represented by a maxillary fragment with a partial P4 (NMMNH 25308) that compares well in size and other features to Lynx rexroadensis, known from late Hemphillian and Blancan faunas (MacFadden and Galiano, 1981). Two pocket gopher mandibles (NMMNH 25324, 25329) from Hatch are similar to mandibles referred to Geomys (Nerterogeomys) paenebursarius from the Belen and Pajarito Faunas in the Albuquerque basin (Morgan and Lucas, 2000a).

The age of the Hatch LF is somewhat uncertain because most of the mammalian taxa are not identifiable below the generic level. A medial Blancan age is suggested by the faunal and stratigraphic similarity to the nearby Tonuco Mountain LF (Morgan et al., 1998). A pre-late Blancan age is indicated by the lack of South American immigrant mammals that appeared in faunas after 2.7 Ma. A magnetostratigraphic section in the Camp Rice Formation at Hatch Siphon, located several kilometers west of the badlands that produced the Hatch LF, samples essentially the entire Gauss Chron, including both the Kaena and Mammoth Subchrons (Mack et al., 1993). The lowermost part of the section does not cross the Gilbert/Gauss boundary and is thus younger than 3.6 Ma, whereas the upper 10 m samples the Matuyama Chron. The Hatch Siphon section is similar to the magnetostratigraphic section for Cedar Hill that contains the medial Blancan Tonuco Mountain LF (Mack et al., 1993; Morgan et al., 1998; see below), further supporting a medial Blancan age for the Hatch LF.

Rincon Arroyo:

Several fragmentary teeth and partial metapodials of Equus were collected from the Camp Rice Formation in Rincon Arroyo by Paul Knight and Paul Sealey. Rincon Arroyo is in the Hatch-Rincon basin in northern Doña Ana County about 10 km east of Hatch (fig. 12.1, site 24). The most significant aspect of Rincon Arroyo is the detailed geochonologic data available for the stratigraphic section, including magnetostratigraphy (Mack et al., 1993) and a dated pumice-clast conglomerate (Mack et al., 1998). The lower half (about 50 m) of the Rincon Arroyo section is in the Gauss Chron and the upper portion (also about 50 m) is in the Matuyama Chron, and includes the Reunion, Olduvai, and Jaramillo subchrons (Mack et al., 1993). Between the Olduvai and Jaramillo subchrons in the upper part of the section is a pumice-clast conglomerate dated at 1.60 Ma, which correlates with the lower Bandelier eruption (Mack et al., 1998). The Rincon Arroyo section crosses the Blancan/Irvingtonian boundary at the top of the Olduvai Subchron in the upper third of the section. Unfortunately, Rincon Arroyo is not particularly fossiliferous, and the few fossils recovered so far are not age diagnostic. Future field work in Rincon Arroyo hopefully will yield additional mammal fossils from this well-dated section.

Tonuco Mountain:

The Tonuco Mountain LF is a medial Blancan vertebrate assemblage from the Cedar Hill area southeast of Tonuco Mountain (also called San Diego Mountain) in northern Doña Ana County, southern New Mexico (Morgan et al., 1998; fig. 12.1, site 25). The fossils are derived from the axial river facies of the Camp Rice Formation in the western Jornada Basin. The stratigraphic section of the Camp Rice Formation at Cedar Hill consists of about 50 m of sandstone and conglomerate, with a minor component of sandy mudstone, and is unconformably underlain by Miocene red beds of the Rincon Valley Formation (Morgan et al., 1998).

The Tonuco Mountain LF is composed of 16 species: the mud turtle Kinosternon, the land tortoises Gopherus and Hesperotestudo, a duck, and 12 mammals including an indeterminate rabbit; the badger Taxidea; the coyote-like canid Canis lepophagus; the borophagine canid Borophagus sp.; the horses Nannippus peninsulatus, Equus simplicidens, and E. scotti; the peccary Platygonus cf. P. bicalcaratus; the camelids Camelops, Hemiauchenia blancoensis, and a small Hemiauchenia; and the gomphothere Cuvieronius. Among these taxa, Borophagus, C. lepophagus, N. peninsulatus, E. simplicidens, P. bicalcaratus, and H. blancoensis are indicative of the Blancan, and several of these taxa help to further limit the age of this fauna within the Blancan. E. simplicidens is absent from very early Blancan faunas, Platygonus and Camelops supposedly do not appear until the beginning of the medial Blancan (Lindsay et al., 1984), and most Blancan records of Nannippus in the southwestern United States predate the Gauss/Matuyama magnetic reversal at 2.6 Ma. The absence of South American immigrants suggests the fauna is older than 2.7 Ma, the earliest date for the onset of the Great American Interchange. The biostratigraphic data restrict the age of the Tonuco Mountain LF to the medial Blancan (between 3.6 and 2.7 Ma). A magnetostratigraphic study of the Camp Rice Formation at Cedar Hill helps to further constrain the age of this fauna (Mack et al., 1993). The entire Cedar Hill section is within the Gauss Chron (younger than 3.6 Ma), and the fossiliferous interval is below the top of the Kaena Subchron (older than 3.0 Ma). The combination of biostratigraphic and magnetostratigraphic data limit the age of the Tonuco Mountain LF to the medial Blancan, between 3.6 and 3.0 Ma.

Tortugas Mountain:

Vertebrate fossils have been known for 40 years from gravel pits in the northern Mesilla Basin (Ruhe, 1962; Hawley et al., 1969). The Tortugas Mountain LF includes fossils collected from two gravel pits near Tortugas Mountain, several kilometers east of Las Cruces in Doña Ana County (Lucas et al., 1999, 2000; fig. 12.1, site 26). These pits expose axial river sands and gravels of an ancestral Rio Grande in the upper part of the Camp Rice Formation. A palate of Stegomastodon, two mandibles of Cuvieronius, and isolated teeth of Mammuthus were collected from a gravel pit on the northwestern side of Tortugas Mountain (NMMNH site L-3537). A palate of Cuvieronius, a Mammuthus tooth, and several teeth and postcranial elements of Equus were found from about the same stratigraphic level in the Inman Gravel Pit (NMMNH site L-3649), about 3 km north of Tortugas Mountain (Hawley et al., 1969; Tedford, 1981; Vanderhill, 1986; Lucas et al., 1999, 2000).

With the exception of several Equus fossils figured in Hawley et al. (1969) that we cannot locate, the Tortugas Mountain LF consists of three species of proboscideans (Lucas et al., 1999, 2000), the gomphotheres Cuvieronius tropicus and Stegomastodon mirificus and the mammoth Mammuthus imperator. Tortugas Mountain is only the second site that records the co-occurrence of Cuvieronius, Stegomastodon, and Mammuthus, the other being the early Irvingtonian Gilliland LF from the Seymour Formation in Texas (Hibbard and Dalquest, 1966). Cuvieronius is not particularly age diagnostic in North America, as this genus first appears in the Blancan and survives until the early Rancholabrean (Morgan and Hulbert, 1995). However, the association of Stegomastodon and Mammuthus is important biochronologically because these two genera have only a limited period of overlap in the early Irvingtonian, after the arrival of Mammuthus across the Bering Land Bridge from Eurasia (about 1.6 Ma) and before the extinction of Stegomastodon (about 1.2 Ma). The presence of Mammuthus clearly establishes an Irvingtonian age for the Tortugas Mountain LF. At present there are no confirmed records of mammoths in North America older than 1.6 Ma (Lucas, 1995, 1996; Cassiliano, 1999). The mammoth teeth from the Tortugas Mountain LF, identified as M. imperator by Lucas et al. (1999, 2000), are very similar to mammoth teeth from other early Irvingtonian sites, including M. haroldcooki from Gilliland, Texas (Hibbard and Dalquest, 1966) and Holloman, Oklahoma (Dalquest, 1977) and M. hayi from Leisey Shell Pit, Florida (Webb and Dudley, 1995). The youngest well-dated record of Stegomastodon mirificus is from Tule Canyon in the Texas Panhandle, where this species is associated with mammoths and several species of horses in a volcanic ash at the base of the Tule Formation dated at between 1.3 and 1.2 Ma (Izett, 1977; Tedford, 1981; Madden, 1983). An age between 1.6 and 1.2 Ma is indicated for the Tortugas Mountain LF based on the association of Mammuthus and Stegomastodon.

Mesilla Basin Fauna A:

There are three vertebrate faunas from the Camp Rice Formation in the southern Mesilla basin, from Chamberino south to Santa Teresa near the border with Mexico (Vanderhill, 1986). Faunas A and B are late Blancan in age, and Fauna C is early Irvingtonian. Mesilla Basin Fauna A (hereafter shortened to Mesilla A) is the oldest and least well known of the three southern Mesilla basin faunas, probably because exposures of the lower part of the section are less extensive than in the upper part of the sequence (Vanderhill, 1986; fig. 12.1, site 27). The vertebrate assemblage from Mesilla A is composed of only six species (table 12.1; from Vanderhill, 1986): the catfish Ictalurus, the softshell turtle Apalone (= Trionyx), the glyptodont Glyptotherium, the horses Nannippus peninsulatus and Equus cf. E. calobatus, and the llama Hemiauchenia blancoensis. The presence of Glyptotherium confirms a late Blancan or younger age (younger than 2.7 Ma) for this fauna, as the first appearance of South American immigrants, including Glyptotherium, in North American faunas defines the beginning of the late Blancan. The presence of Nannippus indicates an age greater than 2.2 Ma.

Magnetostratigraphic data for the Camp Rice Formation strata from which the Mesilla A fauna was derived further restrict its age (Vanderhill, 1986). Mesilla A occurs in normally magnetized sediments in the upper Gauss Chron between the Kaena Subchron (3.04 Ma) and the Gauss/Matuyama boundary (2.58 Ma). Mesilla A is one of only six southwestern Blancan faunas that documents the presence in the upper Gauss Chron (between 3.0 and 2.6 Ma) of South American immigrant mammals. Although 2.7 Ma is often cited as the first appearance datum for South American immigrants in North America, and is used here as the boundary between the medial and late Blancan, the timing of the first appearance in southwestern Blancan faunas of mammals that participated in the Great American Interchange is not firmly established, but occurred sometime between 3.0 and 2.6 Ma. There are no confirmed records of Interchange mammals in Blancan faunas north of Mexico that predate the Kaena Subchron at 3.04 Ma. However, several new records of South American immigrants from central Mexico appear to be early Blancan (Miller and Carranza-Castañeda, 2001, 2002).

Mesilla Basin Fauna B:

Mesilla Basin Fauna B (hereafter shortened to Mesilla B) is the most diverse late Blancan fauna in New Mexico (fig. 12.1, site 28). Vanderhill (1986: table 1) listed 16 species from Mesilla B: two turtles, the aquatic emydid Trachemys and the land tortoise Hesperotestudo; and 14 mammals including Glyptotherium arizonae, the mylodont gound sloth Paramylodon cf. P. harlani, the rabbits Aluralagus virginiae and a larger Lepus or Sylvilagus, the small cat cf. Lynx rufus, the sabercat Smilodon gracilis, the horses Equus cf. E. calobatus and E. scotti, the large tapir Tapirus haysii, the camelids Blancocamelus meadei, Gigantocamelus cf. G. spatula, and Camelops, the deer Odocoileus, and the gomphothere Cuvieronius. Our recent fieldwork in the Camp Rice Formation near La Union has added four species of large mammals to the Mesilla B fauna (table 12.1).

We recently discovered a new microvertebrate site near Chamberino in the southern Mesilla basin (NMMNH site L-2971), from the same stratigraphic level as Vanderhill's Mesilla B. This site occurs stratigraphically about 5 m below the lowest occurrence of Mammuthus in the Chamberino section. The Chamberino microsite is being actively screenwashed at the present time. Preliminary identifications add at least 11 species to the Mesilla B fauna: two snakes, a lizard, a small passerine bird, and seven mammals including the mole Scalopus, the ground squirrel Spermophilus, the pocket gopher Geomys, the heteromyid rodent Dipodomys, the cotton rat Sigmodon, the grasshopper mouse Onychomys, and the skunk Spilogale. Combining Vanderhill's Faunule B with our Chamberino site, the Mesilla B fauna now has 25 species of mammals, making it the most diverse Blancan mammal fauna in New Mexico.

Mesilla B has a diverse assemblage of mammals, but only a few taxa are age diagnostic. Many species occur in both late Blancan and early Irvingtonian and younger faunas. Vanderhill (1986) correlated Mesilla B with the transitional interval between the Blancan and Irvingtonian (2.2–1.8 Ma). Two genera typical of the Blancan, the giant camels Blancocamelus and Gigantocamelus, occur in Mesilla B. The presence of the South American immigrant xenarthrans Glyptotherium arizonae and Paramylodon cf. P. harlani confirms that Mesilla B is late Blancan or younger. G. arizonae is supposedly restricted to early Irvingtonian sites elsewhere in the southwest (Gillette and Ray, 1981). However, the type locality of G. arizonae, Curtis Ranch in the San Pedro Valley of Arizona, although considered earliest Irvingtonian by most workers (e.g., Lundelius et al., 1987; Repenning, 1987), is best regarded as latest Blancan owing to the absence of Mammuthus and other typical Irvingtonian genera (Lindsay et al., 1990). There are also late Blancan records of G. arizonae from Florida (Morgan and Hulbert, 1995). The rabbit Aluralagus virginiae appears to be restricted to Blancan/Irvingtonian transitional faunas. The only other faunas where A. virginiae has been identified (referred to Hypolagus virginiae by Tomida, 1987), Curtis Ranch and the San Simon Power Line, both in southern Arizona, are similar in age to Mesilla B. Smilodon gracilis and Tapirus haysii are typical of early Irvingtonian faunas, but both are known from the late Blancan. Previous southwestern records of S. gracilis are restricted to the early Irvingtonian, but this sabercat occurs in several Florida late Blancan faunas (Berta, 1987; Morgan and Hulbert, 1995). T. haysii is known from late Blancan faunas in Texas and Colorado (Strain, 1966; Hager, 1974; Hulbert, 1995).

Magnetostratigraphic data for Mesilla B (Vanderhill, 1986) indicate that the zone containing this fauna spans the time period from the Gauss/Matuyama boundary to the Olduvai Subchron (between about 2.6 and 1.8 Ma). Faunal evidence suggests that Mesilla B falls in the younger half of this interval, as typical Blancan taxa that went extinct by about 2.2 Ma (Hypolagus, Nannippus, and Rhynchotherium) are absent from this fauna. Pending the discovery of additional age-diagnostic taxa in the Mesilla B fauna, particularly among small mammals, this fauna is here considered to be latest Blancan (latest Pliocene) in age, between 2.2 and 1.8 Ma.

Mesilla Basin Fauna C:

The uppermost portion of the Camp Rice Formation in the southern Mesilla basin (fig. 12.1, site 29) contains Mammuthus, and is thus Irvingtonian or younger in age. Mesilla Basin Fauna C (hereafter shortened to Mesilla C) is correlative with the early Irvingtonian Tortugas Mountain LF near Las Cruces (Lucas et al., 1999, 2000) in the northern Mesilla basin. Among Irvingtonian faunas in New Mexico, Mesilla C has the most diverse sample of large vertebrates (table 1, from Vanderhill, 1986), consisting of 17 species: the land tortoises Gopherus and Hesperotestudo and 15 species of mammals, including Glyptotherium arizonae; Paramylodon harlani; the megalonychid ground sloth Megalonyx wheatleyi; the beaver Castor canadensis; the wolf Canis armbrusteri; a smaller coyote-like Canis sp.; the bobcat Lynx rufus; three horses, Equus cf. E. calobatus, E. scotti, and a large Equus, Camelops cf. C. hesternus; the cervid Navahoceros lascrucensis; the deer Odocoileus; the gomphothere Cuvieronius tropicus; and the mammoth Mammuthus cf. M. meridionalis.

The first appearance of Mammuthus in North America as an immigrant from Eurasia occurred sometime around 1.6 Ma (Lucas, 1995, 1996; Cassiliano, 1999). The primitive mammoth Mammuthus meridionalis, identified from Mesilla C (Vanderhill, 1986), occurs only in early Irvingtonian faunas, and is quickly replaced by (or evolved into) more advanced mammoths variously referred to M. haroldcooki, M. hayi, and M. imperator, which are also present in the early Irvingtonian. Although Glyptotherium arizonae is supposedly restricted to early Irvingtonian faunas in the southwestern United States (Gillette and Ray, 1981), fossils referred to this species from Mesilla B and Virden appear to be latest Blancan. The large wolflike canid Canis armbrusteri is found only in Irvingtonian faunas, with the oldest record of this species from the early Irvingtonian Leisey Shell Pit LF in Florida (Berta, 1995). The living beaver Castor canadensis is restricted to Irvingtonian and younger faunas in North America. A supposed late Blancan record of C. canadensis from the Haile 15A LF in Florida (Robertson, 1976) is based on a femur that is not diagnostic at the species level.

Mesilla C is most similar to early Irvingtonian faunas from the southern Great Plains, such as Gilliland and Rock Creek in Texas (Hibbard and Dalquest, 1966) and Holloman in Oklahoma (Dalquest, 1977), that contain Glyptotherium arizonae, Paramylodon harlani, Equus scotti, E. calobatus, and Mammuthus sp. Magnetostratigraphy for the interval that produces the Mesilla C fauna (Vanderhill, 1986) suggests correlation with the upper Matuyama Chron, spanning the time interval from just after the Olduvai Subchron (1.81 Ma) to just prior to the Jaramillo Subchron (1.07 Ma). Mack et al. (1996) obtained Ar/Ar dates on three pumice beds from the Camp Rice Formation near La Union, in the vicinity where many of the fossils from both Mesilla B and C were collected. A pumice dated at 1.59 Ma occurs about 30 m below the La Mesa surface (local top of the Camp Rice Formation in the La Union section) and a pumice dated at 1.32 Ma occurs about 10 m below the top of the section (Mack et al., 1996: fig. 2). Although these radioisotopic dates cannot currently be precisely correlated with the magnetostratigraphy and biostratigraphy, it appears that all of the Mesilla C fossils were collected from above the 1.59 Ma pumice date, which provides a maximum age for this fauna. It is likely that some fossils assigned to Mesilla C were collected from strata above the 1.32 Ma pumice date. The combination of biostratigraphy, magnetostratigraphy, and radioisotopic dates suggests an age between 1.6 and 1.1 Ma for Mesilla C (Vanderhill, 1986; Mack et al., 1996).

Buckhorn:

The Buckhorn LF is derived from 14 sites located between 3 and 10 km northwest of Buckhorn in northern Grant County, southwestern New Mexico (Morgan et al., 1997; fig. 12.1, site 30). The fossils occur in unconsolidated sands, silts, and muds through a stratigraphic interval of about 20 m in the upper part of the Gila Group in the Mangas basin. The abundance of aquatic vertebrates in several of the Buckhorn sites, including fish, frogs, flamingos, rails, and ducks, as well as the lithology of the sediments, suggests a freshwater depositional environment, possibly a large lake. The Buckhorn LF is the most diverse Blancan vertebrate faunal assemblage known from New Mexico, composed of 33 species, including 14 lower vertebrates and 19 mammals (Morgan et al., 1997). The nonmammals include three species of small fish, the frog Rana, the salamander Ambystoma, two unidentified colubrid snakes, a lizard, and six birds—the flamingo Phoenicopterus, two ducks, a rail, the turkey cf. Meleagris, and a small perching bird. The mammalian fauna of 19 species includes a microchiropteran bat; a rabbit; the ground squirrel Spermophilus cf. S. bensoni; Repomys cf. R. panacensis; the vole Mimomys poaphagus, Peromyscus sp., and Baiomys sp.; four carnivores including the badger Taxidea, a bear, a small feline, and a large machairodontine; two horses, Nannippus peninsulatus and Equus simplicidens; a peccary, three camelids, Camelops sp., Hemiauchenia blancoensis, and a small Hemiauchenia, an indeterminate ruminant, and Stegomastodon.

Many of the mammals from the Buckhorn LF are indicative of a Blancan age, including Mimomys, Repomys cf. R. panacaensis, Nannippus peninsulatus, Equus simplicidens, and Hemiauchenia blancoensis. Several taxa allow for a more precise placement within the Blancan. The presence of a primitive species of the arvicoline rodent Mimomys and the absence of Neotropical immigrants suggest a pre-late Blancan age (older than 2.7 Ma). The occurrence of Equus simplicidens and a large species of Mimomys (subgenus Ogmodontomys) excludes very early Blancan faunas. The evolutionary stage of two of the Buckhorn rodents, Mimomys (Ogmodontomys) poaphagus and Repomys cf. R. panacaensis, is most consistent with a late early Blancan age (between about 4.2 and 3.6 Ma; Blancan II of Repenning, 1987) for the Buckhorn LF.

The Buckhorn LF and the Truth or Consequences LF from the central Rio Grande Valley both appear to be early Blancan in age on the basis of comparisons with early Blancan faunas outside of New Mexico; however, the mammalian faunas from these two sites have virtually no age-diagnostic taxa in common, particularly among the small mammals. Mimomys, the extinct “cricetid” genus Repomys, and the tiny sigmodontine Baiomys occur in the Buckhorn LF, whereas the Truth or Consequences LF is dominated by the wood rat Neotoma, the cotton rat Sigmodon, and the small extinct sigmodontine genus Jacobsomys. The Buckhorn and Truth or Consequences faunas must have sampled different habitats or perhaps differ enough in age to have allowed significant change in the small mammal fauna. Other early Blancan faunas that are broadly correlative with Buckhorn are the Verde LF in Arizona (Czaplewski, 1987, 1990), the Panaca LF in Nevada (May, 1981; Mou, 1997, 1999), and the Rexroad 3 and Fox Canyon faunas from Kansas (Hibbard, 1938, 1941, 1950, 1967; Repenning, 1987).

There are currently no radioisotopic dates or magnetostratigraphic data that would provide additional information on the age of the Buckhorn LF. There are several volcanic ash beds in the Buckhorn section, but they are too altered to provide accurate radioisotopic dates. The latest Hemphillian Walnut Canyon LF occurs in Gila Group strata about 25 km southeast of the Buckhorn LF (Morgan et al., 1997; fig. 12.1, site 31). The latest Hemphillian age (earliest Pliocene; 5.0–4.5 Ma) of the Walnut Canyon LF provides a maximum age for the Buckhorn LF. These two faunas have no taxa in common, indicating that the Hemphillian/Blancan boundary occurs somewhere in the section separating them. Further fieldwork in the region between Buckhorn and Walnut Canyon should allow a more precise stratigraphic placement of these two faunas.

Pearson Mesa:

Exposures on Pearson Mesa south of the Gila River in the Duncan basin along the New Mexico-Arizona border have produced a diverse assemblage of late Blancan vertebrate fossils from strata of the Gila Group (fig. 12.1, sites 32, 33). Most of Pearson Mesa is located in Hidalgo County in southwestern New Mexico, but the northwestern point of the mesa extends into Greenlee County in southeastern Arizona. About three-fourths of the 76 NMMNH fossil sites located on Pearson Mesa are in New Mexico and the rest are in Arizona. The stratigraphic section at Pearson Mesa consists of more than 60 m of sandstones, mudstones, and sedimentary breccias referred to the Gila Group. The lower 15 m of this section contains the three-toed horse Nannippus peninsulatus and the horse Equus simplicidens, together with rarer specimens of many other taxa, the most significant of which is the mylodont ground sloth Glossotherium cf. G. chapadmalense. This assemblage, the Pearson Mesa LF (Tomida, 1987; Morgan and Lucas, 2000b; fig. 12.1, site 32), is indicative of a late Blancan age. A latest Blancan fauna, the Virden LF (fig. 12.1, site 33), occurs in the upper 20 m of the section at Pearson Mesa, and is discussed below.

The Pearson Mesa LF consists of 16 species of vertebrates: the land tortoise Gopherus, a large and a small species of the land tortoise Hesperotestudo, the box turtle Terrapene, a heron, and 11 mammals, including Glossotherium cf. G. chapadmalense; a small cat and a larger machairodontine sabercat; the pocket gopher Geomys (Nerterogeomys) cf. G. persimilis; the horses Nannippus peninsulatus, Equus cumminsii, E. scotti, and E. simplicidens; the peccary Platygonus bicalcaratus; the camelid Hemiauchenia blancoensis; and the gomphothere Stegomastodon rexroadensis. The presence of Nannippus in the Pearson Mesa LF indicates an age greater than 2.2 Ma, whereas Glossotherium does not appear in North American faunas until the late Blancan, indicating an age less than 2.7 Ma. Other taxa from the Pearson Mesa LF that provide some indication of age include Geomys (N.) persimilis, Platygonus bicalcaratus, and Stegomastodon rexroadensis. G. (N.) persimilis occurs in medial Blancan through early Irvingtonian sites in the southwestern United States. Tomida (1987) noted that the presence of this pocket gopher in the lower part of the Pearson Mesa section suggested placement in the uppermost Gauss Chron. Platygonus bicalcaratus apparently is restricted to medial and late Blancan faunas, and Stegomastodon rexroadensis occurs in the early and medial Blancan. The mandible of Stegomastodon upon which this record is based (F:AM 23338) is from an imprecisely located site in the vicinity of Pearson Mesa, and is of unknown stratigraphic position. A mandible of similar morphology, referred to S. rexroadensis, occurs in the medial Blancan Cuchillo Negro Creek LF (Lucas and Oakes, 1986). The medial Blancan Duncan Fauna is located near Duncan, Arizona, less than 10 km northwest of Pearson Mesa (Tomida, 1987), and thus it is possible that the S. rexroadensis mandible was derived from lower in the Gila Group section than strata that produce the Pearson Mesa LF.

The association of Nannippus with Glossotherium provides the most significant biostratigraphic information regarding the age of the Pearson Mesa LF. The first appearance of South American immigrants in North American faunas, following the onset of the Great American Interchange at about 2.7 Ma, defines the beginning of the late Blancan. The only time interval during which these two genera coexisted in southwestern faunas was between 2.7 Ma (the beginning of the Interchange) and 2.2 Ma (the Nannippus extinction datum of Lindsay et al., 1984). A paleomagnetic section from Pearson Mesa (Tomida, 1987) further restricts the age of the lower portion of the section, including the Pearson Mesa LF, to the upper Gauss Chron (3.0–2.6 Ma). Combining the biostratigraphic and magnetostratigraphic data for the Pearson Mesa LF would seem to tightly constrain the age of this fauna between 2.7 Ma (earliest appearance of Glossotherium) and 2.6 Ma (Gauss/Matuyama boundary). Although the timing of the first arrival of South American immigrants in North America is not precisely known, Pearson Mesa is important because it is one of only six sites in the southwestern United States that records the presence of South American Interchange mammals in the Gauss Chron, indicating an age greater than 2.6 Ma.

Virden:

A substantial thickness (about 30 m) of Gila Group strata in the middle of the Pearson Mesa section has not yet produced fossils, including a 9-m-thick breccia at the top of this unfossiliferous interval that may represent a hiatus (Morgan and Lucas, 2000b). Just above this breccia, at about the 45 m level in the section and continuing for about the next 10 m, vertebrate fossils occur at several localities on Pearson Mesa. This fauna differs significantly from the underlying Pearson Mesa LF, and is here named the Virden Local Fauna for the nearby village of Virden, New Mexico (fig. 12.1, site 33). Exposures in the upper portion of the Pearson Mesa section are not as extensive and tend to be more vertical than lower in the section, and consequently the Virden LF is not as abundant or diverse as the Pearson Mesa LF. Nonetheless, the presence of Glyptotherium arizonae and the absence of Nannippus in the Virden LF indicate that this fauna is younger than the Pearson Mesa LF, either latest Blancan or earliest Irvingtonian in age. The only taxa these faunas share are a large land tortoise and Equus scotti.

The Virden LF consists of seven species: the land tortoise Hesperotestudo and six mammals, including Glyptotherium arizonae, the coyote-like canid Canis lepophagus, Equus scotti, Camelops, a small Hemiauchenia, and an indeterminate proboscidean. The glyptodonts in the Virden LF, represented by osteoderms, a cranial fragment, and several postcranial elements, are referable to the large species G. arizonae. Glyptodonts first appear in southwestern faunas in the early late Blancan (upper Gauss Chron) in sites equivalent in age to the Pearson Mesa LF (e.g., Hudspeth and Cita Canyon in Texas and 111 Ranch in Arizona), but these occurrences represent the smaller species G. texanum. Glyptodonts are unknown from the Pearson Mesa LF. Although previous records of G. arizonae indicated that this species was restricted to the early Irvingtonian in the southwestern United States (Gillette and Ray, 1981), there are now late Blancan records of this species from Curtis Ranch in Arizona, Mesilla B, and Virden, as well as Florida (Morgan and Hulbert, 1995).

Associated mandibles of a medium-sized canid, similar in size and morphology to the coyote-sized species Canis lepophagus, were found in association with Glyptotherium osteoderms in the upper portion of the Pearson Mesa section. C. lepophagus is restricted to the Blancan, including the late Blancan Cita Canyon and Red Light faunas in Texas (Akersten, 1972) and the late Blancan 111 Ranch fauna in Arizona (Galusha et al., 1984; Tomida, 1987). A pair of lower jaws from Virden represents a small species of Hemiauchenia. These mandibles are from an adult with well-worn teeth, but are very small compared to other known species of Hemiauchenia, such as the Blancan H. blancoensis and the Irvingtonian and Rancholabrean H. macrocephala. A small Hemiauchenia occurs in several New Mexico Blancan faunas, including Cuchillo Negro Creek, Tonuco Mountain, and Buckhorn, as well as two early Irvingtonian sites in Florida (Morgan and Hulbert, 1995). Equus scotti is one of the most common horses found in southwestern early Irvingtonian faunas, but this species also occurs in medial and late Blancan sites in New Mexico.

The presence of Glyptotherium arizonae and the absence of diagnostic Blancan indicators (e.g., Nannippus) suggests that the Virden LF is distinctly younger than the underlying Pearson Mesa LF, probably either latest Blancan or earliest Irvingtonian (between 2.2 and 1.6 Ma). The presence of Canis lepophagus, a species unknown from the Irvingtonian, suggests that a latest Blancan age is more likely. Tomida's (1987) Pearson Mesa paleomagnetic stratigraphy does not include the upper 20 m of the section (Morgan and Lucas, 2000b), and thus we currently have no magnetostratigraphic data for this fauna. Paleomagnetic samples from the upper portion of the section recently collected by G. Mack should help clarify the age of the Virden LF.