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Fossil fauna and flora from late pleistocene Cerros Negros locality, Pinal County, Arizona: with update of Mammuthus and all Mammut localities from Arizona.

Records of mastodons Mammut americanum have been published for only six localities in Arizona: Davidson Canyon, Pima County (MacDougal, 1908); Greenbush Draw, Cochise County (Haury et al., 1953); Saint Johns, Apache County (Saunders, 1970); Billings, Santa Cruz County (Minckley et al., 1997); Lehner Ranch, Cochise County (Mead et al., 1979); and M and M site, Yavapai County (Minckley et al., 1997). Lehner Ranch and the M and M site are the only published localities in Arizona documented by verifiable remains of Mammut in collections or repositories. These sites are also the only reported localities for mastodons in Arizona with associated pollen and molluscan data, plus radiocarbon dates. This report documents the Cerros Negros locality (Fig. 1), which constitutes a new stratified deposit containing remains of mastodons (Mammut), mammoths (Mammuthus columbi), horses (Equus), pollen, and snails. In addition, radiocarbon dates have been established.

Remains of mammoths have been documented from >96 localities in Arizona (Saunders, 1970; Agenbroad and Mead, 1989; Agenbroad et al., 1992). Cerros Negros is the second reported locality for mammoths, as well as the first reported locality for mastodons, from the central portion of the San Pedro River valley.

Materials and Methods--The Cerros Negros locality (UALP A65) is in Pinal County, Arizona (Fig. 1; 32.53TN, 110.56[degrees]W). Deposits containing fossils are remnants of horizontal sedimentary beds of late Pleistocene age that have been dissected, but are preserved as remnants inset against Tertiary sediments within a canyon known as Stratton Wash, a western tributary to the San Pedro River. At an elevation of 960 m above sea level, these deposits presently are surrounded by typical creosotebush-dominated desert scrub (Larrea tridentata) of the Sonoran Desert.

Deposits in Cerros Negros pond were discovered and mapped as part of a doctoral dissertation (Agenbroad, 1967). Deposits represent stratified, water-laid sediments such as would be deposited in an arroyo impoundment within a drainage incised into an older, Tertiary pediment. Deposits are on the west side of the San Pedro valley, 138 m higher than the San Pedro River 1.6 km to the east. The lacustrine deposits have a total thickness of 4 m.

A pale-brown, pebble-cobble conglomerate with a calcareous, silty matrix and cement is the basal unit (<1 m thick). This basal conglomerate grades upward into a pale-brown silty, clayey marl ([+ or -]2.5 m thick) overlain by a white, massive marl ([+ or -]1.5 m) containing lesser amounts of clay than the lower marl. Snails are in the upper 3 m of marls, and the total thickness of 4 m has been dated at 27,000 to 12,000 years before present (Fig. 2). This is the same age, with similar stratigraphy, as the Coro Marl of the Murray Springs Formation (Haynes, 1968, 2007) 140 km to the south (upriver). Similarities of the stratigraphic sequences and radiocarbon dates of the two outcrops argue for similar deposition and correlation. Haynes (1968) suggested a large lake throughout the upper San Pedro Valley. Later he abandoned that interpretation to one that calls for synchronous small lakes or ponds (Haynes, 2007). The updated interpretation matches well with there being a pond at Cerros Negros. A recent study by Pigati et al. (2009) indicated the Coro Marl and similar deposits may have started forming as early as 50,000 years before present.


No trace of a dam remains. An exposure of granite and a diorite dike cropout in the floor of the wash, about mid-length of the existing late Pleistocene deposits. It is not believed that the igneous outcrops caused the damming, although they may have created a local knick point in the drainage that was dammed by mass wasting of canyon walls, farther to the east, sometime before 27,000 years ago. Correlation of deposits at Cerros Negros with the Coro Marl of the upper San Pedro Valley implies a low-gradient valley floored with numerous spring-fed ponds and marshes (Haynes, 1968, 2007; Pigati et al., 2009) created by rising groundwater tables. Later incision of the valley has left these marl deposits stranded on margins of the dissected valley floor.


Results--Remains of mammoths and horses were found in the conglomerate-lower-marl contact. A mastodon tooth, recovered from upper marl units, was associated with mollusks. At time of discovery, tooth enamel of Mammuthus and teeth of Equus were collected from the deposit. LDA and the late John Lance returned to the locality and collected the mandible of Mammuthus columbi (Agenbroad, 1967). Haynes (1968) produced a stratigraphic section of the Cerros Negros deposits. Radiocarbon analyses of marl carbonates provide a basal age of 27,060 [+ or -] 1,080 years before present (Tx-973) for the deposit and a minimum age from the upper marl unit of 13,630 [+ or -] 200 years before present (Tx-976; Velastro et al., 1975). A later radiocarbon age of 12,000 [+ or -] 300 years before present (A-854) was derived from the upper 10 cm of the marl (Haynes et al., 1971). These dates imply 15,000 years for duration of the local impoundment. A molluscan fauna also was noted, but it remained unstudied until the 1980s when the site was revisited to collect samples of pollen and mollusks.

At least one individual of Mammuthus columbi was represented by a mandible (UALP 2464) containing two last molars (M6). The mandible was at the contact of the lower marl carbonate and the basal conglomerate. An estimated age is 27,000 years before present based on stratigraphic position at the basal conglomerate-lower marl contact (Fig. 2).

It was during a return visit to the locality in December 1982 by LDA and JIM that material was collected involving at least one individual Mammut americanum represented by a mandibular tooth (MSM p7699) and fragments of enamel. This specimen was in the upper lacustrine marl. Age is estimated as 12,000-21,000 years before present as per the stratigraphic section (Fig. 2).

At least 26 localities with Mammuthus have been reported in Arizona since the compilation by Saunders (1970). Minckley et al. (1997) provided an update of Mammut with six localities in the state. Distribution ofthe 26 new (post-1970) localities for mammoths and the eight localities for mastodons in Arizona are depicted in Fig. 1.

Mollusks recovered from the Cerros Negros locality are from a small, single-matrix sample from around the remains of Mammut. Mollusks recovered (Mead, 1991) included four terrestrial gastropods (Deroceras, Succinea, Gastrocopta, and Vertigo), two aquatic snails (Fossaria and Gyraulus), and a fingernail clam (Pisidium). Identifications of specimens follow Pilsbry (1948) for gastropods and Herrington (1962) for clams. All molluscan specimens are consistent with a reconstruction of a pond environment with near-shore vegetation producing shade and leaf litter. Riparian woodland adjacent to a small pond is a possible scenario. These remains were presented previously in an overview of molluscan faunas in southeastern Arizona (Mead, 1991). Reconstruction of the riparian and pond community at Cerros Negros fits the pattern observed elsewhere along the San Pedro River Valley for the late Pleistocene.

Pollen was extracted from samples of sediments of various stratigraphic units of the Cerros Negros site using standard procedures (Gray, 1965). Units were rich in carbonates, necessitating prolonged and repeated treatment with hydrochloric acid. Preservation of pollen was poor in all samples. It was possible to identify >100 pollen grains in only one sample; that was from the marl unit in close association with remains of vertebrates. Pollen in this sample was composed predominantly of grasses (Gramineae), with moderate proportions of juniper (Juniperus), cheno-ams (Chenopodiaceae and Amaranthaceae), and Asteraceae (Compositae). Low proportions of pollens of pine (Pinus), oak (Quercus), hackberry (Celtis), walnut (Juglans), mesquite (Prosopis), ash (Fraxinus), and sedge (Carex) also were present.

When compared to pollen from the slightly younger Lehner Ranch and Naco Clovis sites (Mehringer and Haynes, 1965), it is apparent that pollen from Cerros Negros has more grass, juniper, oak, and hackberry, and less sunflower and cheno-am than other localities to the south. Proportions of oak and pine are about equal to those observed in examples from the Sonoran and Chihuahuan deserts, as well as some samples from desert grasslands, but proportions of juniper pollen are comparable only to those of modern juniper savannas of northern Arizona. Proportions of grass are matched only with samples from modern oak woodlands of southern Arizona. The particular combinations of types and proportions of pollen have not been observed in soils of modern plant communities thus far sampled in Arizona (Martin, 1963; Hevly, 1964; Hevly et al., 1965; Martin and Mehrenger, 1965). Fossil pollen at Cerros Negros appears to reflect a grassland with junipers, mesquites, and possibly oaks in proximity to a hygric system with walnuts, hackberries, ashes, and sedges.

Discussion--Extinct mammals at Cerros Negros and nearby localities indicate the fauna apparently lived in a more mesic environment than presently exists in the San Pedro Valley. This environment ended as ponds and marshes dried and were eroded in the late Pleistocene, possibly due to less precipitation and onset of degradation of the valley floor. Clovis hunters killed mammoths and bison during the terminal late Pleistocene (Haury et al., 1953, 1959; Hemmings and Haynes, 1969; Hemmings, 1970, 2007). Pollen from the Naco and Lehner sites (Mehringer and Haynes, 1965) indicate increasing aridity at the end of the Clovis occupation of the valley, as does geological and palynological data at nearby Wilcox Playa (Hevly and Martin, 1961; Haynes et al., 1987) and Murray Springs during the Clovis drought (Haynes, 1991, 2007).

At the Cerros Negros locality, analyses of pollen and molluscan studies provide insight into the local paleoenvironment. Mollusks are consistent with a riparian and pond community similar to other late Pleistocene samples from the San Pedro Valley. The paleoenvironment at Cerros Negros reflects a grassland with similarities to juniper savannas of northern Arizona, as well as oak woodlands of southern Arizona. The environment at Cerros Negros supported walnuts, hackberries, ashes, and sedges. Types, abundances, and combinations of pollens suggest a more mesic environment that rarely is encountered in the Southwest at present.

Remains of mammoths and mastodons have been found in close association with large drainage systems in Arizona (Fig. 1). The Fain-Jordan mammoth site in the upper Agua Fria drainage (Pasenko and Agenbroad, 2012) provides the youngest (11,560 years before present) locality of mammoths in Arizona not associated with hunting activity of humans. The M and M site (Minckley et al., 1997) provides the youngest (10,360 years before present) locality for mastodons in Arizona.

Submitted 2 June 2009. Accepted 9 September 2012. Associate Editor was Gary D. Schnell.


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Larry D. Agenbroad,* Jim I. Mead,C.Vance Haynes,Jr., and Richard H. Hevly

Mammoth Site of Hot Springs, South Dakota, P.O. Box 692, Hot Springs, SD 57747 (LDA) Department of Geosciences, East Tennessee State University, Johnson City, TN 37614 (JIM) Department of Anthropology and Department of Geosciences, University of Arizona, Tucson, AZ 85721 (CVH) 257 Forest Highlands, Flagstaff, AZ 86001(RHH)

* Correspondent:
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Author:Agenbroad, Larry D.; Mead, Jim I.; Haynes, C. Vance, Jr.; Hevly, Richard H.
Publication:Southwestern Naturalist
Article Type:Report
Geographic Code:1USA
Date:Mar 1, 2013
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