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Revised Geochronology of the Casamayoran South American Land Mammal Age: Climatic and Biotic Implications

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Revised Geochronology of the Casamayoran South American Land Mammal Age: Climatic and Biotic Implications Revised geochronology of the Casamayoran South American Land Mammal Age: Climatic and biotic implications Richard F. Kay*†, Richard H. Madden*, M. Guiomar Vucetich‡, Alfredo A. Carlini‡, Mario M. Mazzoni‡§, Guillermo H. Re¶, Matthew Heizlerʈ, and Hamish Sandeman** *Department of Biological Anthropology and Anatomy, Duke University, Durham, NC 27710; ‡Facultad de Ciencias Naturales y Museo, Universidad Nacional de La Plata, La Plata, Argentina; ¶Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Buenos Aires, Argentina; ʈDepartment of Earth and Environmental Sciences, New Mexico Institute of Mining and Technology, Socorro, NM 87801; and **Department of Geological Sciences, Queen’s University, Kingston, ON, Canada K7L 3N6 Edited by David L. Dilcher, University of Florida, Gainesville, FL, and approved July 21, 1999 (received for review April 20, 1999) Isotopic age determinations (40Ar͞39Ar) and associated magnetic Methods and Results polarity stratigraphy for Casamayoran age fauna at Gran Barranca A stratigraphic interval (75 m) of the Gran Barranca, encompassing (Chubut, Argentina) indicate that the Barrancan ‘‘subage’’ of the the Gran Barranca and Puesto Almendra members and including Casamayoran South American Land Mammal ‘‘Age’’ is late Eocene, 18 the Barrancan, Mustersan, and Astraponote´en plus Supe´rieur to 20 million years younger than hitherto supposed. Correlations of faunas, was sampled for radiometric age determination and mag- the radioisotopically dated magnetic polarity stratigraphy at Gran netic polarity stratigraphy. Three 40Ar͞39Ar dates were obtained Barranca with the Cenozoic geomagnetic polarity time scale indicate from a sample of dark glass-rich pyroclastic tuff, MZ-7 level that Barrancan faunal levels at the Gran Barranca date to within the [stratigraphic levels in the measured composite section of the magnetochronologic interval from 35.34 to 36.62 megannums (Ma) or Sarmiento Formation at the west end of Gran Barranca are staked, 35.69 to 37.60 Ma. This age revision constrains the timing of an labeled, numbered consecutively, and designated MZ by one of the adaptive shift in mammalian herbivores toward hypsodonty. Specif- authors of the present paper (M.M.M.)], equivalent to the ‘‘steel EVOLUTION ically, the appearance of large numbers of hypsodont taxa in South gray band’’ (9) within the Gran Barranca member and within the America occurred sometime between 36 and 32 Ma (late Eocene– sampled range of the Barrancan fauna (Fig. 1). Bulk plagioclase and early Oligocene), at approximately the same time that other biotic glass separates from MZ-7 were analyzed by 40Ar͞39Ar laser-fusion and geologic evidence has suggested the Southern high latitudes and step-heating age spectrum methods, respectively. The age experienced climatic cooling associated with Antarctic glaciation. spectrum for glass did not yield a plateau age, but climbed to an age of 32.74 Ϯ 0.90 Ma for the final 30% of the released 39Ar (Table mammalian evolution ͉ South America ͉ Eocene ͉ paleontology 1; Fig. 2 Upper). The age gradient for glass suggests argon loss, so the maximum age recorded by the high temperature steps is he isotopic ages of Eocene mammalian faunas of South Amer- interpreted to be a minimum age for the glass. Plagioclase was also analyzed by laser fusion (Fig. 2 Lower). Seven splits yielded ana- ica are poorly known: there are no radiometric dates from rock T lytically indistinguishable total-fusion ages with a weighted mean of units containing fossil mammals. Two time-successive faunas, the 36.01 Ϯ 0.44 Ma (Table 2). Given the fine-grained nature of the Casamayoran and the Mustersan South American Land Mammal plagioclase, we cannot rule out xenocrystic contamination during Ages (SALMAs), conventionally are regarded as representing early postdepositional reworking and͞or from the vent during eruption. [55–50 megannums (Ma)] and middle (45–40 Ma) Eocene, respec- Assuming no argon loss, the plagioclase age is interpreted to tively. A third, the Divisaderan, found only at one locality, is represent a maximum age. On the basis of these results, the age considered to be late Eocene (1–6). We report newly dated rocks range for the volcanic eruption producing the MZ-7 tuff is between from the Gran Barranca, a cliff south of Lake Colhue Huapi (S45° Ј Љ Ј Љ 32.7 and 36.0 Ma. A whole rock sample from MZ-7 was analyzed 42 49 , W68° 44 16 in Chubut Province, Argentina) where there with the total fusion 40Ar͞39Ar method by a second laboratory, occurs an important Cenozoic fossil-mammal succession (Figs. 1 yielding an age of 38.1 Ϯ 3.0 Ma (Table 3). The whole-rock data and 2). The Gran Barranca consists of pyroclastic sediments of the suggest that the true age is closer to the plagioclase than to the glass Sarmiento Formation, which is subdivided into three members (7, age (i.e., nearer 36 Ma). 8). (i) The oldest Gran Barranca member contains a fauna upon Rock samples were collected for paleomagnetic study (five which is based the Barrancan ‘‘subage’’ of the Casamayoran samples for each site from MZ-1 to MZ-16) through the Gran SALMA, the younger of two subages (Vacan and Barrancan) of the Barranca and Puesto Almendra members (Fig. 1). The sampling Casamayoran (9). (ii) The intermediate Puesto Almendra member interval between sites averaged 10 meters. From each sample, two contains the type assemblage of the Mustersan SALMA (3). specimens were obtained and the magnetic susceptibility, intensity, Toward the top of this member occurs an as-yet poorly sampled fauna called the ‘‘Astraponote´en plus Supe´rieur’’ of Ameghino††, similar faunally to the Tinguiririca fauna of central Chile (10, 11). This paper was submitted directly (Track II) to the PNAS office. The Puesto Almendra member is disconformably overlain by basalt Abbreviations: Ma, megannum, 1 million years in the radioisotopic time scale; GPTS, Ϯ iii geomagnetic polarity time scale; MZ, Mazzoni stratigraphic level; SALMA, South American flows dated (K–Ar) at 28.8 0.9 Ma (12). ( ) The youngest Land Mammal Age. exposed unit is the richly fossiliferous Colhue-Huapi member, †To whom reprint requests should be addressed. E-mail: Rick࿝[email protected]. which contains a major locality for mammals of the Colhuehuapian §Deceased October 1, 1999. SALMA. At Gran Barranca this latter fauna is dated to between 20.5 and 19.1 Ma. (13). Thus, Gran Barranca samples all Eocene ††Bond, M., Reguero, M. A. & Vizcaino, S. F., Boletin de Resumenes, XIII Congresso Brasileiro de Paleontologia y I Simposio de Paleonto´logia do Cone Sul, Universidade do through Oligocene SALMAs except possibly the Deseadan. (Mam- Vale do Rio dos Sinos, Sao Leopoldo, Brasil, September 19–26, 1993, p. 93 (abstr.). mals of the Deseadan age may occur at the Barranca accompanying The publication costs of this article were defrayed in part by page charge payment. This stringers of basalt pebbles associated with the initial stages of the article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. erosion of the basalt flows.) §1734 solely to indicate this fact. PNAS ͉ November 9, 1999 ͉ vol. 96 ͉ no. 23 ͉ 13235–13240 Downloaded by guest on September 24, 2021 Fig. 1. Stratigraphic profile of the west end of the Gran Barranca, south of Lake Colhue´-Huapı´, Chubut Province, Argentina, indicating MZ levels, lithostratigraphy, stratigraphic nomenclature (7, 8), dated levels, magnetostratigraphy, Simpson’s informal stratigraphic terminology, and the Barrancan faunal zone. The profile on which the figure is based is 0.5 km east of Cifelli’s (9) Section I. MZ levels sampled for magnetic polarity stratigraphy, radioisotopic dating, lithostratigraphy, and phytolith stratigraphy. Radioisotopic dates are from this paper and refs. 11, 12, and 44. 40Ar͞39Ar and 40K–40Ar dates are given in Ma. declination, and inclination of the natural remnant magnetization ature of 700°C in a Schonstedt (Kearneysville, WV) SSM2 furnace. were measured by using a 2-G cryogenic magnetometer at the Magnetic susceptibility was measured after each heating step to University of Buenos Aires. Half of the specimens were subjected determine possible changes in magnetic mineralogy. The remaining to demagnetization in 15 incremental steps to a maximum temper- specimens were subjected to alternating field demagnetization. Up Table 1. Furnace argon isotopic results for MZ-7 glass 36 39 39 40 39 Temp, Ar͞ Ar ArK Ar*, Ar, Age, Ϯ1 ␴, Step °C 40Ar͞39Ar 37Ar͞39Ar (ϫ10Ϫ3) (ϫ10Ϫ15 mol) K͞Ca % % Ma Ma A 550 10385 0.2738 35070 0.132 1.9 0.2 0.4 32 504 B 600 134.3 0.5938 435.1 1.03 0.86 4.3 3.1 8.4 1.5 C 650 69.96 0.6300 201.8 2.71 0.81 14.8 10.3 14.96 0.51 D 700 52.46 0.6468 131.1 2.43 0.79 26.2 16.7 19.81 0.53 E 730 53.10 0.7097 121.3 6.31 0.72 32.6 33.5 24.90 0.32 F 760 47.85 0.7493 96.78 4.84 0.68 40.3 46.3 27.75 0.27 G 800 53.06 0.7971 107.3 6.25 0.64 40.3 62.9 30.75 0.29 H 830 59.22 0.8182 125.6 5.39 0.62 37.4 77.2 31.83 0.34 I 870 71.23 0.8627 164.1 4.11 0.59 32.0 88.1 32.74 0.45 J 900 118.9 0.9004 329.1 2.40 0.57 18.3 94.5 31.17 0.76 K 950 318.3 0.9827 1007.8 1.39 0.52 6.5 98.2 29.6 2.6 L 1000 1302.7 0.8560 4331 0.688 0.60 1.8 100.0 33.0 14.0 Total gas age 37.7 0.67 27.3 2.5 MZ-7 glass, sample mass ϭ 19.73 mg, J (radiation parameter for neutron flux) ϭ 0.0008025, Laboratory sample 8955–01.
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