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

^*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; ^‖Department 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)

Abstract

Isotopic age determinations (⁴⁰Ar/³⁹Ar) and associated magnetic polarity stratigraphy for Casamayoran age fauna at Gran Barranca (Chubut, Argentina) indicate that the Barrancan “subage” of the Casamayoran South American Land Mammal “Age” is late Eocene, 18 to 20 million years younger than hitherto supposed. Correlations of the radioisotopically dated magnetic polarity stratigraphy at Gran Barranca with the Cenozoic geomagnetic polarity time scale indicate that Barrancan faunal levels at the Gran Barranca date to within the magnetochronologic interval from 35.34 to 36.62 megannums (Ma) or 35.69 to 37.60 Ma. This age revision constrains the timing of an adaptive shift in mammalian herbivores toward hypsodonty. Specifically, the appearance of large numbers of hypsodont taxa in South America occurred sometime between 36 and 32 Ma (late Eocene–early Oligocene), at approximately the same time that other biotic and geologic evidence has suggested the Southern high latitudes experienced climatic cooling associated with Antarctic glaciation.

Footnotes

↵ † To whom reprint requests should be addressed. E-mail: Rick_Madden{at}baa.mc.duke.edu.
↵ § Deceased October 1, 1999.
This paper was submitted directly (Track II) to the PNAS office.
↵ ‡‡ Bond, M., Reguero, M. A. & Vizcaino, S. F., Boletin de Resumenes, XIII Congresso Brasileiro de Paleontologia y I Simposio de Paleontólogia do Cone Sul, Universidade do Vale do Rio dos Sinos, Sao Leopoldo, Brasil, September 19–26, 1993, p. 93 (abstr.).
↵ †† Madden, R. H., and Williams, S. H. & Kay, R. F. (1999) in Programa y Resumenes, Congreso Internacional Evolución Neotropical del Cenozoico, Academia Nacional de Ciencias and Museo Nacional de Historia Natural, La Paz, Bolivia, May 19–22, 1999, p. 29 (abstr.), and p. 46 (abstr.), respectively.
Abbreviations:

Ma,

megannum, 1 million years in the radioisotopic time scale;

GPTS,

geomagnetic polarity time scale;

MZ,

Mazzoni stratigraphic level;

SALMA,

South American Land Mammal Age