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5-1985

Magnetic Polarity Stratigraphy And Mammalian Fauna Of The Deseadan (Late Oligocene-Early Miocene) Salla Beds Of Northern Bolivia

Bruce J. Macfadden University of Florida, Gainesville

Kenneth E. Campbell Jr. Natural History Museum of Los Angeles County

Richard L. Cifelli National Museum of National History

Oscar Siles Servicio Geol6gico de Bolivia

Noye M. Johnson Dartmouth College

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Macfadden, Bruce J.; Campbell, Kenneth E. Jr.; Cifelli, Richard L.; Siles, Oscar; Johnson, Noye M.; Naeser, Charles W.; and Zeitler, Peter K., "Magnetic Polarity Stratigraphy And Mammalian Fauna Of The Deseadan (Late Oligocene-Early Miocene) Salla Beds Of Northern Bolivia" (1985). USGS Staff -- Published Research. 496. https://digitalcommons.unl.edu/usgsstaffpub/496

This Article is brought to you for free and open access by the US Geological Survey at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in USGS Staff -- Published Research by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. Authors Bruce J. Macfadden, Kenneth E. Campbell Jr., Richard L. Cifelli, Oscar Siles, Noye M. Johnson, Charles W. Naeser, and Peter K. Zeitler

This article is available at DigitalCommons@University of Nebraska - Lincoln: https://digitalcommons.unl.edu/ usgsstaffpub/496 VOLUME 93 NUMBER 3

THE JOURNAL OF GEOLOGY

May 1985

MAGNETIC POLARITY STRATIGRAPHY AND MAMMALIAN FAUNA OF THE DESEADAN (LATE OLIGOCENE-EARLY MIOCENE) SALLA BEDS OF NORTHERN BOLIVIA1

BRUCE J. MACFADDEN, KENNETH E. CAMPBELL, JR., RICHARD L. CIFELLI, OSCAR SILES, NOYE M. JOHNSON, CHARLES W. NAESER, AND PETER K. ZEITLER FloridaState Museum, Universityof Florida,Gainesville, FL 32611(U.S.A.) NaturalHistory Museumof Los Angeles County, Los Angeles, CA 90007 (U.S.A.) Division of Mammals,National Museum of NationalHistory, SmithsonianInstitution, Washington, D.C. 20560(U.S.A) Servicio Geol6gico de Bolivia, La Paz (Bolivia) Departmentof Earth Sciences, DartmouthCollege, Hanover, NH 03755 (U.S.A.) Branchof Isotope Geology, U.S. GeologicalSurvey, Denver, Colorado80225 (U.S.A.) ResearchSchool of Earth Sciences, AustralianNational University, Canberra,A.C.T. 2601 (Australia) ABSTRACT

The Salla Beds containa rich assemblageof Deseadanmammals that traditionally has been consideredof early Oligoceneage. These deposits, located 90-100 km SE of La Paz, Bolivia, consist of more than 540 m of principallyfluviatile clays and silts with numerousinterbedded tuffs. Paleomagneticsamples were col- lected from 104 sites spaced at stratigraphicintervals of about 6 m. As a result of thermaldemagnetization (at 400°Cor greater),unambiguous polarities were determinedfor 76 of the 104originally sampled sites. The patternof reversals,stage of evolutionof the fossils, and preliminaryisotopic dates (includinga K/Ar age of 26.4 ± 1.0 Ma and zircon fission trackages of ca. 22 Ma)from interbedded tuffs suggestthat the Salla Beds correlateto chrons C10 throughC6 of the magneticpolarity time scale, or an absolute time intervalfrom about 28.5 to 24 Ma. Pyrotheriumand caviomorphrodents occur throughoutthe section; the primate Branisellais known to occur at about 27 Ma. The Salla faunas seem to representan earlier part of the Deseadanthan faunasof this Land MammalAge from Argentina.These data suggestthat the early Desea- dan should be shifted significantlyupwards (by ca. 10 m.y.) to late Oligocene-earlyMiocene time.

INTRODUCTION sites in other countries. Although richly fos- the sediments and faunas During most of the Cenozoic, South siliferous, Tertiary until America drifted as an isolated continent, re- of Bolivia have been virtually unstudied a recent surge of field within the last sulting in the evolution of a highly unique and activity endemic terrestrial fauna. In contrast to quarter century. important sites in Holarctica, the fossil record of the Age of One of the most Tertiary Mammals in South America is extensive but South America is Salla, located in northwest- Bolivia. It has attracted considerable poorly known. Much of our knowledge of the ern South American succession comes from attention because it contains a very rich De- the first occur- Argentina, Brazil, and some scattered critical seadan assemblage, including rence of a South American primate (Brani- Hoffstetter Wolff and a I Manuscriptreceived October22, 1984;revised sella, 1969; 1985) (Hoff- January29, 1985. diverse suite of caviomorph rodents stetter 1976; Hoffstetter and Lavocat 1970; p. [JOURNALOF GEOLOGY,1985, vol. 93, 223-250] Lavocat Patterson and Wood 1982). © 1985 by The University of Chicago. All rights 1976; reserved. Other aspects of the Salla fauna have been 0022-1373/85/9303-/006$1.00 published to date (see references below), but 223 224 BRUCE MACFADDENET AL.

a detailed litho- and biostratigraphy and asso- Ma; meg-annum, or million years before pres- ciated chronological framework of the Salla ent on the radiometric time scale. area has previously not been presented. MPTS; Magnetic Polarity Time Scale Other Oligocene vertebrate faunules or isolated specimens have been reported from PREVIOUS AND CURRENT INVESTIGATIONS Bolivia (Lacayani, see Hoffstetter et al. 1971; Middle Tertiary vertebrates from Bolivia Petaca Formation, see Sanjin6s-S. and were completely unknown until the discovery Jimenez-M. 1978), but the rich mammalian of the Salla localities by G. Bejarano in 1962 assemblage from Salla remains the standard (Hoffstetter 1968). Vertebrate fossils were for comparison with the classic Deseadan se- collected from the Salla-Luribay basin during quence of Argentina. the mid 1960's for Princeton University by L. This report presents the results of an inter- Branisa with funds provided by the Gordon disciplinary analysis that suggests a new age Barbour bequest. During the late 1960s, and for the sediments and faunas of the Salla early 1970's, Hoffstetter of the Institut de Beds. In addition, this study integrates the Paleontologie, Paris, and associates made preliminary mammalian biostratigraphy into collections from Salla. Subsequent field work a temporal scheme based on our collections was also done there by Hartenberger of Mont- and existing archival data associated with the pellier, France, and Villarroel (formerly of more extensive Branisa collection housed at GEOBOL, La Paz). The present paper has Princeton University. As more vertebrate resulted principally from field collecting and fossil, paleomagnetic, and isotopic dating geological investigations conducted at Salla samples are collected and analyzed from since 1978, although portions are based on Salla, and as knowledge of the sequence in the PU collection. Argentina increases, the present study will Preliminary accounts of the geology of the serve as a fundamental chronological founda- Salla-Luribay basin have been presented by tion. Hoffstetter (1968, 1976), Hoffstetter et al. (1971), and Villarroel and Marshall (1982). ABBREVIATIONS Hayashida et al. (1984) presented a study of The following abbreviations are used in the fission track dating and magnetic stratigraphy remainder of the text: of the Salla Beds. Our data indicate that their AF; alternating field (demagnetization). fission track (zircon) dates of 54.0 +2.6 Ma A/m; ampere/meter, equivalent to 10-3 and 52.0 + 2.1 Ma are too old to indicate the Gauss. true time of deposition of the Beds. Although mT; millitesla, equivalent to 10 oersteds. several superposed magnetozones were de- R; resultant vector length, used in Fisher tected by Hayashida et al. (1984), these were (1953) statistics. not correlated to the MPTS. NRM; natural remanent magnetization. The majority of publications on Salla have k; kappa, best estimate of precision parame- been directed toward description of the rich ter, used in Fisher (1953) statistics. vertebrate assemblage. Hoffstetter (1968) GB; paleontology collection, GEOBOL, La presented a preliminary faunal list of the Salla Paz, Bolivia. mammals. This fauna first attracted attention ro- GEOBOL; Servicio Geol6gico de Bolivia, La because of the presence of caviomorph Paz, Bolivia. dents (references cited above), which are MNHN SAL; Salla Collection, Museum Na- among the earliest-known in South America tional d'Histoire Naturelle, Paris, France. and the surprising discovery in the late 1960's UF/LACM/GB; collection from Salla made of the ceboid monkey Branisella (Hoffstetter de- by joint expeditions of the University of 1969; Wolff 1985). Recent papers have Florida, Natural History Museum of Los scribed numerous other aspects of the Salla Angeles County, and GEOBOL, presently fauna, including Argyrolagidae (Wolff 1984), housed at the Florida State Museum, Uni- Pyrotherium (MacFadden and Frailey 1984), versity of Florida, Gainesville. marsupials (Villarroel and Marshall 1982; PU; vertebrate paleontology collection, Patterson and Marshall 1978; Hoffstetter and Princeton University, Princeton, New Petter 1983; Petter and Hoffstetter 1983), Jersey. litopterns (Cifelli and Soria 1983a, 1983b), DESEADAN SALLA BEDS OF BOLIVIA 225

FIG. 1.-General location of the Salla-Luribay basin (insert, lower left corner) and detailed location of the three measured sections discussed in this report, i.e., Pasto Grande, Poco Poconi, and El Planimiento. and a possible condylarth (Soria and Hoff- structural basin consist of deformed and lo- stetter 1983). Despite the lack of precise field cally metamorphosed Paleozoic sediments data for previous collections, some chrono- (Ahlfeld and Branisa 1960; Martinez and To- logical constraints can be placed on many of masi 1978). The initial tectonic phases of ba- those specimens based on our knowledge of sin development are represented by the general field areas (as contained in the ar- Luribay Conglomerate, which lies uncon- chives at Princeton or published references). formably on Paleozoic sediments. The age of this unit is early Tertiary; Hoffstetter (1976) GEOGRAPHIC AND GEOLOGIC SETTING stated that it was probably Eocene based on The Salla Beds are exposed in the Salla- the anecdoctal occurrence of a notoungulate. Luribay Basin, which is located about 90-100 The contact between the Luribay Conglomer- km SE of La Paz in the Cordillera Orientale ates and overlying Salla Beds is gradational physiographic province at a general location and best exposed in the NW portion of the of between lat. 170 8' to 17° 14'S and long. basin along the south side of the Rio Mul- 670 40' to 67° 37'W at an elevation of about lakha Jahuira (fig. 2A). Several lines of evi- 3485 to 4085 m (fig. 1). The margins of the dence now indicate that the Salla Beds are 226 BRUCE MACFADDEN ET AL.

FIG. 2.-A. Lower part of the Salla section looking generally westward with the Rio Chala Jahuira in the middle distance. The approximate contact between the underlying Luribay Conglomerates and Salla Beds is indicated. Branisa's V-12 locality is in this general area. B. View of general structure of center of Salla- Luribay basin looking N along the axis of the anticline formed by the Salla Beds. C. The Principal Guide Level (shown by arrow), a prominent lithostratigraphic marker in the Salla-Luribay basin taken NE of Poco Poconi and Khara Tranca Pata. D. The middle and upper part of the Salla Beds ("El Planimiento" section, see below) exposed near the axis of the principal anticline. The lower part of this section, including the Branisella and Upper White levels, are among the most highly fossiliferous portions of the Salla Beds. E. The middle part of the Salla Beds with the Branisella and Upper White levels looking east (in the general direction of the village of Salla). The arrow labelled BL to the right (south) of our Campsite II indicates the exact locality of the Branisella partial maxilla, UF 27887, collected by one of us (B. J. MacF.) in 1981. In figures 2D and 2E, the grass and low shrub covered surface just below the Branisella level is "El Planimien- to." Abbreviations are as follows: LC, Luribay Conglomerates; PGL, Principal Guide Level; SB, Salla Beds; UWL, Upper White Level; BL, Branisella Level; EP, El Planimiento. late Oligocene to early Miocene in age, which axis trending NNW-SSE (fig. 2B). Locally, is a significant departure from previous inter- the Salla Beds are covered by a thin (usually pretations. Post-depositional deformation re- less than 10 m) unit of unnamed fine-grained sulted in the folding and faulting of these Ter- alluvial or aeolian deposits which are proba- tiary sediments into a broad anticline with an bly of Pleistocene age. DESEADAN SALLA BEDS OF BOLIVIA 227 The Salla Beds consist of at least 540 m of separationbetween sites averages about 6 m, section exposed in the core of the anticline. and in no case does the separationbetween The predominantlithology consists of mod- two superposedsites exceed 10 m. erately to well-induratedred-brown, pinkish- In the laboratorythe paleomagnetic sam- tan, and light gray claystones and siltstones. ples were cut and sanded into 25 mm cubes. Locally, these colors can be either massive or All paleomagneticmeasurements, laboratory variegated in outcrop. There are numerous treatment,and data analyses were conducted tuffaceous horizons which represent either at the University of Florida Paleomagnetic primaryair-falls or secondarilyreworked de- Laboratory. This study employed a Super- posits. A prominentlithostratigraphic mark- conducting Technology two-axis cryogenic er, the "PrincipalGuide Level" (Nivel Guia magnetometer (Goree and Fuller 1976) on in Villarroel and Marshall 1982) is used for which most of the measurementsof magneti- correlation within the Salla-Luribay basin zation were made;a SchonstedtDSM-2 spin- (fig. 2C). Infrequently,the Salla Beds contain ner magnetometer, Schonstedt alternating local lenticular channel conglomerates and field (AF) and thermaldemagnetizers and Ap- thin (ca. 1 m) limestones. These lithologies ple II series microcomputersfor data collec- suggest that the Salla Beds were deposited tion and analyses. principallyin a fluviatile environment. The In order to determine the paleomagnetic variegated horizons probably represent characteristicsfor the sediments of the Salla paleosols. Beds, 18 sites selected from variedlithologies The Salla Beds weather to a characteristic were used for a pilot demagnetizationstudy. badlandstopography. Fossil vertebratesare For eight of these sites, one specimen was relatively rare in the lower and upper one- subjected to step-wise AF demagnetization thirdof our composite section. The principal (at NRM, 10, 20, 30, 40, 50, and 60 mT), and fossiliferoushorizons are located in the mid- one specimen was subjected to step-wise dle one-thirdof the Salla Beds (figs. 2D and thermaldemagnetization (at NRM, 100, 200, 2E). The fossils are usually isolated, often- 300, 400, 500, 550, 600, 620, and 640°C).The times occurring as fragmentary dentitions additional10 sites were subjectedto the com- and postcranialelements dispersedrandomly plete step-wise thermal demagnetizationre- within these zones. Infrequently,articulated gime. The results of the step-wise AF pilot specimens have been found. Our field work demagnetizationindicate that in many cases, suggests that there are few, if any, horizons a high coercivity magnetic component was amenableto quarrysampling. Therefore, our presentthat could not effectively be removed Salla collections made thus far are the results by this technique (fig. 3A, 3B, 3D). AF de- of surface prospecting. magnetization effectively isolated a stable component in only one of the eight pilot specimens as evidenced by a general linear PALEOMAGNETIC PROCEDURES AND ANALYSES decay towardsthe originin vector demagneti- In 1981 paleomagnetic samples were col- zation (Zijderveld)diagrams (fig. 3C). The re- lected from three measuredstratigraphic sec- sults of the step-wise thermal demagnetiza- tions in the Salla Beds (fig. 1). These sections tion (fig. 4) indicate that, in general, after a were chosen because: (1) they collectively low blocking temperaturecomponent is re- span what seems to representthe most com- moved at temperaturesof about 100-200°C,a plete composite sequence available;(2) they stable magnetic component seems to be are relatively continuousand consist of sedi- isolated between 3000 and 550°C. Above ments potentiallyamenable to paleomagnetic 550°C the magnetic component usually sampling; (3) they include important fos- continues to decay in the same direction as siliferous, marker,and ash horizons; and (4) between 300°-550°C (fig. 4A, 4C, and 4D) they are located in the undeformedportion of or sometimes becomes unstable with no the basin. A total of 104 paleomagneticsites linear decay towards the origin (fig. 4B). were collected from the three sections which NRM intensities of magnetizationusually fall total a composite thickness of 540 m. A in the 10-7 and 10-8 A/m range. After ther- minimumof three separately-orientedhand mal demagnetizationat 4000C,and for some sampleswere taken at each site. The vertical specimens(see above) at 500°C,intensities of AF DEMAGNETIZATION A B C D N(UP) N(UP) N(UP) N(UP)

FIG. 3.-AF demagnetization characteristics for four representative samples from the Salla Beds, 8A (A), 46B (B), 68B (C), and 83A (D). Closed and open squares represent, respectively, horizontal and vertical components on these vector demagnetization (Zijderveld) diagrams.

THERMAL DEMAGNETIZATION C

A B

FIG. 4.-Thermal demagnetization characteristics for four representative samples from the Salla Beds, 14A (A), 29C (B), 49C (C), and 59C (D). Conventions for these vector demagnetization diagrams are the same as in figure 3. DESEADAN SALLA BEDS OF BOLIVIA 229 A. NRM B. OPTIMUM THERMAL DEMAGNETIZATION

FIG. 5.-Stereographic projections of the statistically significant site mean directions for the Salla Beds. A. NRM. B. After "optimum" thermal demagnetization (the data set with greater precision, see text). Closed and open circles represent, respectively, sites of positive and negative inclinations. After demagnetization, the population of sites in the northern hemisphere with negative inclinations and those in the southern hemisphere with positive inclinations are interpreted to be of, respectively, normal and reversed polarity. The four sites represented by asterisks are statistically significant but of dubious polarity. magnetization usually fall in the 10- 8 and versed polarity). The separation into two 10-9 A/m range. populations suggests that in most cases, the After thermal demagnetization and data thermal demagnetization regime used here analysis using Fisher (1953) statistics, of the satisfactorily isolated the characteristic com- 104 sites sampled, 76 were used to interpret ponent of magnetization. However, sites of the magnetic polarity zonation presented be- intermediate distribution between the two low. Following the criteria of Opdyke et al. populations suggest that our thermal demag- (1977), 63 fell into the Class I category (statis- netization did not remove completely all sec- tically significant), one into Class II (only two ondary components of the NRM. In addition, samples analyzed from the site), and 12 into four Class I sites, although statistically Class III (two of three samples with concor- significant, were of ambiguous polarity (decli- dant directions). nations in northern hemisphere with positive The directional data for the Class I sites for inclinations or declinations in southern hemi- NRM and "optimum" demagnetization are sphere with negative inclinations). These presented in figure 5. For sites demagnetized sites were not used to interpret the magnetic at two steps, i.e., 4000C and 500°C, the data polarity zonation presented below. were chosen (as "optimal") with the higher The mean directional data for the 42 Class I precision (k). Sites were rejected from the demagnetized sites of normal polarity is dec- Class I category if they did not meet the reli- lination 357.21°, inclination -24.890 (R = ability criteria set forth in Irving (1964, p. 68) 39.24, k = 14.87, 095 = 5.920). The mean where for N = 3; R > 2.62 and N = 4; R > directional data for the 30 Class I demag- 3.10. For the NRM site mean data, there is netized sites of reversed polarity is declina- considerable scatter of directions with sites tion 169.45°, inclination 45.02° (R = 27.25, k of both positive and negative inclinations, = 10.55, a95 = 8.50). These two populations particularly in the northern hemisphere (fig. do not pass a "reversals test" at the 95% 5A). After thermal demagnetization, the sites confidence level (McElhinny 1973). The re- tend to separate into two populations, one in versed and normal populations are signifi- the northern hemisphere with negative incli- cantly different from each other and from the nations (interpreted to be of normal polarity) dipolar field (31.44o) calculated for the and one in the southern hemisphere with latitude of Salla (17°S) (using the formula tan positive inclinations (interpreted to be of re- I = 2 tan X, where X = known present-day 230 BRUCE MACFADDEN ET AL. PASTO GRANDE

STRAT LITHO- FOSSIL DECLINATION VGP LATITUDE MAGNETIC THICKNESS STRAT. AND MARKER POLARITY HORIZONS ZONATION M FT 270° 0° 90° 180° 270° +90 +45 0 -45 -90 300

905 BASE OF UPPER WHITE LEVEL

800

PRINCIPAL 200 GUIDE LEVEL

600 170m LEVEL

400

100m LEVEL 100

200

30-45m LEVEL GRADATIONAL CONTACT

LURIBAY CONGLOMERATE SYMBOLS

CONGLOMERATE CLAYSTONE NORMAL POLARITY

SANDSTONE VOLCANIC ASH REVERSED POLARITY

CALCAREOUS FOSSIL HORIZON SILTSTONE ZONE

FIG. 6.-Lithostratigraphy, important horizons, and magnetostratigraphic data for the Pasto Grande section. All paleomagnetic sites are Class I unless otherwise indicated (i.e., II or III). latitude and I = predicted inclination). These section and include known important fossil, divergences from expected inclinations prob- marker, and tuffaceous horizons. ably result from incomplete removal of sec- Pasto Grande.-This section (fig. 6), ondary components of magnetization. Never- located at lat. 17° 9' 45" S, long. 670 38' 30" W theless, as mentioned above, the polarities (fig. 1), consists of about 265 m of the Salla are unambiguously determined for 76 of the Beds representing about the lower one-third 104 originally sampled sites. of the known stratigraphic extent of this unit. The lower contact with the underlying LITHOSTRATIGRAPHY AND MAGNETIC Luribay Conglomerates is gradational (fig. POLARITY STRATIGRAPHY 2A). Locally, the Salla Beds have dips of 10° The locations of the three measured strati- N or less, and they consist principally of graphic sections, Pasto Grande, Poco Poconi, moderately indurated reddish brown silty and El Planimiento are shown in figure 1. To- claystones and clay siltstones. Other smaller gether, they comprise 540 m of composite units consist of poorly indurated gray sands, DESEADAN SALLA BEDS OF BOLIVIA 231 POCO POCONI

STRAT. LITHO FOSSIL DECLINATION VGP LATITUDE MAGNETIC AND POLARITY THICKNESS STRAT. MARKER 270 0o 90° 180° 270o +90 +45 0 +45 -90 HORIZONS ZONATION

M FT TOP OF CERRO 60 { 200

- 30- 100 (PRINCIPAL GUIDE LEVEL

0-- 0 ARBITRARY BASE OF LOCAL SECTION FIG. 7.-Lithostratigraphy, important horizons, and magnetostratigraphic data for the Poco Poconi sec- tion. See figure 6 for symbols and conventions. lenticularconglomerates, and several thin (1 tion (fig. 6). Twelve paleomagneticsites were m) tuffaceous horizons. At 205-210 m there originallycollected from the lowermost77 m is a prominentwhite band of silty, bentonitic of the Salla Beds. However, because of claystone that weathers to a characteristic numerous single site polarity zones in that "popcorn" surface. This unit, the "Principal part of the section, this portion of the mag- Guide Level," is one of the most prominent netic polarity zonation is considered here to lithostratigraphicmarker horizons in the be indeterminate(more samplingis required Salla-Luribaybasin (fig. 2C). The top of the to extend the paleomagneticcolumn further Pasto Grande section was arbitrarilyplaced down the section). at the base of anotherprominent stratigraphic Poco Poconi.-This section (fig. 7), marker,the "Upper White Level." located at lat. 17° 10' 15"S, long. 670 38' W Although not as rich nor as intensively (fig. 1), consists of 60 m of the Salla Beds. prospectedas certain other areas in the Salla This section was chosen because it includes Beds, fossil vertebrateshave been collected importantfossil localities. Its base was arbi- from several horizons in the Pasto Grande trarilystarted 25 m below the PrincipalGuide section. These levels occur at 35-40 m, 100 Level. In this section the dips are less than m, 170 m, and 205-210 m (PrincipalGuide 100.The sediments of the Salla Beds at Poco Level) above the base of the section (fig. 6). Poconi consist predominantlyof moderately In addition, Branisa's notes and correspon- induratedreddish-brown silty claystones and dence (on file in the Departmentof Geology, clayey siltstones with some interbeddedlight PrincetonUniversity) indicate that certain of gray well-indurated calcareous fine sand- his localities (V-12, Anchallani, and Que- stones and tuffaceous poorly indurated brada Chala Jahuira)occur within the lower sands. The prominent marker horizon, the one-third of the Salla Beds either at Pasto PrincipalGuide Level, is present in this sec- Grande or from stratigraphicallyequivalent tion. sections. In particular,the correlativesection Two fossil horizons occur in the Poco Po- at QuebradaChala Jahuira(figs.1, 2A) is im- coni section. The lower one is at the Principal portant because it records the lowest strati- GuideLevel. The mainconcentration, named graphiclevel of rodents in the Salla Beds. the Branisella Level (see below), occurs in Three magnetozones(2N and 1R) are rep- the upper one-quarterof the section above a resented by 21 sites in the Pasto Grandesec- gray-brown, well indurated, ledge-forming, 232 BRUCE MACFADDEN ET AL. EL PLANIMIENTO

STRAT. LITHO- FOSSIL, RADIO- DECLINATION VGP LATITUDE MAGNETIC THICKNESS STRAT. METRIC, AND MARKER POLARITY HORIZONS ZONATION

M FT 270* 0o 90* 180" 270* +90 +45 0 -45 -90 400

1200 FISSION TRACK

300 1000

800

200

600

FISSION TRACK

400

100 UPPER WHITE LEVEL

BRANISELLA 200 LEVEL

PRINCIPAL GUIDE LEVEL - K/AR=26.4±1.0 Ma

0 0 ARBITRARY BASE OF LOCAL SECTION FIG. 8.-Lithostratigraphy, important horizons, and magnetostratigraphic data for the El Planimiento section. See figure 6 for symbols and conventions. The levels of the fission track dates are indicated; however, these were not used in our present chronologyfor the reasons discussed in the text.

0.3 m thick calcareous sandstone located at close to the lowest stratigraphic level at 50 m above the local stratigraphic base. which rodents are abundant. Based on seven superposed sites, this entire El Planimiento.-This section (fig. 8), section consists of one zone of normal polar- located between lat. 170 11' and 170 12' S, ity. long. 67° 37' 30"W, consists of about 365 m of Recently, a highly fossiliferous new hori- the Salla Beds. It was chosen because it rep- zon, Calabozo Pata, located 45 m below the resents a long portion of the upper part of the base of the Principal Guide Level (below our Salla Beds that includes important marker previously measured section), was worked in and ash horizons, and because the most the region of Poco Poconi and Khara Tranca abundant and diverse suite of mammals Pata (figs. 1, 2C). Calabozo Pata is biostrati- known from the basin are found there. The graphically very important because it seems base of this section was arbitrarily taken 25 m DESEADAN SALLA BEDS OF BOLIVIA 233

TABLE 1

K/AR ANALYTICAL DATA FOR UNIT 5 BIOTITE

ANU Ka Radiogenic 40Ar Radiogenic 40Ar Age lab no. (wt %) (10-10 mol/g) (% of total) (Ma ± lo) 84-4(1)b 6.343, 6.385 2.900 69.0 26.1 ±0.4 84-4(2)c 6.343, 6.385 2.972 92.3 26.7 +0.3 Mean = 26.4 ± 1.0 (2cr)

NoTE.-Unit 5 biotite is located 20 m above the local base of the El Planimiento section and 195 m above the base of the composite section of the Salla Beds. 10-10 Ae + = 0.581 x 10-0 yr-'; hp = 4.962 x yr '; 4K/K = 1.167 x 10-4 mol/mol. a K measuredhe' by flame photometry. As measured by isotope dilution using a VG Isotopes MM1200 mass spectrometer operated in static mode. b + 152 (m; 0.09758 g. c + 152 Fim; 0.16269 g. below the PrincipalGuide Level. This section netozones (6N, 6R) are representedin the El was measured along the axis of the general Planimientosection. Five of these zones (2N anticlinal structure of the basin (fig. 2B), and 3R), i.e., R2, N4, R4, R5, and N7, are where the sediments are flat-lying.The prin- based on Class I single sites. Because of the cipal lithology in the lower half of the Salla possibility that these sites could represent Beds in this section is reddishbrown, moder- spurious magnetic polarities, furtherlabora- ately induratedsilty claystones and clay silt- tory analysis was performedfor all samples stones. In the upperhalf of this local section, from each site. All samples were measured there is a distinct color change to pale brown for NRM, demagnetizedat 4000Cand 500°C and pinkish tan although the predominant (as was the routine procedure described grain sizes are the same as in the lower half. above), and then were also demagnetizedat Throughout there are numerous inter- 550°C,600°C, 620°C, and 6400C.In all cases, bedded light colored well-induratedcalcare- these sites remainedof the same polarity as ous sandstones(two of which, between 195- determined from demagnetizationat lower 200 m, form the resistant base of "El Plani- temperatures. miento," fig. 2D and 2E), claystones, tuffs, Given the results of this additionallabora- and tuffaceous silty clays. tory analysis and using the criteriadescribed Our Upper White Level, at about 83 m above for acceptance/rejectionof sites (Op- above the local base, is a prominent litho- dyke et al. 1977), a non-arbitraryscheme stratigraphicmarker in the centralpart of the does not exist for rejectingthese five sites in Salla basin (fig. 2). It consists of white, dark an interpretationof the magneticpolarity zo- gray, and red bentoniticclays that weatherto nation. These single site polarity zones are a characteristic"popcorn" surface. Our sec- thus considered to be short-termreversals. tion ends at the top of an unnamedridge crest Problemshave been encounteredin previous at an elevation of 4078 m. studies with the use of single site polarity Very rich fossil horizonsoccur in the lower zones in correlation.Therefore, only polarity one-thirdof this section. As shown in figures transitions between zones represented by 2D, 2E, and 8, these include the Principal two or more sites are used below. In this Guide Level at 25-30 m, our "Branisella way, the short-term(i.e., single site) polarity Level" [TapialPampa of Branisa(correspon- zones do not affect our correlation of the dence at PU) and Villarroel and Marshall Salla magneticpolarity zonation to the stan- 1982] and our Upper White Level (Cebadal dard MPTS. Churu, Huichinca and equivalents of Villar- roel and Marshall 1982). The upper two- ISOTOPIC DATING thirds of this local section is not highly fos- The Salla Beds contain numerousvolcanic siliferous, althoughsome unidentifiablebone tuffs and bentonized tuffs that are amenable scraps have been encountered during field to isotopic dating. We report one K/Ar date work. on biotite (table 1) and two fission-trackdates Comments on Single-Site Polarity Zones. on zircons (table 2). The analytical data for -Based on 51 paleomagneticsites, 12 mag- these ages are internally consistent. They therefore seem to approximate closely the real ages of the associated air-fall events. The K/Ar date was obtained from air-fall U 350 320 biotites found at the 20 m level above the (ppm)c local base in the El Planimiento section (fig. 8). Two splits were dated with good repro- ducibility (table 1). The mean age for these is rb .92 .99 26.4 + 1.0 (2a) Ma. These analyses were per- formed at the isotope geology laboratory at the Australian National University. The two fission-track determinations on 6 6 zircon were obtained from tuffaceous units at numbergrains the 135 m and 355 m levels above the local base in the El Planimiento section (fig. 8). The analytic data for these dates, obtained SECTION with the external detector method (Naeser 2oa Ma 1.9 2.0 1979), are presented in table 2. The zircon + grains were mounted in FEP teflon at 300°C. The grain mount was then polished, and the fossil tracks etched. The zircons were etched PLANIMIENTOT Ma 22.2 22.0 for 34 hours in a eutectic melt of KOH and EL NaOH at 215°C to develop the tracks THE (Gleadow et al. 1976) and then were covered with an external detector of muscovite to re- FROM 10'5 cord the induced fission tracks. This package .940 .940 2 x n/cm2 was then irradiated with NBS glass standard 962 in the USGS Reactor at Denver, Col- orado. Following irradiation, the muscovite TABLECOLLECTED 106 detector was etched in 48% HF at 20°C for 15 Pi 9.38 x t/cm210.46 (1235) (1064) minutes. The neutron dose was determined ASHES by counting the number of tracks in a musco- FOR vite detector which covered the glass stan- dard during the irradiation. The NBS Cu cali- 106 DATA bration was used in the dose. Ps 4.14 3.68 determining x t/cm2 (977) (835) The error reported in table 2 was determined AGE by using Poisson statistics based on the total number of tracks counted. TRACK Discussion.-The analysis for both the K/ Ar and fission-track dates indicates precise zirconzircon Mineral the dates derived from these FissioN ages; however, two techniques are not mutually compatible with the chronology presented here. The '7/yr. fission-track ages from Salla sample 8 and aboveabove Salla sample 10 (table 2), which are separated base base 7.03e- m m counted. (in direct superpositional sequence) by 210 m Location 135local 345 local of section, essentially yield equivalent ages of tracks constant:coefficient. ca. 22 Ma. Given the fact that seven polarity deviation.of transitions separate these two dates, and us- ing the stochastic model of geomagnetic field standardnumber correlation): reversals (Johnson and McGee 1983), it is a: r: ( NOTE.-Decaya b c clear that a greater period of SampleSalla-8 DF-4718Salla-10 DF-4719 significantly time is actually represented between these two tuffs (more sampling and analysis is re- quired to provide temporal discrimination be- DESEADAN SALLA BEDS OF BOLIVIA 235 tween these sites). We view the fission-track bly in the extremely fossiliferous zone be- dates as general corroborationof the K/Ar tween the PrincipalGuide Level throughthe age, the latterof which we chose to accept for Upper White Level. It is not possible to de- calibrationof our magneticpolarity zonation termine how far above the Upper White and correlationto the MPTS. Level Branisa'scollection extends; however, In summary, both of these isotopic tech- based on the fact that the upper one-thirdof niques indicate that the middle to upper part the Salla Beds is poorly fossiliferous, there of the Salla Beds, and their Deseadanfossils, probablyare not many specimens from this are in the mid to low 20's Ma age range. This part of the section. Given these constraints, is substantially younger than previously the preliminarybiostratigraphy (fig. 9) is di- stated for Deseadan fossils, i.e., ca. 33-35 vided into two informalparts, the lower and Ma (Marshallet al. 1983). It is improbable upperSalla Beds, with the transitionbetween that both of our isotopic methods have sys- these taken arbitrarilyat the PrincipalGuide tematic errors of this magnitude. Never- Level (fig. 10). theless, we are presently resamplingand re- The stratigraphic distribution of fossil datingthese and other tuffs to validatefurther mammalsat Salla is presented in figure 10. the K/Ar date used here for calibrationof the Hoffstetter (1968, 1976) presented a prelimi- magneticpolarity zonation. nary, annotatedfaunal list for the mammals from Salla. Since that time manygroups have MAMMALIAN FAUNA been describedfrom that fauna, i.e., primates The UF/LACM/GB and PU collections (Hoffstetter 1969; Wolff 1985) marsupials were studied to determine the composition (Villarroeland Marshall 1982;Patterson and and stratigraphicdistribution of the mammals Marshall 1978; Hoffstetter and Petter 1983; collected from the Salla Beds. With respect Petter and Hoffstetter 1983); litopterns to the UF/LACM/GB collection, fossil levels (Cifelli and Soria 1983a, 1983b)and a possi- are located throughoutthe composite section ble condylarth(Soria and Hoffstetter 1983). as follows (fig. 9): (1) the lowermost at 30-45 The taxonomy of many of the Salla mammals m above the base; (2) the 100 m level; (3) the has recently been studied, so that their bio- 160 m level (CalabozoPata); (4) 170 m level; stratigraphic utility is fairly good. Other (5) PrincipalGuide Level at 205-210 m; (6) groups from Salla, particularlythe edentates 230-240 m level, which is equivalent to the and notoungulates, are virtually unstudied. Branisella Level (= Tapial Pampa sensu The following list (also see table 3) includes stricto, see Villarroeland Marshall1982); and preliminaryidentifications based on compari- (7) 260-270 m level or Upper WhiteLevel (= sons with Deseadan mammals from Argen- Branisa's white-gray gypsiferous clay; = tina mostly resulting from the unpublished Huichinca of Villarroeland Marshall 1982). work of one of us (R.L.C.). In the PU Bransiacollection many specimen localities are cited merely as within the gen- MARSUPIALIA: eral Salla area. However, correspondence Paraborhyaena boliviana.-Hoffstetter between Branisa and Dr. G. L. Jepsen (late and Petter (1983) and Petter and Hoffstetter Professor of Geology at Princeton), associ- (1983) describe this large borhyaenid from ated photographs,and our knowledge of the Salla (also see description of Proborhyaena generalfield relationsindicate that several of gigantea in Patterson and Marshall 1978). Branisa's more specific localities can be This same taxon recently was reported as placed in our stratigraphicscheme (fig. 9): (1) Proborhyaena gigantea from Argentina V-12 (= Quebrada Chala Jahuira = Anchal- (Bond and Pascual 1983, Marshall pers. lani) is located above the Luribay Conglom- comm.). The hypodigm from Salla is small, erates in sediments equivalent to the lower consisting of four specimens (two in MNHN part of the section, and it possibly extends and two in the PU collection, see Petter and stratigraphicallyas high as, although almost Hoffstetter 1983).None of the MNHN or PU certainly not above, the Principal Guide specimens have specific locality data so that Level. (2) V-2 and V-3 are located above the this species is listed as occurring from the Principal Guide Level. Most of the fossils general "Salla" area in figure 10. collected from these "localities" are proba- Pharsophorus lacerans.-Patterson and POLARITY COMPOSITEMAGNETICZONATION

LOCALITIES

PRINCETON INCOLLECTION polarity BRANISA'S

magnetic

FOSSIL HORIZONS composite ASH and UF/LACM/GBAND

sections,

measured

three the of

MAGNETICCORRELATION

AND correlation SECTIONS, Beds. ZONATION,

MEASURED Salla POLARITY the for 9.-Biostratigraphy,

FIG. zonation

BEDS ABOVE SALLA OF COMPOSITE STRATIGRAPHIC THICKNESS BASE DESEADAN SALLA BEDS OF BOLIVIA 237

LOWER SALLA BEDS UPPER SALLA BEDS 30-45M 100M 170M PGL BL UWL

Paraborhyaena boliviana Pharsophorus lacerans Notogale mitis Sallacyon hoffstetteri Borhyaeninae, gen.et sp. indeterminate Palaeothentes boliviensis ?Polydolops sp. or ? Palaeothentinae Proargyrolagus bolivianus Dasypodidae cf. Peltiphilus sp. cf. Glyptatelus sp Megalonychidae, n gen. et n. sp. cf. Octodontotherium sp. Salladolodus deuterotheroides Branisella boliviana Migraveramus beatus Sallamys pascuali Incamys bolivianus aff. Neoreomys sp. Cephalomys bolivianus Branisamys luribayensis ?Coniopterium primitivum Thadanius hoffstetteri Tricoelodus boliviensis Isotemnidae, gen. and sp. uncertain ?Rhynchippus cf. equinus ?Rhynchippus cf. pumulis Nesodontinae gen. and sp. uncertain Prohegetotherium sp. A (large) Prohegetotherium sp. B (small) Archaeohyrax sp. Protypotheriinae, n. gen. and n. sp. Argyrohyrax sp. Trachytherus sp. A (small) Trachytherus sp. B (large) Astrapotheriidae, indeterminate Pyrotherium romeri cf. Acamana sp.

TAXON OCCURS IN LOWER OR UPPER SALLA BEDS. IN PU BRANISA (OR MNHN) COLLECTION OR FROM ALSO OCCURS AT KNOWN LEVEL AS REPRESENTED PUBLISHED ACCOUNT. BUT ONLY KNOWN FROM IN PU OR UF/LACM/GB COLLECTIONS. GENERAL SALLA LOCALITY.

FIG. 10.-Preliminary biostratigraphic distribution of mammalian taxa in the Salla Beds based on study of the UF/LACM/GB and PU collections and published accounts (see references cited in text). The arbitrary division of the lower and upper Salla Beds (with the transition at the Principal Guide Level) allows recognition of many of Branisa's localities and integration of many of the PU specimens into this scheme.

Marshall (1978) recognized this species from with any of the specimens. Notogale mitis is Salla based on two specimens, PU 20551 and also known from the Deseadan Cabeza PU 21865 (also see Petter and Hoffstetter Blanca local fauna in Patagonia, Argentina. 1983). Branisa's field notes indicate that these Sallacyon hoffstetteri.--This species was specimens were collected from his locality V- described from three specimens from Salla, 12A in the lower part of the section. Phar- GB 539 (Villarroel and Marshall 1982) and sophorus lacerans is also known from the De- MNHP SAL 92 and 93 (junior synonym, seadan Cabeza Blanca local fauna of "Andinogale sallensis"; Hoffstetter and Pet- Patagonia, Argentina. ter 1983). It has recently been reported from Notogale mitis.-This small marsupial is La Flecha, Argentina (Marshall et al. 1984). represented by about 15 specimens from The Salla sample is of unknown provenience. Salla preserved in the PU, MNHP and Villarroel and Marshall (1982) suggest Salla- GEOBOL collections (Patterson and Mar- cyon hoffstetteri as the probable ancestor of shall 1978; Villarroel and Marshall 1982; Pet- the later Tertiary borhyaenids Perathereutes ter and Hoffstetter 1983). Unfortunately, and (or) Sipalocyon. there are no specific locality data associated Borhyaeninae, gen. et sp. indet.- 1983

(large)(small) 19831983 Soria, 18971897 A B (small)(large) sp.sp. and sp. A B EXAMINATION Soria,Soria, sp. sp. 1889 ON n. sp.sp. and 18971897 Cifelliand n. 1962 Ameghino,Ameghino,uncertain 1897 18891889 al. BASED uncertain and CifelliCifelli sp. Sediset REFERENCES sp. gen. Ameghino, BEDS equinuspumulisand Ameghino,Ameghino, primitivum and n. cf. cf. Ameghino, indet. gen. sp. Ameghino,Ameghino, romeriIncertaeSimpson SALLA boliviensis PUBLISHED hoffstetteri gen. THE Family ALL MacraucheniidaeAdianthidae IsotemnidaeNotohippidae ToxodontiaeHegetotheriidae ArchaeohyracidaeInterotheriidae Mesotheriidae Astrapotheriidae Pyrotheridae Acamana FROM Litopterna Notoungulata AstrapotheriaPyrotheria and AND ?ConiopterniumIhadaniusIricoelodus isotemnid, ?Rhynchippus?Rhynchippus nesodontine,Prohegetotherium Prohegetotherium Archaeohyraxprotypotheriine,Argyrphyrax Trachytherus Trachytherus astrapotheriid, Pyrotheriumcf. 3 FamilyFamily FamilyFamily FamilyFamily FamilyFamily Family Family Family Order Order Order Order Order

TABLERECOVERED COLLECTIONS

PU

MAMMALS AND 1983 1978 1970 FOSSIL 1983 indet. 1970 THE 1982 sp. 1982 1970 et Hoffstetter, Lavocat, FORUF/LACM/GB Marshall, 1895 gen. and Wood, Lavocat, and LISTTHE Hoffstetter,1897 and Marshall, 1984 Lavocat, 1976 & 1969 and OF & Soria and 1887 1897 sp. and FAUNAL 1897) n. Wolff, Ameghino, Petter indet.Patterson Protolitopternidae Hoffstetter Ameghino, Lavocat, Villarroel Caenolestidae and sp. or Patterson sp. or Hoffstetter, Hoffstetter et ?Palaeothentine, Ameghino, Litopterna Hoffstetter CURRENT Ameghino, gen. boliviana or bolivianus or sp. lacerans(Ameghino, boliviensis sp. sp. n. beatus gen. sp. deuterotheroides bolivianusluribayensis spp. boliviana mitishoffstetteri pascualibolivianus Borhyaenidae CaenolestidaePolydolopidaeArgyrolagidae Dasypodidae Peltephilidae Glyptodontidae Megalonychidae Mylodontidae ?Didolodontidae CebidaeOctodontidae EchimyidaeDasyproctidae Dinomyidae PeltiphilusGlyptatelus Octodontotherium Neoreomys Marsupialia Edentata CondylarthraPrimates Rodentia ParaborhyaenaPharsophorusNotogaleSallocyonborhyaenine, Palaeothentes ?Polydolops Proargyrolagus dasypodidcf, cf. megalonychid,cf. Salladolodus Branisella Migraveramus SallamysIncamysaff. Cephalomys Branisamys Family FamilyFamily Family FamilyFamily Family Family Family Family Family FamilyFamily Family Family Order Order Order Order Order DESEADAN SALLA BEDS OF BOLIVIA 239

Pattersonand Marshall(1978) originallyrec- Argyrolagidaehad its earliest occurrence in ognized this taxon as Proborhyaena gigantea Huayquerian (late Miocene) deposits from based on two specimens from Salla, PU Argentina.The phylogeneticposition of argy- 21866and PU 21922 (no specific locality data rolagidsis somewhatproblematical, although available). However, Petter and Hoffstetter most workers follow Simpson (1970) in re- (1983) concluded that the assignmentto that ferringthe Argyrolagidaeto the Marsupialia. species (as describedfrom Patagoniaby Pat- terson and Marshall1978) was unconvincing. EDENTATA: Given the poorly preserved material from General Comments.-The edentates rep- Salla, we follow this more recent interpreta- resent one of the major unstudiedgroups at tion. Additionalmaterial must be found in or- Salla. Ourtaxonomic and biostratigraphicob- der to elucidate the taxonomic and biostrati- servations on the Salla forms are therefore graphicutility of this species. preliminary. Palaeothentes boliviensis.-Patterson and Dasypodidae.-Several jaws and many Marshall(1978, also see Villarroeland Mar- dermalscutes occur from several localities in shall 1982) originally described this species the Salla Beds. Hoffstetter (1968, 1976)lists based on materialfrom Salla. It recently has the possibilityof at least three generaof dasy- been reported from La Flecha, Argentina podids including "Prozaedius," Proeutatus, (Marshallet al. 1984). The hypodigm from and an undescribedform. Salla consists of two specimens, PU 21977 Cf. Peltephilus sp.-A small sample (in- and GB SAL 621, from, respectively, cluding a relatively well preserved skull) is Branisa's V-2 locality and Tapial Pampa. representedin the PU collection. These oc- With regardto the biochronology,Patterson currences are from Branisa's V-2 locality in and Marshall (1978, p. 45) state that: the upper part of the section. "Palaeothentes boliviensis is slightly smaller Cf. Glyptatelus sp.-A single specimen, than, but is structurally similar to P. PU 22164, consisting of a caudal fragment chubutensisfrom Cabeza Blanca. The possi- with vertebrae and dermal scutes is known bility exists that P. boliviensis is ancestralto from the Branisa's V-12 locality in the lower P. chubutensis, making the fauna from part of the section. Cabeza Blanca slightly later than that of Megalonychidaen. gen. et n. sp.-A sin- Salla." gle lower jaw of an undescribedtaxon, PU ? Polydolops sp. or ?Palaeothentinae, gen. 20552, is representedfrom Branisa'sV-12 lo- et sp. indet.-Patterson and Marshall(1978) cality. This very interestingspecimen, primi- and Marshall(1982) described ? Polydolops tive in many aspects, is currentlyunder study sp. from Salla based on a single fragmentary by Dr. George F. Engelmannat the Univer- lower jaw, PU 21998, collected by Branisa sity of Nebraska, Omaha. from his locality V12F (in the lower part of Cf. Octodontotherium sp.-A single lower the section). Villarroel and Marshall (1982) jaw with two posterior teeth, PU 21972, is described the presence of ?Palaeothentinae referableto this genus from Salla with, how- gen. et sp. indet., GB 088, from the "nivel ever, no specific locality data. Marshallet al. superior" at Kolla Huichinca (a probable (1983) consider this genus to be a Deseadan equivalentof our Upper White Level). Those index taxon. authors suggested that PU 21998 may actu- ally representthe lower dentitionof an inde- PRIMATES: terminate palaeothentine. The exact tax- Branisella boliviana.-Hoffstetter (1969) onomic assignment and biostratigraphic described this genus and species of ceboid utilityof this marsupialmust await the collec- monkey from Salla based on a partial left tion of more specimens. maxilla containing well-preserved p4M2. Proargyrolagus bolivianus.-Wolff (1984) Since the original description three other described this new species based on two in- specimens have come to light: a fragmentary complete rami containing teeth, UF 27895 lower jaw with M2, PU 21861, with no and UF 27896, from the Branisella Level. specific locality data, a partial right maxilla, This is a rathersurprising discovery for a De- UF 27887, and a fragmentaryright mandible, seadan fauna because previously the family UF 27888 (Wolff 1985). The UF specimens 240 BRUCE MACFADDENET AL. were both collected from the Branisella taxon are representedfrom the upperpart of Level at 230-240 m (see fig. 2E). Branisella the Salla Beds. Pattersonand Wood (1982, p. boliviana is known only from Salla and re- 382) state that: "We conclude that the oc- mains to date the only known Deseadan pri- todontidsare the most primitiveof the Desea- mate. This taxon has generatedmuch discus- dan rodents, and the nearestknown approach sion concerning the timing of dispersal of to the ancestral condition is representedby platyrrhineprimates into South Americaand Migraveramus." the phylogenetic origins of ceboid monkeys Sallamys pascuali.-Hoffstetter and La- (e.g., Ciochon and Chiarelli1981). vocat (1970)described this echimyidfrom the generalSalla area. The PU collection consists ?CONDYLARTHRA OR LITOPTERNA: of 12 specimens referableto this taxon from Salladolodus deuterotheroides.-Soria and Branisa's Upper White Level and general Hoffstetter (1983) describe this new genus Salla area (Patterson and Wood 1982). The and species based on two isolated teeth (one UF/LACM/GB collection contains the upper and one lower, MNHN SAL 266 and largest sample of Sallamys from several 267, respectively) from the generalSalla area stratigraphichorizons (fig. 10). No specimens (no specific locality known). They refer this of Sallamys are known to occur from the taxon to the OrderCondylarthra, Family Di- lower part of the section. dolodontidae, and assert that it is inter- Incamys bolivianus.-Patterson and Wood mediate in morphology between that group (1982)describe the largestknown sample (ca. and the prototheriidlitopterns. However, this 90 dentitions)of this species which occurs in poses a numberof characterconflicts at vari- several horizons in the upperpart of the sec- ous levels within both groups. The assign- tion. Of this sample, about one-thirdwas col- ment of Salladolodus to the early Tertiary lected from the Upper White Level. The UF/ litoptern family Protolipternidaecannot be LACM/GB specimens come from both the ruled out because the dentitionof that family Branisella and Upper White levels. A single is didolodont-like (i.e., primitive). Neither specimen, PU 21970, was supposedly col- family is otherwise known from the Desea- lected from Branisa's V-12 locality. If the dan; protolitopternidsare known only from field data are correct, then this representsthe the Riochicanand possibly the Casamayoran only known occurrence of this rodent from (Cifelli 1985a); didolodonts are primarily the lower part of the Salla Beds. known from these early Tertiaryfaunas but Aff. Neoreomys sp.-Patterson and Wood persisted into the Friasian in the form of (1982) state that a single specimen, MNHN Megadolodus (McKenna 1956). Depending SAL 118, previously referred to Incamys on its affinities,Salladolodus may representa bolivianus(Lavocat 1976), represents a new late survivor of an early Tertiarygroup or a dasyproctidgenus close to Neoreomys. This primitive member of the advanced litoptern specimen was collected from the general family Proterotheriidae. It should also be Salla area (no specific locality known). No noted here that several fragmentary, pro- other specimensreferable to this taxon are as terotheriid-likelower molars are present in yet recognized from either the PU or UF/ the Salla collections. These are of appropri- LACM/GBcollections. ate size for Salladolodus. If these specimens Cephalomys bolivianus.-Lavocat (1976) do indeed belong to this genus then its pro- described the presence of this dasyproctid terotheriidaffinities are strengthenedand the from the general Salla area based on a single speci- phylogenyof advancedlitopterns needs to be specimen, MNHN SAL 164. No other reconsidered. mens referableto this taxon have been recog- nized from either the PU or UF/LACM/GB RODENTIA: collections. Migraveramus beatus.-Patterson and Branisamys luribayensis.-Based on spec- Wood (1982) described this new species of imens from Salla, Hoffstetter and Lavocat octodontid based on the one specimen avail- (1970) described this large rodent, which able to them, PU 21948, from the general probablyhas affinitieswith the Dinomyidae. Salla area. Several additional specimens in A moderate-sized sample of this species is the UF/LACM/GB collection of this rare representedin the PU collection (Patterson DESEADAN SALLA BEDS OF BOLIVIA 241 and Wood 1982)from the upper fossiliferous this relatively rare species is recorded from portions of the Salla Beds. Numerous speci- the Salla general locality and from V-12, the mens of B. luribayensis in the UF/LACM/ lower part of the section. Two morphsof ?C. GB collection come from the same partof the primitivumare known from Salla and may be section (includingthe Branisella Level), and specificallydistinct (Cifelli and Soria 1983a). recent field work in September, 1984 indi- It is noteworthythat the single specimen as- cates that, althoughrare, this genus also oc- signableto stratigraphiclevel (V-12)pertains curs from the lower partof the section (Queb- to the more primitiveof the two morphs. In rada Chala Jahuira). Hartenberger (1975) its known morphology, ?C. primitivum, described Villarroelomys bolivianus from which is only known from Salla, is more Salla based on a single specimen, GB 014. He primitivethan C. andinumfrom Patagonia. stated that this form was a member of the Thadanius hoffstetteri.-Cifelli and Soria Hydrochoeridae based on dental pattern. (1983b) originally described this small, rare However, subsequent workers (Lavocat adianthidlitoptern based on materialin the 1976; Patterson and Wood 1982) concluded PU collection from Salla (specificlocality not that GB 014 representsa deciduouspremolar known). Specimens of T. hoffstetteri in the of Branisamys luribayensis. UF/LACM/GB collection are known to oc- General Comments on Salla Rodents.- cur from the Branisella level. This species is The Salla rodents are of prime importance only known from Salla; it is the most primi- to an understanding of the origin of the tive of undoubted, i.e., Deseadan and later, Caviomorpha,and, as such, these mammals Adianthidae(Cifelli and Soria 1983b). Hoff- have been extensively studied (references stetter (1976) listed the similar-sizedDesea- cited above). Based on the several hundred dan adianthidProadianthus among the Salla relevant specimens of known stratigraphic fauna;however in light of recent studies it is context, rodents seem to be rare in the lower probably T. hoffstetteri. part of the Salla Beds. They first appear in Tricoelodus boliviensis.-Cifelli and Soria abundanceat Calabozo Pata about 45 m be- (1983a)described this pygmy litopternbased low the base of the PrincipalGuide Level. on materialin the PU collection from Salla The composition of the Salla rodent fauna (specific locality not known). Additionalun- is strikinglydifferent from that of the Desea- described specimens from the Branisellaand dan of Argentinaboth in terms of taxonomy Upper White levels are represented in the and relative abundances (Wood and Patter- UF/LACM/GB collection. Tricoelodus boli- son 1959;Patterson and Wood 1982).In gen- viensis is more primitive than the also rare eral, the Salla rodents are less hyposodont but much better known type species, T. than those from the Deseadan of Argentina. bicuspidatus, from Patagonia (Cifelli and Hoffstetter and Lavocat (1970) suggest that Soria 1983b). these are principallyecological differences. Hartenberger(1975) suggested that the stage NOTOUNGULATA: of evolution of the Salla rodents was inter- Isotemnidae, genus and species undeter- mediate between Scarritt Pocket (also see mined.-Several isolated upper and lower Wood and Patterson1959) and the typicalDe- molarsfrom the PU and UF/LACM/GB col- seadan La Flecha and Cabeza Blanca lo- lections are referableto the Isotemnidae.The calities of Patagonia. As mentioned above, PU specimens are only known from the gen- Pattersonand Wood (1982)suggest that Mig- eral Salla area and those from the UF/ raveramusfrom Salla is primitive and very LACM/GBcollection are from the lower part close to the ancestry of other Caviomorpha. of the Salla Beds. Affinities within the Isotemnidaeremain to be determined.Hoff- LITOPTERNA: stetter (1976) referred materialsof probably ?Coniopterium primitivum.-Cifelli and the same taxon to Henricofilholia,species of Soria (1983a) described this primitive mac- which are synonymous with various species raucheniidlitoptern based on materialin the of Leontinia and Ancylocoelus, large and PU and MNHN Salla collections. Additional small (respectively)leontiniids typical of De- undescribed specimens occur in the UF/ seadan local faunas of Patagonia (Cifelli, LACM/GB collection. So far as is known, pers. obs., and Patterson, unpub.). The 242 BRUCE MACFADDEN ET AL.

Isotemnidae, the most primitive notoungulate ters and in relative abundance.) The less family above the notioprogonian level (Simp- complete UF/LACM/GB materials have not son 1967) and very abundant in early Tertiary yet been sorted; however, at least one of faunas, make their last appearance in the De- these forms occurs in the Branisella and Up- seadan. There they are represented by per White levels. The larger of the species Pleurocoelodon and the possibly synony- represented in the PU collection, comparable mous Trimerostephanos. (McKenna, 1956, in size to P. sculptum (a typical species from referred a specimen from the Friasian of Co- Patagonia), is the more abundant of the two, lombia to the family, but the specimen is too being represented by about 140 specimens. fragmentary for confident placement.) Re- The smaller species is known from about 30 mains of Deseadan isotemnids are rare; in specimens. With a few dubious exceptions, Argentina they are known from La Flecha the advanced notoungulate family Hegeto- (the type Deseadan) and Cabeza Blanca. theriidae is first recorded from the Divisadero Cf. Rhynchippus.-Two notohippid spe- Largo local fauna in the form of cies, differing principally in size, are present Ethegotherium (Simpson et al. 1962). In addi- at Salla and are tentatively referred to the tion to Prohegetotherium, most Deseadan lo- species Rhynchippus equinus and R. pumi- cal faunas of Patagonia also include one or lus, otherwise known from Patagonia. These more members of the somewhat aberrant and taxa are represented in both the PU and UF/ strictly Deseadan hegetothere group which LACM/GB collections and they occur in includes the possibly distinct genera Pro- numerous horizons in the Salla Beds. As with sotherium and Propachyrucos. However, most Deseadan local faunas, notohippid ma- these are conspicuously absent at Salla. terials are rather common at Salla, with the Archaeohyrax sp.-An undescribed spe- large species being the more abundant of the cies of Archaeohyracidae is represented in two. Notohippids are most abundant and di- the PU and UF/LACM/GB Salla collections. verse during the Deseadan; perhaps four Archaeohyrax sp. is known to occur in the valid genera of Rhynchippinae and two of Principal Guide, Branisella, and Upper White Notohippinae are present in local faunas of levels. It is smaller than, but clearly-a close that age in Patagonia. The primitive subfam- ally of, the type species A. patagonicus from ily Rhynchippinae is not recorded after the Cabeza Blanca, Chubut (Patterson and Deseadan. Cifelli, in prep.). This is by far the single most Nesodontinae, genus and species uncer- abundant species at Salla, being represented tain.-This toxodontid is represented by by more than 500 specimens in the PU and fragmentary specimens known from several UF/LACM/GB collections. This abundance horizons within the Salla Beds. These may be is startling in view of the scarcity of speci- referable to the Deseadan and Colhuehuapian mens pertaining to Deseadan and earlier genus Proadinotherium as indicated by Hoff- members of this family in Patagonia. stetter (1976), but we are at present unable to Interatheriidae.-The Salla interatheres characterize nesodontine genera on the basis are here referred to two taxa, both belonging of isolated molars. Nesodontine toxodontids to the group including Protypotherium and al- are among the many advanced notoungulate lies (Cifelli 1985a) within the advanced in- groups that make their first appearance in the teratheres, subfamily Interatheriinae. The Deseadan, and their remains are found in larger and more abundant of the Salla moderate abundance at most localities of this species, probably a new genus, was referred age in Argentina. by Hoffstetter (1968, 1976) to Plagiarthrus Prohegetotherium spp.-The large series but differs from that genus and other In- of relatively complete specimens of this teratherium allies in the molarization of the genus in the PU collection sort into two size posterior premolars. The smaller Salla groups, here tentatively recognized as dis- species, considerably rarer, is probably re- tinct species. (Although the extent of sexual ferable to the Argyrohyrax, also known from dimorphism in notoungulates remains to be Patagonia [including Progaleopithecus and properly assessed, this seems unlikely in the possibly also Archaeophylus (Marshall et al. present case. In addition to size differences, 1985)] although it is distinct from the species the taxa differ in other morphological charac- A. proavunculus). Both taxa are known to DESEADAN SALLA BEDS OF BOLIVIA 243

occur in the upper part of the Salla Beds as "couches a Pyrotherium") of the Deseadan indicated by specimens in the PU and UF/ fauna in Argentina, although it may not ex- LACM/GB collections. Interatheriids are tend into the latest part of this Land Mammal first known in the late Casamayoran (Cifelli Age (Marshall et al. 1985). 1985a) of Patagonia, but advanced forms, Cf. Acamana sp.-One PU specimen, con- subfamily Interatheriinae, do not appear until sisting of an edentulous premaxilla from an the Deseadan, when they underwent an ex- undetermined level in Salla, is tentatively re- tensive radiation (Cifelli 1985b). ferred to Acamana, otherwise known only Trachytherus spp.-The Mesotheriidae are from a single, similar specimen from the Di- represented in the PU and UF/LACM/GB visadero Largo local fauna of Argentina collections by two taxa that differ in size and (Simpson et al. 1962). Due to the fragmentary several morphological features (such as the nature of the known specimens, the affinities relative proportions of the anterior pre- of Acamana are problematical. McKenna molars). These taxa probably both occur (1981) suggested that it might be a trigonosty- throughout the section at Salla. These were lopid. The only known trigonostylopid skull previously referred to Trachytherus spegaz- with a rostrum, that of Eoastrapostylops zinianus and an undescribed genus (Hoffstet- riolorense, unfortunately was not figured by ter 1976). Both are here more conservatively Soria and Powell (1982) in ventral view. considered as undetermined species of Among South American fossil mammals, the Trachytherus pending detailed systemic stud- specialized anterior dentition of Acamana is ies. Trachytheriine mesotheriids, probably similar to Simpsonontus, a ?henricosborniid represented in the Deseadan of Argentina notoungulate known from the Riochican only by the type genus Trachytherus, are a Mealla Formation of northwestern Argentina rather rare but characteristic element of al- (Pascual et al. 1978). The Henricosborniidae, most all Deseadan local faunas. The presence which includes the most primitive notoungu- of two species at Salla, which are represented lates, is restricted to the Riochican and by rather complete and abundant remains, is Casamayoran and, with several dubious ex- unusual. Mesotheriids are first known to oc- ceptions, is not encountered in Patagonia af- cur in the Divisadero Largo local fauna, rep- ter the early Casamayoran (Cifelli 1985a). resented by ?Trachytherus (Simpson et al. 1962). However, Cifelli (1985b) suggested that SUMMARY: lower molars assigned to the Mustersan ar- The faunas from the Salla Beds seem to chaeohyracid Pseudhyrax eutrachytheroides change in taxonomic composition about one- may pertain to the family. Mesotheres are not third of the way up the measured section at encountered in subsequent faunas until the about the Principal Guide Level. Our prelimi- Friasian. nary assessments of these faunas are as fol- Astrapotheriidae, indeterminate. -Two lows (also see fig. 10): fragmentary dentitions in the PU Salla collec- Lower Salla Beds.-This part of the sec- tions are referable to this taxon, PU 22055 tion is either pre-Deseadan (?Divisideran) or and 22192 (specific locality unknown). One earliest Deseadan in age. The choice between astrapothere specimen in the UF collection, these two possibilities depends on the UF 68054, occurs from the lower Salla Beds definitions of (1) the Divisideran, which is equivalent at Quebrada Chala Jahuira. In either early Deseadan or latest pre-Deseadan contrast to their common occurrence in De- (Simpson et al. 1962) or a Land Mammal Age seadan local faunas from Argentina, as- distinct from the Deseadan (Pascual et al. trapothere remains are rare at Salla. 1965, 1966) and (2) the paleontological crite- Pyrotherium romeri.-MacFadden and ria used to define the base of the Deseadan. Frailey (1984) refer the Salla pyrothere to the Regardless of the ultimate definitions of these classic Deseadan species P. romeri. This tax- biochronological units (which must come on is fairly well represented by dental and from the Argentine sequence), the lower postcranial remains, and it is known from Salla Beds have some notable biostrati- several horizons within the Salla Beds (fig. graphic characteristics. 10). This genus is the principal guide taxon Among the edentates, an undescribed and original name bearer (Ameghino's primitive taxon of Megalonychidae is only 244 BRUCE MACFADDEN ET AL. known from Branisa's V-12 locality. Of the Certainelements of the fauna, eitherby pres- several hundred specimens of rodents col- ence/absence, or stage of evolution, appear lected from Salla, only a few (one of Incamys to reflect an age difference with respect to and several of Branisimys)occur within this those of Patagonia. Acamana, the ?Poly- part of the section. Trachytherus,which oc- dolops/palaeothentine,isotemnid, Salladolo- curs in both the Divisideranand Deseadan of dus, and Trachytherus belong to archaic Argentina (Marshall et al. 1983) is known groups not known from the late Deseadan fromthis partof the Salla Beds. Pyrotherium, ScarrittPocket local fauna or (with one ex- the principalindex taxon to the Deseadan in ception) later in the fossil record (Marshallet Argentina,is known to occur here. Rhynchip- al. 1985). The litopterns, which have been pus is very common in the lower Salla Beds. compared closely with Argentineforms, in- The rare astrapothereis only known to occur clude the most primitivemembers of their re- from near the base of the Salla Beds at Que- spective lineages (Thadanius hoffstetteri, bradaChala Jahuira. Tricoelodus boliviensis, and ?Coniopternium Upper Salla Beds.-The most dramatic primitivum).MacFadden and Frailey (1984) differencebetween the lower and upperparts noted that Salla Pyrotherium romeri seems of the section is the very commonoccurrence slightly smaller (and ?possibly more primi- of rodents in the upper Salla Beds. Another tive) than this species as it is known from important occurrence in this zone is Argentina. These data suggest that Salla is Branisella,which has received wide attention slightly older than other local faunas of De- as the earliest ceboid primate (e.g., seadan age known from Argentina. Ciochonand Chiarelli1981). As was noted by Hoffstetter(1968, 1976),many of the remain- CORRELATION TO THE TIME SCALE ing Salla mammalsare characteristicDesea- Three separate lines of evidence are used dan taxa. Of the generic list of Deseadan to correlate the Salla Beds to the standard mammalsthat occur in Argentina(Marshall magnetic polarity scale (MPTS). These in- et al. 1983) Palaeothentes, Prohegeto- clude the following:(1) The stage of evolution therium, and Archaeohyrax seem to be re- of the fossil mammals,which indicatea prob- stricted to the upper part of the Salla Beds. able early Deseadan age for the Salla Beds. Trachytherus and Pyrotherium occur through- Marshallet al. (1977)originally published K/ out the known fossiliferous portions of the Ar dates for the Deseadan from Argentinaof Salla Beds. 35.4 -0.4 and 33.6 -0.4 Ma. However, addi- It seems clear that the upper part of the tional dates have become available since Salla Beds is Deseadan in age. However, the those were published (Marshallet al. 1983; temporal placement of the Salla fauna with Marshallet al. 1985) indicating:(a) that the respect to the local faunas of Deseadan age original dates are probably several million from Patagonia remains problematic. Some years too old; (b) it is not certain if the dates of the unusual features of the Salla fauna publishedby Marshallet al. (1977)do, in fact, have been noted previously, with opinionbe- date the Land MammalAge because no De- ing divided as to whether these differences seadan fossil mammals are definitely re- are ecological (Patterson, in Patterson and cordedin direct stratigraphiccontext with re- Wood 1982;Hoffstetter and Lavocat 1970)or spect to the basalts in question; and (c) temporal (Wood, in Patterson and Wood volcanics interbeddedwith fossils at Scarritt 1982; Hartenburger1975). The presence of Pocket, Argentina,which on faunalevidence primates,abundance of Trachytherusand Ar- appears to be slightly younger than typical chaeohyrax, and the scarcity or absence of Deseadan local faunas, yield K/Ar dates in astrapotheriid,leontiniid, and toxodontid re- the 24-21 Ma range (Marshallet al. 1985). mains is regardedhere to reflect ecological, 2) A K/Ar date of 26.4 + 1.0 Ma was pro- or zoogeographic differences between Salla duced from a tuff located at 195 m above the and the localities in Patagonia. The base (ca. 10 m below the Principal Guide hegetotheres, interatheres,notohippids, and Level) of the Salla section. This tuff directly archaeohyracidare not, at the present state underlies the main fossiliferous concentra- of knowledge, suggestive of a precise age tions in the El Planimiento section. The (other than the fact that they are Deseadan). fission track dates located much higherin the DESEADAN SALLA BEDS OF BOLIVIA 245 same section, althoughthey are not used here the El Planimientosection may elucidate the in the correlationto the MPTS, suggest ages upper age and extent of the Deseadan Land younger than the K/Ar date. This age rela- Mammal Age as represented in the Salla tionship is obviously concordant with the Beds. The data and our chronological hy- superpositionalorder of the relevant tuffa- pothesis presented here have significant ceous horizons. ramificationswith regardto the calibrationof 3) The sequence and pattern of reversals the Deseadan Land MammalAge and associ- for the Salla Beds is representedby 14 mag- ated paleontologicalhypotheses. netic polarityzones (figs. 9, 11). Most of the reversals in the Salla Beds are concentrated CONCLUSIONS, SIGNIFICANCE AND DISCUSSION in the lower two-thirds of the section, Deseadan Calibration.-The present whereas the upper one-thirdis characterized study suggests a significantrecalibration of by almost continuous reversed polarity this Land MammalAge. Marshallet al. (1983; (zones R6 and R7). This long period of re- our fig. 12) indicate the Deseadan spanned a versed polarity imparts a relatively distinc- time from about 37 to 27 Ma, which made it tive signatureto the Salla magnetic polarity the longest of the South American Land zonation. Mammalages. The stage of evolution of the Given items 1 and 2 above, we may infer Salla Mammals and correlation to the time that the Salla Beds should lie withinthe 20 to scale suggest that the basal Deseadan should 30 Ma range. Within this time period, a rel- be no older than ca. 27-28 Ma (our "pre- atively long, reversed signatureon the MPTS ferredhypothesis" in fig. 12). The Salla Beds lies within the lower two-thirdsof Chron C6 are of added importancebecause they appear between 24 and 26 Ma. We assert that the to representthe earliest Deseadan, which is a long reversed polarity zone represented by period of time not recognized in Argentina. Salla zones R6-R7correlates to Chron6 (fig. With the results from this study (along with 11). Using this correlation as our principal those of Marshallet al. 1985), the temporal anchor-point, figure 11 indicates our pre- limits of the Deseadan must be revised ferredcorrelation of the Salla magneticpolar- significantly,i.e., shifted upwardsby ca. 10 ity zonation with the MPTS. It should be m.y. from early Oligocene to late Oligocene- noted that the details of the MPTS of early Miocene. Berggren et al. (1984) are at considerable Pascual et al. (1965, 1966)suggest that the variancewith that of Mankinenand Dalrym- Divisideranis a distinct, pre-DeseadanLand ple (1979)for this same time interval(fig. 11). MammalAge. In their primitiveaspects, sev- Despite these complications, the correlation eral elementsof the lower Sallafauna are sug- we propose in figure 11 seems correct and is gestive of "Divisideran" (fig. 12, alternate corroboratedby the data presentedhere (i.e., hypothesis). However, because the status principallythe K/Ar date of 26.4 + 1.0 Ma and limits of the Divisideranare unclear, we and the distinctivecorrelation between zones provisionallyplace the entire Salla fauna in R6-R7at Salla and C6 on the MPTS). the Deseadan, because of its close affinities Therefore, our current data suggest that with local faunas of that age in Patagonia(fig. the Salla Beds span an intervalfrom at least 12, preferredhypothesis). If the lower Salla 28.5 Ma to about 24 Ma (fig. 11), i.e., from Beds are indeed Divisideran,then there is no late Oligocene to early Miocene time. We need to invoke a hiatus between this Land reiteratethat this is not a unique correlation Mammalage and the Deseadan (fig. 12; Mar- to the MPTS; however, it is our preferred shall et al. 1977). However, because the in- hypothesis based on the K/Ar date and the dex taxon Pyrotheriumoccurs in the lower stage of mammalianevolution. As noted, the part of the section, we believe that our first fission-trackdates in the upper part of the hypothesisis correct. The ultimateresolution section are ca. 2-3 m.y. youngerthan would of this problem must come from a critical be expected from this correlation. Recently reassessment of the Divisideranand Desea- we have furthersampled relevant tuffaceous dan ages as representedin Argentina. horizonsfor additionalcorroborative isotopic Given the upward shift in the Deseadan, age determinations.Also, the complementary several other related considerations should searchfor fossil mammalsin the upperpart of be addressed:(1) The upperextent of the De- see

1979, AL.

ETSCALE Dalrymple, TIME and

BERGGREN (1984)

& Mankinen or SCALE (1979) 1984, MANKINENDALRYMPLETIME al., et

Berggren

(using MAGNETICPOLARITYZONATION

MPTS VERY the to DESEADAN) (EARLY DESEADAN)

(PROBABLYEARLY correlation and

zonation, BIOSTRATIGRAPHY

polarity

magnetic

Beds.

COMPOSITE1HICKNESS Salla STRATIGRAPHIC the I1.-Biostratigraphy, for FIG. text) DESEADAN SALLA BEDS OF BOLIVIA 247

t M U MARSHALL, PREFERRED IMPORTANT BIO- P p H HOFFSTETTER AND HYPOTHESIS ALTERNATE STRATIGRAPHIC 0o T R C T 0 PASCUAL (1983) (THIS PAPER) HYPOTHESIS EVENTS AT SALLA H SN N M 1 _ SANTACRUCIAN SANTACRUCIAN 21- 20 0 SANTACRUCIAN COLHUEHUAPIAN COLHUEHUAPIAN C 22- E 21 N 23- E 22 COLHUEHUAPIAN

24- C6 DESEADAN DESEADAN 25- HIATUS 26- C7 BRANISELLA 27- C8 28- "DIVISIDERAN" O0 29- L C9 G 30- O C C1C 31- E N E 32- C11 DESEADAN

33-

34- C12

35-

36- C13

37-

38- 015

39- C16 HIATUSIDERAN 40- C17 41- DIVISIDERAN 018

FIG. 12.-Comparisons of previous assessments of the age of the Deseadan Land Mammal age (Marshall et al. 1983) and the revised interpretations based on the present study. The dashed zones in the Marshall et al. (1983) correlation indicate postulated hiatuses. seadan remains questionable as represented cian is usually stated to be early Miocene in the Salla sequence. Our present chronol- with no significant hiatus between this Land ogy suggests that known fossil levels extend Mammal Age and the Colhuehuapian. The upwards to Chron C7 at about 26 Ma; how- Santacrucian has been considered as a rela- ever, the top of the Salla Beds are at least as tively long Land Mammal Age with a dura- young as about 24 Ma. To date this part of the tion of 6 m.y. from about 22 to 16 Ma. Mar- Salla Beds seems relatively unfossiliferous; it shall et al. (1977) list two K/Ar dates will not add to our knowledge of the later associated with Santacrucian fossils of 21.7 Deseadan until more fossil mammals and as- +0.3 Ma and 18.5 ±0.2 Ma. Work in prog- sociated radiometric dates are collected. ress (Marshall pers. comm.) indicates from a Also, given the shift of the Deseadan up- new suite of K/Ar age determinations that the wards in time, what happens to the Col- Santacrucian is certainly no older than ca. 21 huehuapian and Santacrucian Land Mammal Ma, and it probably extends upward in time Ages? The Colhuehuapian is usually stated to to ca. 15 Ma. Therefore our revised calibra- be late Oligocene with a significant hiatus be- tion of the Deseadan based on the Salla study tween this Land Mammal Age and the Desea- is plausible. dan. The Colhuehuapian has been considered Dispersal of Mammals into South to be a very short Land Mammal Age of ca. America.-Since Ameghino's time, paleon- 1-2 Ma duration (Marshall et al. 1983), al- tologists studying Cenozoic mammals in though there are no radiometric (or South America have recognized the first ap- paleomagnetic) data associated with known pearance of rodents (and, more recently the Colhuehuapian local faunas. The Santacru- primate Branisella from Salla) in the Desea- 248 BRUCE MACFADDEN ET AL. dan (see e.g., summary of the "Second spect to future hypotheses of the timing and Faunal Stratum" in Simpson 1980). This is origin of South American rodents and pri- generally accepted to represent allochthon- mates. ous dispersal from Africa or North America ACKNOWLEDGMENTS.-Wethank Carlos (there is by no means a consensus on this Villarroel, Ronald G. Wolff, Carl David subject, see, e.g., Ciochon and Chiarelli Frailey, and Annalisa Berta for their help in 1981). Until now, precise dating of these De- the field. Special thanks goes to Carlos Villar- seadan events was unavailable. Because the roel for his insight on the geology of Salla. Deseadan has been equated with "early Leonardo Branisa, John J. Flynn, Robert Oligocene," and given the fact that the first Hoffstetter, Larry G. Marshall, Steven R. set of published dates for this interval were May, S. David Webb, and A. E. Wood pro- 33.6 ±0.4 and 35.4 +0.4 Ma (Marshall et al. vided helpful comments that improved the 1977), these dispersal events were believed to manuscript. Dr. Donald Baird allowed us to have occurred at about 35 to 37 Ma. At Salla, study the PU Salla collection. Funds to de- rodents are known to occur throughout the fray field expenses for the PU Branisa Collec- fossiliferous horizons (although they are rare tion were provided by the Gordon Barbour from the lower Salla Beds), which would indi- bequest. Dr. A. E. Wood and Gary S. Mor- cate that they had arrived in South America gan provided identifications for the rodents. before 28.5 Ma. The known stratigraphic oc- This research was made possible through the currence of Branisella is 27 Ma. These new funds and logistic support provided by the data place revised constraints on the poten- University of Florida, LACM, GEOBOL, tial timing and origins of these groups that and the U.S. National Science Foundation could be 10 m.y. later than has been previ- Grants DEB 79-05861 and EAR 8318903. Car- ously believed. So far as is known, our data Special thanks are extended to Ing. Raul suggest that the base of the "Second Faunal rasco of GEOBOL for his support during this Stratum" could be as young as late (not project. This report is University of Florida early) Oligocene. This revised chronology Contribution to Vertebrate Paleontology should have significant ramifications with re- number 240.

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