Asociaci6n Paleontol6gica Argentina. Publicaci6n Especial 7VII International Symposium on Mesozoic Terrestrial Ecosystems: 185-189. Buenos Aires , 30-6-2001

The evolutionary impact of an epeiric seaway on LateCretaceous and Paleocene palynofloras of South America

Gregory P. WILSON! and Nan Crystal ARENS l

ISSN 0328-347X

Abstract. Paleogeographie reconstructions hypothesize that during the Cretaceous, South America wassplit into northern and southern porti ons by an epeiric seaway. Although the location, extent, and dura­tion of this ancient seaway is debated, some propose that the resulting separation produced a northernSouth American biota that more closely resembled other equatorial biotas, distinct from a southern SouthAmerican biota that more closely resembled other austral biotas. Palynological data from nine SouthAmerican countries, five equatorial representatives (including the southeastern U.S. and northwesternAfrica), and three austral repre sentatives (Antarctica, Australia, and New Zealand) were assembled into adatabase that includes more than 450 genera from more than 150 localities spanning the Late Cretaceousand Paleocene epochs. Principal components and cluster analyses of the palynological data separatenorthern South America from southern South America during the Maastrichtian and Paleocene. Duringthese epochs, northern South America clusters with the equatorial representatives; whereas southernSouth America clusters with austral representatives. These results suggest that biogeographic barriers,such as epeiric seaw ays, may have pla yed a significant role in the evolution of distinct terre strial biotas inSouth America during the Late Cretaceous and Paleocene.

Keywords. South America. Cretaceous. Paleocene. Biogeography. Palynology. Epeiric seaway.

Introduction

Although the historical patterns of terrestrial bio­geography generally followed that of large-scalecontinental movements, the distribution of organ­isms has also been sensitive to transient, less easilydetectable geographic features, such as climaticzones, mountain ranges, and epeiric seaways. For ex­ample, recently discovered vertebrate faunas fromGondwana suggest a Mesozoic biogeography morecomplex than plate reconstructions alone would pre­dict (Sereno et al., 1998; Goodwin et al., 1999). Here,we investigate the effect of one such geographic bar­rier -an hypothesized seaway bisecting SouthAmerica during the Late Cretaceous and Paleocene.

Sedimentological data indicate that during theLate Cretaceous and Paleocene, South America expe­rienced a marine transgression that produced anepeiric seaway isolating Argentina and Chile fromthe remainder of South America (Malumian et al.,1983; Smith et al., 1994; Scotese, 1997). Based on thedistribution of sedimen tary facies, Uliana and Biddle(1988) suggest maximum epeiric flooding during theMaastrichtian. During this interval, they concluded

'Department of Integrative Biology and University of CaliforniaMuseum of Paleontology, 1101 Valley Life Sciences Building,Berkeley, California 94720-4780, USA.

©Asociaci6n Paleontol6gica Argentina

Although the historical patterns of terrestrial bio-- ~ ~ ..

that southern South America was characterized by awestern peninsula and southeastern archipelago.Malumian and Carames (1995) used similar data toshow that despite a series of eastern embayments,southern South America was a largely continuouspeninsula by the Paleocene. Some propose that suchmarine incursions produced a northern SouthAmerican biota that more closely resembled otherequatorial biotas (e.g., northern Africa and southeast­ern North America), distinct from a southern SouthAmerican biota that more closely resembled otheraustral biotas (e.g., Antarctica and Australia;Zinsmeister, 1987; Pascual et al., 1992). We test thishypothesis through a quantitative comparison ofLate Cretaceous and Paleocene palynofloras fromSouth American, austral, and equatorial representa­tives. We chose pollen and spore, rather thanmacrofloral or faunal, data to evaluate this hypothe­sis because palynological data offered denser andmore complete sampling in both space and time.

Methods and materials

We assembled a data set from the published liter­ature that included more than 450 pollen and sporegenera from more than 150 localities spanning theLate Cretaceous and Paleocene epochs. Our datacovered nine of the thirteen South American coun-

0328-347X/Ol$00.00+50

western peninsula and southeastern archipelago.Malumian and Carames (1995) used similar data to

186 G.P. Wilson and N.C. Arens

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ral similarity, \principal comp'exception was Vformed a distiruCampanian (ligsepara te austra.2.A-B). Howeveare unexpectedlal rep resentativand Gab6n).

In con trast,and Paleocene stral and equatorcomponent (lilPses results showdivisions (figunlynofloras fromlocalities (ArgerZealand) . Soullargely isolated

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PCA and cluster analysis graphically illustrate thedegree of compositional similarity am ong geograph­ic samples. In the absence of the proposed seaway,the paleolatitudinal and climatic effects would likelyhave produced a gradual compositional transitionbetween northern and southern biotas within SouthAmerica. A more discrete barrier, such as the pro­posed seaway, might permit evolutionary divergencebetween biotas, which would emerge in these analy­ses as more discrete biotic provinces.

In the Albian through the Campanian, PCA re-

o• Colombia • C. Chile A N.E. Australia• Venezuela 0 N. Argentina . S.C.Australia• Guyana , C. Argentina 0 NewZealand

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• Colombia ° S. Chile 0 Gabon• Venezuela , C. Argentina . S.E. Australia

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• C, Chile D Senegal

Figure 1. Principal component analysis perform ed in SPSS 8.0. The data set was analyzed according to the following time intervals: A,Albian-Coniacian; H, Santonian-Campanian; C, Maastrichtian; and D, Paleocene.

tries, five northern African countries, the southeast­ern United States, Antarctica, New Zealand, andAustralia. We chose to record genera, rather than pa­

. Iynospecies, to minimize taxonomic variation be-tween workers in different regions. Unique appear­ance data were removed from the study to reduceoutlier bias in principal components analysis (PCA).A full data set with references is available from thesenior author upon request. Data were assembled in­to a presence-absence matrix that was analyzed bytime interval. We chose broad time intervals to ac­commodate uncertainty in age-dating of some locali­ties. PCA and the cluster analyses were performed inSPSS 8.0 and SYSTAT 6.0 respectively. PCA solutionswere not rotated. For cluster analyses, Pearson prod­uct moment correlation coefficients for binary data

A.p.A. Publicaci6n Especial 7, 2001

Late Cretaceous epeiric seaway and South American palynofloras 187

A BN. Peru S. Colombia,

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Distances Distances

Figure 2. Cluster analyses performed in SYSTAT 6.0. The data set was analyzed according to the following time intervals: A, Albian­Coniacian; B, Santonian-Campanian; C, Maastrichtian; and D, Paleocene.

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suits showed a north-to-south gradient in palynoflo­ral similar ity, without distinct p rovinces on bothprincipal component axes (figures LA-B). The oneexception was the southeastern Uni ted States, whichformed a distinct province during the Santonian andCampanian (figure 1.B). Cluster analyses did showsepara te au str al and equatorial groupings (figures2.A-B). However, the palynofloras from central Chileare un expectedly positioned among typical equatori­al representatives (Colombia, Brazil, Peru, Nigeria,and Gabon).

In con tras t, PCA results from the Maastrichti anand Paleocene show distinct separation between aus­tral and equa torial palynofloras on the first principalcomponen t (figures 1.C-D). Likewise, cluster analy­ses results show distinct austral and equatorial sub­divisions (figures 2.C-D). In both time intervals, pa­lynofloras from central Chile lie among other aus trallocalities (Argentina, An tarctica, Australia, and NewZealand) . Southeastern United States remainedlargely isolated from Gondwan an palynofloras.

Discussion

The north-to-south gradient present from theAlbian through the Campanian supports the hypoth­esis that biogeographic distributions were more cos­mopolitan during this interval, and largely con­trolled by paleolatitudinal and associated climatic in­fluen ces. Although cluster analyses produced aus tra land equatorial group ings, the position of centralChile within the equatorial group suggests that nostrong physical barrier to biotic exchan ge existednorth of central Chile. Because the clustering algo­rithm forces dichotomy, cen tral Chile was likelyplaced among the equator ial localities be cause it isgeographically intermediate between northern andsouthern sites. This pattern follows the predictions ofa biogeographic distribution shaped primarily by lat­itudinal variation.

Conversely, more distinct grou pings of palynoflo­ras during the Maa strichtian and Paleocene supportthe hypothesis that distributions were shaped by a

A.p.A. Publicaci6n Especial 7, 2001

rj

188 G.P. Wilson and N.C. Arens

ba rrier to biotic exchange. Geographically intermedi­ate palynofloras, such as northern Argentina andsoutheastern Brazil, do not span the similar ity gapbetween the geographic extremes as predicte d by alati tudinal or clima tic effect. Likewise, the clustersshow distinct subdivisions with geographic interme­diates well nested within their respective provinces.This lack of similarity, despi te geographic proximity,argues for a physical division, between neighboringSouth American palynofloras. Additionally, our re­sults indicate that the equatorial province did not be­come phy togeographically di stinct from the australp rov ince until the Maast rich tian , contrary toSrivas tav a (1994) who suggested an Early Cretaceousorigin for these provinces, as well as the inclusion ofsoutheastern United Sta tes in the equatorialprovince.

Although Haq et al. (1987) argued for a eustaticsea leve l low d uring the Maas trichtian, sedirnento ­logical evidence ind icates that continental bed s cov­ered m uch of the Northwestern Argentine Basin,wes tern Para guay and southern Bolivia from theCenomanian through Campanian (Malumian et al.,1983). By the early Maastrichtian, a transgression hadinunda ted mo st of these intracratonic basins andmuch of southern Argen tina (Uliana and Biddle,1988). Marine facies extended over most of northernPatagonia and, at minimum, to the borders ofArgen tina, Uru guay, Paraguay, and Bolivia. This ma­rin e incursion con tin ued th rough the middlePaleocene.

Available vertebrate data also suppor tMaas trichtian p rov incializa tion. Pascual et al. (1992)noted that faunas recovered from upp er Cretaceousand Paleocene rocks of Patagonia show greater over­all affinity w ith eastern Gondwan an biotas(Antarctica, Indi a, Aus tralia, and Madagascar) thanwith fossil assemblages from Laguna Umayo (Peru)or Tiupampa (Bolivia). For example, gond ­wana theres we re present in India, Mad agascar, andsouthern South America during the Campanian andMaastr ichtian (Bonaparte, 1990, 1996). However,gondwanatheres we re absen t in northern Sou thAmerica during the Paleocene (Krause et al., 1997).Although this is negative evidence, it is consistentwith a Maastrichtian and Paleocene barrier to north­ward migration . Collectively, the independent geo­logical and biogeographic evidence po int toward aSignificant degree of evolu tionary isolation for north­ern and southe rn South Am erica bio tas d uring theMaastrich tian and Paleocene . Despit e noted climaticd ifferences between high and low latitudes duringthe Maastrich tian (Parrish, 1987; Askin and Spicer,1995), this biogeographic pattern is best exp lained bya more discrete biogeog raphic barrier, such as anepe iric seaway. However, closely related elements

A.P.A. Publicaci6n Especial 7, 2001

among the ear ly Paleocene !taborai (Brazil), LasFlores (Pa tagonia, Argen tina), and Tiupampa(Bolivia) faun as (Pascual ~t al., 1996) suggest tha twhatever the nature of the barrier, it is best describedas inhibiting, rather than completely blocking, dis­persal.

Our results must be considered in light of the lim­itations of the data. First, pollen and spore genera aredescrip tive form taxa (para taxa, as specified by theInternational Code of Botanical Nomenclature) anddo not trans late directly int o evolutionary groups.Morphological simil arity due to plesiomorphy orconvergence may misrep resent evolutionary related ­ness. Second, pal ynological studies may be ham­pered by variation in taxonomic ass ignment byworkers in d ifferent regions. Third, this study couldbenefit from grea ter temporal resolution, but wehave conservatively divide d our data set into largetime intervals, reflecting the un cert ain ty in the ageassign men ts of some palynofloras.

C on cl us ions

Plate tectonics has contribu ted much to our un­derstandin g of paleobiogeographic pattern s. Howe­ver, organisms are sensitive to a variety of barriersthat exist within plate boundaries. Biogeographic hy­potheses must incorporate knowledge of regionaland sometimes transient geographic features, such asmajor ecotones and epeiric seaways. We bring an ad­ditional line of evidence -the palynofloral record- tothe discuss ion of biogeographic evolution of SouthAm erican bio tas. These data, combined with sedi­mentological and faunal evidence, suggest that anepeiric seaway played an important role in the diver­gent evolutionary pathways of terrestrial biotas innorthern and southern South American during theMaastrichtian and Paleocene.

Acknow ledgments

We gratefully acknowledge helpful d iscussion w ith Dr. W.A.Clemens, Ora. P. Holroyd, T.A. Stidha m, and CA.E. Stromb erg.Support for th is stud y wa s prov ided by the Univers ity ofCaliforn ia Museum of Paleon toJogoy and a National ScienceFoundation Gradua te Research Fellowship (to GPW). This isUniversity of Califo rnia Museum of Pa leon tology contribution no.1740.

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Accepted: November I:?, 2000.

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