Asociación Paleontológica Argentina. Publicación Especial 7
VII International Symposium on Mesozoic Terrestrial Ecosystems:185-189. BuenosAires,30-6-2001
ISSN0328-347X
The evolutionary impact of an epeiric seaway on Late Cretaceous and Paleocene palynofloras of South America
Gregory P.WILSON1and Nan Crystal ARENS1
Abstract. Paleogeographic reconstructions hypothesize that during the Cretaceous, SouthAmerica was split into northern and southern portions by an epeiric seaway.Although the location,extent, and dura- tion of this ancient seaway isdebated, some propose that the resulting separation produced a northern South American biota that moreclosely resembled other equatorial biotas,distinct from a southern South American biota that more closely resembled other austral biotas. Palynological data from nine South American countries, five equatorial representatives (including the southeastern U.S.and northwestern Africa),andthree austral representatives (Antarctica, Australia,andNew Zealand) wereassembled into a database that includes more than 450 genera from more than 150 localities spanning theLate Cretaceous and Paleocene epochs. Principal components and cluster analyses of the palynological data separate northern South America from southern South America during the Maastrichtian and Paleocene.During these epochs, northern South America clusters with the equatorial representatives; whereas southern South America clusters with austral representatives. These results suggest that biogeographic barriers, such as epeiric seaways, may have played asignificant role in the evolutionof distinctterrestrialbiotas in South America during the LateCretaceous and Paleocene.
Introduction
Keywords. South America. Cretaceous. Paleocene. Biogeography. Palynology. Epeiric seaway.
Although the historical patterns ofterrestrial bio- geography generally followed that of large-scale continental movements, the distribution of organ- isms has also been sensitive to transient, less easily detectable geographic features, such as climatic zones,mountain ranges, and epeiricseaways.For ex- ample, recently discovered vertebrate faunas from Gondwana suggest a Mesozoicbiogeography more complexthan plate reconstructions alone would pre- dict (Sereno et al., 1998;Goodwin et al., 1999).Here, we investigate the effect ofone such geographic bar- rier -an hypothesized seaway bisecting South America during the Late Cretaceousand Paleocene.
Sedimentological data indicate that during the Late Cretaceous and Paleocene,South America expe- rienced a marine transgression that produced an epeiric seaway isolating Argentina and Chile from the remainder of South America (Malumián et al., 1983;Smith et al., 1994; Scotese, 1997).Based on the distribution of sedimentary facies,Uliana and Biddle (1988)suggestmaximum epeiricflooding during the Maastrichtian. During this interval, they concluded
'Department of Integrative Biology and University of California Museum of Paleontology, 1101 Valley Life Sciences Building, Berkeley,California 94720-4780, USA.
©Asociación Paleontológica Argentina
that southem SouthAmericawas characterized by a western peninsula and southeastern archipelago.
Malumián and Caramés (1995) used similar data to show that despite a series of eastern embayments, southern South America was a largely continuous peninsula by the Paleocene.Somepropose that such marine incursions produced a northern South American biota that more closely resembled other equatorial biotas (e.g.,northern Africa and southeast- ern North America),distinct from a southern South American biota that more closely resembled other austral biotas (e.g., Antarctica and Australia;
Zinsmeister, 1987;Pascual et al., 1992). We test this hypothesis through a quantitative comparison of Late Cretaceous and Paleocene palynofloras from South American, austral, and equatorial representa- tives. We chose pollen and spore, rather than macrofloral or faunal, data to evaluate this hypothe- sis because palynological data offered denser and more complete sampling in both space and time.
Methods and materials
We assembled a data set from the published liter- ature that included more than 450pollen and spore genera from more than 150 localities spanning the Late Cretaceous and Paleocene epochs. Our data covered nine of the thirteen South American coun-
0328-347X/Ol$00.00+50
186 G.p.Wilson and N.e. Arens
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N.E. Brazil <> CamerounFigure 1. Principal component analysis performed in SPSS 8.0. The data setwas analyzed according to the following timeintervals: A, Albian-Coniacian; B, Santonian-Campanian; e,Maastrichtian; and D, Paleocene.
tries, fivenorthem African countries, the southeast- em United States, Antarctica, New Zealand, and Australia. We chosetorecord genera,rather thanpa- lynospecies, to minimize taxonomic variation be- tween workers in different regions. Unique appear- ance data were removed from the study to reduce outlier bias in principal components analysis (PCA).
A full data set with references is available from the seniorauthor upon request.Data were assembled in- to a presence-absence matrix that was analyzed by time interval. We chose broad time intervals to ac- commodate uncertainty in age-dating of some locali- tiesoPCA and the cluster analyses wereperformed in sPSS 8.0 and
svsrxr
6.0 respectively.PCA solutions werenot rotated. Forcluster analyses, Pearson prod- uct moment correlation coefficientsforbinary data APA Publicación Especial 7, 2001were used as distance metrices, with single linkage amalgamation.
Results
PCA and cluster analysis graphically illustrate the degree of compositional similarity among geograph- ic samples. In the absence of the proposed seaway, thepaleolatitudinal and climatic effects would likely have produced a gradual compositional transition between northem and southem biotas within South America. A more discrete barrier, such as the pro- posedseaway, might permit evolutionary divergence betweenbiotas, which would emerge inthese analy- sesasmore discrete biotic provinces.
In the Albian through the Campanian, PCA re-
Late Cretaceous epeiric seaway and South American palynofloras
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187
Figure 2. Cluster analyses performed in SYSTAT6.0.The data set was analyzed according to the following time intervals: A, Albian- Coniacian; B,Santonian-Campanian; C, Maastrichtian; and D, Paleocene.
sults showed a north-to-south gradient in palynoflo- ral similarity, without distinct pravinces on both principal component axes (figures 1.A-B).The one exceptionwas the southeastern United States, which formed a distinct province during the Santonian and Campanian (figure 1.B).Cluster analyses did show separate austral and equatorial graupings (figures 2.A-B).However, the palynofloras from central Chile are unexpectedly positioned among typical equatori- al representatives (Colombia, Brazil, Perú, Nigeria, and Gabán).
In contrast, PCA results from the Maastrichtian and Paleocene show distinct separation between aus- tral and equatorial palynofloras on the first principal component (figures 1.C-D). Likewise, cluster analy- ses results show distinct austral and equatorial sub- divisions (figures 2.C-D).In both time intervals, pa- lynofloras from central Chilelie among other austral localihes (Argentina,Anrarclica, Australia, and New Zealand). Southeastern United States remained largely isolated from Gondwanan palynofloras.
Discussion
The north-to-south gradient present from the Albian through the Campanian supports the hypoth- esis that biogeographic distributions were more cos- mopolitan during this interval, and largely con- trolled by paleolatitudinal and associated clirnatic in- fluences. Although cluster analyses produced austral and equatorial groupings, the position of central Chile within the equatorial group suggests that no strong physical barrier to biotic exchange existed north of central Chile. Because the clustering algo- rithm forces dichotomy, central Chile was likely placed among the equatorial localities because it is geographically intermediate between northern and southern sites.This pattern follows the predictions of a biogeographic distribution shaped primarily by lat- itudinal variation.
Conversely, moredistinct groupings oípalynoílo- ras during the Maastrichtian and Paleocene support the hypothesis that distributions were shaped by a
A.P.A. Publicación Especial 7, 2001
188 G.P.Wilson and N.e.Arens
barríer tobíotíc
exchange. GeQgmphit;;nlly
íntermedí-ate palynofloras, such as northern Argentina and southeastern Brazil, do not span the similarity gap between the geographic extremes as predicted by a latitudinal or climatic effect. Likewise, the clusters show distinct subdivisions with geographic interme- diates well nested within their respective pravinces.
This lack of similarity, despite geographic proximity, argues for a physical division, between neighboring South American palynofloras. Additionally, our re- sults indicate that the equatorial pravince did not be- come phytogeographically distinct from the austral province until the Maastrichtian, contrary to Srivastava (1994) who suggested an Early Cretaceous origin for these provinces, as well as the inclusion of southeastern United States in the equatorial pravince.
Although Haq et al. (1987) argued for a eustatic sea level low during the Maastrichtian, sedimento- logical evidence indicates that continental beds cov- ered much of the Northwestern Argentine Basin, western Paraguay and southern Bolivia from the Cenomanian through Campanian (Malumián et al., 1983).By the early Maastrichtian, a transgression had inundated most of these intracratonic basins and much of southern Argentina (Uliana and Biddle, 1988).Marine facies extended over most of northern Patagonia and, at minimum, to the borders of Argentina, Uruguay, Paraguay, and Bolivia. This ma- rine incursion continued through the middle Paleocene.
Available vertebrate data also support Maastrichtian provincialization. Pascual et al. (1992) noted that fauna s recovered from upper Cretaceous and Paleocene rocks ofPatagonia show greater over- all affinity with eastern Gondwanan biotas (Antarctica, India, Australia, and Madagascar) than with fossil assemblages from Laguna Umayo (Perú) or Tiupampa (Bolivia). For example, gond- wanatheres were present in India, Madagascar, and southern South America during the Campanian and Maastrichtian (Bonaparte, 1990, 1996). However, gondwanatheres were absent in northern South America during the Paleocene (Krause et al., 1997).
Although this is negative evidence, it is consistent with a Maastrichtian and Paleocene barrier to north- ward migration. Collectively, the independent geo- logical and biogeographic evidence point toward a significant degree of evolutionary isolation for north- ern and southern South America biotas during the Maastrichtian and Paleocene. Despite noted climatic differences between high and low latitudes during the Maastrichtian (Parrish, 1987; Askin and Spicer, 1995), this biogeographic pattern is best explained by a more discrete biogeographic barrier, such as an epeiric seaway. However, closely related elements A.P.A. Publicación Especial 7, 2001
nmQng
theearíy Paleoce e
ltªbQmí (ñrazil),Las
Flores (Patagonia, Argentina), and Tiupampa (Bolivia) faunas (Pascual et al., 1996) suggest that whatever the nature of the barrier, it is best described as inhibiting, rather than completely blocking, dis- persal.
Our results must be considered in light ofthe lim- itations of the data. First, pollen and spore genera are descriptive form taxa (parataxa, as specified by the International Code of Botanical Nomenclature) and do not translate directly into evolutionary groups.
Morphological similarity due to plesiomorphy or convergence may misrepresent evolutionary related- ness. Second, palynological studies may be ham- pered by variation in taxonomic assignment by workers in different regions. Third, this study could benefit from greater temporal resolution, but we have conservatively divided our data set into large time intervals, reflecting the uncertainty in the age assignments of some palynofloras.
Conclusions
Plate tectonics has contributed much to our un- derstanding of paIeobiogeographic patterns. Howe- ver, organisms are sensitive to a variety of barriers that exist within plate boundaries. Biogeographic hy- potheses must incorporate knowledge of regional and sometimes transient geographic features, such as major ecotones and epeiric seaways. We bring an ad- ditionalline of evidence -the palynofloral record- to the discussion of biogeographic evolution of South American biotas. These data, combined with sedi- mentological and faunal evidence, suggest that an epeiric seaway played an important role inthe diver- gent evolutionary pathways of terrestrial biotas in northern and southern South American during the Maastrichtian and Paleocene.
Acknowledgments
We gratefully acknowledge helpful discussion with Dr. W.A.
Clemens, Dra.P. Holroyd, T.A.Stidham, and c.A.E. Strbmberg.
Support for this study was provided by the University of California Museum of Paleontologoy and a National Science Foundation Graduate Research Fellowship (to GPW). This is University of California Museum of Paleontology contribution no.
1740.
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Accepted: November 17'h,2000.
A.P.A.Publicación Especial 7, 2001
