A Paleocene lowland macrofl ora from Patagonia reveals signifi cantly greater richness than North American analogs

Few South American macrofloras of Paleocene age are known, and this limits our knowledge of diversity and composition between the end-Cretaceous event and the Eocene appearance of high floral diversity. We report new, unbiased collections of 2516 compression specimens from the Paleocene Salamanca Formation (ca. 61.7 Ma) from two localities in the Palacio de los Loros exposures in southern Chubut, Patagonia, Argentina. Our samples reveal considerably greater richness than was previously known from the Paleocene of Patagonia, including 36 species of angiosperm leaves as well as angiosperm fruits, flowers, and seeds; ferns; and conifer leaves, cones, and seeds. The floras, which are from siltstone and sandstone channel-fills deposited on low-relief floodplain landscapes in a humid, warm temperate climate, are climatically and paleoenvironmentally comparable to many quantitatively collected Paleocene floras from the Western Interior of North America. Adjusted for sample size, there are >50% more species at each Palacio de los Loros quarry than in any comparable U.S. Paleocene sample. These results indicate more vibrant terrestrial ecosystems in Patagonian than in North American floodplain environments ∼4 m.y. after the end-Cretaceous extinction, and they push back the time line 10 m.y. for the evolution of high floral diversity in South America. The cause of the dis parity is unknown but could involve reduced impact effects because of greater distance from the Chicxulub site, higher latest Cretaceous diversity, or faster recovery or immigration rates.


INTRODUCTION
Paleocene macrofl oras provide fundamental data regarding ecosystem diversity after the end-Cretaceous extinction (e.g., Wolfe and Upchurch, 1986;Johnson and Ellis, 2002;Barclay et al., 2003;Wilf et al., 2006) and before the major biotic transitions that accompanied global warming across the Paleocene-Eocene boundary (e.g., Wing and Harrington, 2001;Jaramillo, 2002).However, there are virtually no records of confi rmed Paleocene macrofl oras from South America (Burnham and Johnson, 2004;Wing et al., 2004), which harbored elevated plant diversity in both Patagonia and the Neotropics during the Eocene (Berry, 1938;Wilf et al., 2005;Jaramillo et al., 2006).Today, South America has notably high plant diversity in most of its vegetational zones (e.g., Phillips and Miller, 2002).
In southern South America, the few fl oras reported as Paleocene in the literature have either been redated as Eocene (Wilf et al., 2005;Yabe et al., 2006) or have no reliable geo chronol ogy.Most fl oras of possible Paleocene age are known only from small collections that lack stratigraphic context or reliable information about fl oral diversity.A relatively well understood example is the Salamanca Formation fl ora, from central Patagonia (Fig. 1), which is mostly known from palynological and fossil wood studies (Romero, 1968;Archangelsky, 1973;Archangelsky and Romero, 1974;Petriella and Archangelsky, 1975;Archangelsky and Zamaloa, 1986;Brea et al., 2005;Matheos et al., 2005).Compression fl oras from the Salamanca Formation have only been studied by Berry (1937) from a small collection, including 24 fi gured specimens and 11 species, that has never been revised.
Here, we report the fi rst quantitative estimates of plant megafossil diversity for the Paleocene of southern South America, using large, unbiased collections of the Salamanca Formation compression fl ora.We review the stratigraphy and age of the fl ora, analyze fl oral richness, contrast the results with comparable Paleocene fl oras from similar absolute latitudes of the Western Interior of North America, and place these results in the context of early Cenozoic fl oral diversifi cation in South America.

GEOLOGICAL SETTING AND AGE
The new collections come from two localities in the Palacio de los Loros ("Parrot Palace") exposures of the Salamanca Formation.These crop out in the western San Jorge Basin, 36 km south of Sarmiento in southern Chubut Province, Argentina (Fig. 1).
In the local section at Palacio de los Loros (GSA Data Repository Appendix DR1 1 ), the Salamanca Formation, which is extensively exposed in an old valley of the Senguer River, lies unconformably upon the Late Cretaceous Bajo Barrial Formation of the Chubut Group and conformably beneath the continental, 1 GSA Data Repository item 2007233, Appendix DR1 (section of the Salamanca Formation at Palacio de los Loros locality), Appendix DR2 (distinguishing features of angiosperm leaf morphotypes from Palacio de los Loros), and Appendix DR3 (relative abundance of dicot leaf morphotypes for rarefaction analysis), is available online at www.geosociety.org/pubs/ft2007.htm,or on request from editing@ geosociety.org or Documents Secretary, GSA, P.O.Box 9140, Boulder, CO 80301, USA.*E-mail: aiglesias@museo.fcnym.unlp.edu.ar.
A Paleocene lowland macrofl ora from Patagonia reveals signifi cantly greater richness than North American analogs middle and late Paleocene Río Chico Formation.The bulk of the Salamanca Formation is a transgressive marine package, corresponding to the middle and lower Salamanca, but the uppermost Salamanca in the western part of the basin contains mudstone and fi negrained sandstone beds that represent channels and oxbow fi lls from a meandering, low-relief fl uvial environment.The mudstone and some of the sandstone beds contain well-preserved plant compression fossils throughout the exposure area.The two sites reported here, PL1, a channel siltstone, and PL2, a channel siltstone topped by mudstone fi ll at approximately the same stratigraphic level as PL1, had the best preservation of plant remains.The Salamanca Formation is generally assigned to the Danian stage, based on forami nif era and ostracoda from the northern and eastern parts of the basin (Méndez, 1966;Bertels, 1975).
Spe cifi cally, the foraminifera species present, including Globanomalina (Turborotalia ) compressa and Globoconusa daubjergensis, reliably indicate an upper Danian age (zone P1c) for the marine Salamanca (following Olsson et al., 1999).There are three less reliable but con sistent radiometric ages, all from whole-rock K-Ar analyses.First, a tuff reported as being from the uppermost Salamanca Formation, 70 km northeast of Palacio de los Loros, rendered an age of 62.6 ± 5.0 Ma (Andreis, 1977; adjusted using Dalrymple, 1979).Second, two basalt units at or just under the base of the formation yielded ages of 64.0 ± 0.8 Ma and 62.8 ± 0.8 (Marshall et al., 1981).Marshall et al. (1981Marshall et al. ( , 1997) ) used these ages and paleomagnetic data to assign the overlying Banco Negro Inferior (a massive, black, tabular bed conventionally used to identify the Salamanca-Río Chico contact throughout the basin) at several localities to polarity subchron C26r.We also sampled a tuff horizon immediately below the Banco Negro Superior ( Feruglio, 1949), 41.5 m above correlative plant-bearing levels, for 40 Ar/ 39 Ar analyses.The sanidines we retrieved were clearly altered, and the resulting isochron age of 57.80 ± 6.00 Ma has little interpretive value.
Based on this evidence and the lack of signifi cant hiatuses observed between the relevant units, the Palacio de los Loros megafl oras, which locally overlie the marine Salamanca and underlie the Banco Negro Inferior, can be well constrained to an age near the Danian-Selandian boundary.This is equivalent to the magnetic polarity chron 26-27 boundary at 61.7 ± 0.2 Ma (Gradstein et al., 2004).However, a more precise age assignment for the Palacio de los Loros fl oras requires additional work, with particular attention to western strata of the San Jorge Basin.

SALAMANCA FLORA AND PALEOCENE PLANT DIVERSITY
Fossil plants at Palacio de los Loros were discovered by A. Piatnitzky (Feruglio, 1949), who collected the 24 type specimens (Berry, 1937) from an unknown stratigraphic level.Prospecting in this area, we discovered the PL1 and PL2 sites and sampled them using standard bench-quarrying techniques (e.g., Barclay et al., 2003).All identifi able material, including 1119 specimens from PL1 and 1397 from PL2, was prepared and deposited at the Museo Paleontológico Egidio Feruglio (MPEF), Trelew, Argentina.The 2417 leaf specimens were segregated into discrete morphotypes using distinct leaf architectural features (Johnson et al., 1989;Ash et al., 1999;Fig. 2; Appendix DR2 [see text footnote 1]) to allow for paleoecological and paleoclimatic analyses.
From these morphotypes, we estimate the presence of 36 angiosperm leaf species, including 33 dicots (Fig. 2; Appendix DR2) and three monocots.Recognizable angiosperm groups include: a large-leaved Nothofagus (Southern Beech, Fig. 2I); Menispermaceae (moonseed family, Fig. 2Y); Akania (an Australian rain-forest endemic also known from early Eocene Patagonia; Romero and Hickey, 1976; Fig. 2GG); a variable species of Lauraceae (laurel family, Fig. 2CC); at least one species of Urticaceae (nettle family, Fig. 2W); legume leafl ets that represent one of the oldest records of the family (Fabaceae, Fig. 2Z); at least one species of Sapindaceae (litchee family, Figs.2G and 2BB); palmately lobed Malvaceae of the "Sterculia" type (Figs.2B and 2K); and Rosaceae (Fig. 2O).Wellpreserved angiosperm fl owers, fruits, and seeds are also present.Also, the fl ora contain conifers, including Araucariaceae cone scales and Podocarpaceae leaves and cones, and at least two fern species including Lygodium.These are the fi rst occurrences of reproductive and foliar organs of gymnosperms and the fi rst fern foliage found in the Salamanca Formation, corroborating data from wood and palynomorphs ( Archangelsky, 1973;Romero, 1968;Matheos et al., 2005).The recognizable elements, especially Nothofagus, Akania, and the conifer groups, indicate a Gondwanan affi nity for the fl ora.
Using standard techniques of leaf-margin (updated in Wilf, 1997, their equations 2 and 4) and leaf-area (Wilf et al., 1998, p. 204) analyses from the 33 dicot leaves at both Palacio de los Loros outcrops combined (Appendix DR2, see footnote 1), we estimated a mean annual temperature of 14.1 ± 2.6 °C (57.6% of species toothed) and mean annual precipitation of at least 115 cm (+50/-35 cm), consistent with the lack of Andean rain shadow at this time.
The paleoclimate estimates are supported by the presence of diverse thermophilic groups, such as palm organs, which are found throughout the basin (Romero, 1968;Archangelsky, 1973), and alligatorids from the eastern basin, which require a minimum winter isotherm of 10 °C (Bona, 2005).Podocarps nearly exclusively inhabit high-rainfall environments (e.g., Brodribb and Hill, 1999), and Akania today only survives in eastern Australian tropical and subtropical rain forest.Growth-ring studies of petrifi ed wood also suggest a frost-free environment (Brea et al., 2005).
Alpha diversity of dicot leaves was adjusted for sample size using rarefaction (Fig. 3; Appendix DR3) and compared with well-sampled Paleocene fl oras that represent the maximum Paleocene alpha diversity conventionally known (see below) from hundreds of sites in the Western Interior of North America (e.g., Wing et al., 1995;Wilf, 2000;Barclay et al., 2003).These samples come from similar paleolatitudes and paleotemperatures and from comparable lowrelief, fl oodplain paleoenvironments (e.g., Wing et al., 1995); they all were collected using similar, unbiased methods.Our results (Fig. 3) show that diversity at both PL1 and PL2 is more than 50% higher than comparable U.S. fl oras.We note that a suite of diverse Paleocene fl oras, including the Castle Rock fl ora, has been discovered in the Denver Basin of Colorado, proximal to the elevated Laramide Front Range (Johnson and Ellis, 2002;Ellis et al., 2003;Johnson et al., 2003).However, these fl oras clearly are not analogs for Palacio de los Loros because they represent premontane, high-rainfall environments, whereas the samples in Figure 3  Eocene fl oras from Patagonia are known for high diversity far exceeding the Paleocene Sala-manca fl oras (Berry, 1938;Wilf et al., 2005).
Although observed here at a much coarser temporal scale, this pattern is consistent with Paleocene-Eocene diversifi cation seen in Neotropical palynofl oras (Jaramillo, 2002;Jaramillo et al., 2006) and in Northern Hemisphere macrofl oras and palynofl oras (e.g., Wing and Harrington, 2001).The species composition of the Palacio de los Loros fl oras is dissimilar to that of the extremely diverse Laguna del Hunco fl ora, from early Eocene (51.9 Ma) tuffaceous lake beds in northwest Chubut Province (Wilf et al., 2005), although there is taxonomic overlap at the generic and familial level (Appendix DR3; Wilf et al., 2005).Therefore, a number of turnover events occurred between the Paleocene and early Eocene that remain to be documented; globally increasing temperatures during the early Eocene would have facilitated immigration and evolutionary diversifi cation that increased fl oral diversity in Patagonia, as seen in other areas of the Americas (Wilf, 2000;Wing and Harrington, 2001;Jaramillo, 2002;Jaramillo et al., 2006).
The elevated richness of Patagonian Paleogene fl oras adds a new dimension to the history of South American biodiversity.Shortly after the end-Cretaceous event, Patagonia already harbored considerably richer Paleocene plant communities than comparable environments in the Western Interior of North America.This high baseline presumably anchored the evolution of the fl oral diversity seen 10 m.y.later at Laguna del Hunco, apparently mirrored in diverse Eocene Neotropical palynofl oras (Jaramillo et al., 2006).The cause of the Paleocene diversity pattern is not known but may be related to greater distance from the Chicxulub impact, differences in Cretaceous plant diversity, and/or higher immigration or speciation rates.

ACKNOWLEDGMENTS
For generous support, we thank the National Science Foundation (grant DEB-0345750).We also thank the Agencia Nacional de Promoción Científi ca y Técnológica (Project PICT 07-08671); Consejo Nacional de Investigaciones Científi cas y Técnicas; B. Huber for biostratigraphic assistance; M. Caffa, L. Canessa, B. Cariglino, I. Escapa, C. González, R. Horwitt, P. Puerta, and E. Ruigomez for exceptional assistance in the fi eld and laboratory; Secretaría de Cultura de la Provincia de Chubut for permits; and the landowners at Palacio de los Loros, Hugo Visser, Edna de Galáz, and Clara Salazar, for access.This work is part of the Ph.D. thesis of Iglesias at Facultad de Ciencias Naturales y Museo, Universidad Nacional de La Plata.
begins with 7 m of sandstones, fining upward, that represent transgressive marine sedimentation.The section continues with large cycles of regularly sorted, medium to coarse yellow sands with sigmoidal cross beds, interpreted as deltaic, tidally influenced sedimentation (Martinez, 1992;Matheos et al., 2001); these beds preserve large, petrified, transported tree trunks.The uppermost Salamanca Formation contains mudstones and poorly sorted, fine grey sandstones on top of an erosive base, with infilled concave lenses and large lateral accretion sets.These rocks represent a meandering fluvial environment of low energy, with high sinuosity channels.The Palacio de los Loros section is topped by a massive, black, tabular 1.6 m mudstone bed with pedogenic features, root marks, slickensides and high bioturbation.This unit is named Banco Negro Inferior ("Lower Black Bank"; Feruglio,1949), and in the eastern portion of the basin it contains significant mammal fossils representing the Peligran South American land-mammal age (Bonaparte et al., 1993;Flynn and 1995) *Morpotypes that appear to be herbaceous are conventionally excluded (as for one morphotype from PL not analyzed in this paper) from paleoclimate analyses using leaf physiognomy and from rarefaction analysis of diversity (e.g., Wilf, 2000;Barclay et al., 2003). DR2007233 Figure 1.Regional map of the San Jorge Basin and modern location of the fossiliferous Palacio de los Loros outcrops (arrow) in the west-central basin (see also Appendix DR1 [see text footnote 1]).Inset: 62 Ma positions (using Ocean Drilling Stratigraphic Network [2004] online plate reconstruction service, with modern coastlines); box shows study area.
come from basin centers or other low-relief settings.The foliar physiognomy of the Castle Rock fl ora indicates higher temperatures by 8 °C and twice the annual rainfall of the Palacio de los Loros fl oras, indicating benign conditions associated with very high biodiversity today.

Figure
Figure 3. Rarefi ed richness of dicot leaf species, with selected 95% confi dence intervals (Tipper, 1979) and paleolatitudes (using Ocean Drilling Stratigraphic Network [2004] online plate reconstruction service) at Palacio de los Loros (localities PL1 and PL2; Appendix DR3 [see text footnote 1]) and representative and identically collected Paleocene fl oras from single localities in the Western Interior of North America.The West Bijou Creek sample is from Denver Museum of Nature and Science loc.2379, in the D1 unit (ca.65.5 Ma), eastern Denver Basin (Barclay et al., 2003).The 64.4 Ma Mexican Hat sample is from Smithsonian National Museum of Natural History (USNM) loc.42090, from the Lebo Member of the Fort Union Formation, Powder River Basin, southeastern Montana (Wilf et al., 2006).The Polecat Bench sample is the 57.5 Ma Lur'd Leaves site (USNM loc.42042; Wilf et al., 2006) from the Fort Union Formation, Bighorn Basin, Wyoming.The ca. 56.5 Ma Washakie Basin, Wyoming, sample is USNM loc.41270(Wilf, 2000) from the uppermost Fort Union Formation.For West Bijou Creek and the Washakie Basin, which each had multiple published localities, we selected the most diverse localities for this analysis.Mexican Hat, MT 49.4°N Blade trilobate or ovate; primary veins basal acrodromous; secondary veins regulary spaced, numerous, craspedodromous, straight, not ramified; agrophic veins present; petiole long; teeth small, compound, with glandular apices.Size class using the Raunkiaer-Webb system of leaf-area classification(Webb, 1959).Webb, L. J., 1959, A physiognomic classification of Australian rain forests: Journal of Ecology, v. 47, p. 551-570.Data Repository DR3Relative abundance of dicot leaf morphotypes * for rarefaction analysis (Figure3).Palacio de los Loros, Chubut, Argentina *MPEF-Pb, Museo Paleontológico Egidio Feruglio Paleobotany Collection, Trelew, Chubut, Argentina.# DR2007233 Iglesias et al, DR3, p. 1