The Stage for Neotropical Fish Diversification: A

Phylogeny and Classification of Neotropical Fishes. Part 1 - Fossils and Geological Evidence

Reprinted from: Phylogeny and Classification of Neotropical Fishes. 1998. Malabarba, L.R., R.E. Reis, R.P. Vari, Z.M. Lucena & C.A.S. Lucena, (eds). Porto Alegre, Edipucrs, 603p.

The Stage for Neotropical Fish Diversification: A History of Tropical South American Rivers

John G. Lundberg, Larry G. Marshall, Javier Guerrero, Brian Horton, Maria Claudia S. L. Malabarha, and Frank W esselingh Abstract. Today, 93 percent of freshwater drainage off South America runs into the Atlantic. South America's drainage pattern was shaped by its persistent Guyana and Brazilian continental shields, the emerging Andes along its western and northern margins, the fluctuating foreland basin east of the Andes, and several structural arches. South America's plate tectonic setting was established in the Early Cretaceous (Aptian, -118 Ma) with its separation from Africa and opening of the South Atlantic. The continent has long been in a state of west-east compression from which the Andes is one major result. The -90 Myr history of the Andes includes several phases of tectonic uplift that affected large segments of the western and northern continental margin as well as many local events of uplift. Crustal shortening and thickening uplifted the mountains progressively from west to east. Concomitant tectonic loading in the mountains and subsidence (enhanced by sediment loading) eastwardly adjacent and parallel to the thrust front created the foreland basin. When the foreland basin was underfilled with sediment, its axial groove served to guide major rivers northward and southward, to hold large lakes, and to receive several marine transgressions of varying extent from the Caribbean and South Atlantic. The Parana drainage system had an early history of growth northward by watershed capture of a paleo-Amazonas-Orinoco that previously had its headwaters in Chile and Argentina. The modern divide between the Parana and Amazonas systems was established -30 Ma with initiation of a tectonic episode and a major period of bending of the Bolivian orocline. Prior to late Miocene, the foreland basin drained the vast region of western Amazonia, western Orinoco and Magdalena northward into the Caribbean. The Magdalena drainage system was born at - I 0 Ma witll final uplift of the Eastern Cordillera. Continued uplift of the Merida Andes and Eastern Cordillera of Colombia, from -8.5-8 Ma, resulted in closure of the Caribbean portal of the paleo-Amazonas-Orinoco, and by -8 Ma the present day pattern of west-to-east drainage of the Amazonas and Orinoco was established. Neotropical fish diversity has a deep history with some higher endemic clades extending back into the Cretaceous. Some modern, generic-level clades were differentiated by the Paleogene, and the fish fauna was essentially modern by late Miocene. Models for the evolutionary diversification and biogeography of the Neotropical aquatic biota that emphasize single or Late Cenozoic phenomena are incomplete. The long and complex history of South America's landscape and river systems must be considered. Andean uplift provided new upland aquatic habitats, while foreland basin subsidence at times allowed the development of extensive lacustrine habitats. The formation of Andean and other drainage divides, (JGL) Department of Ecology and Evolutionary Biology, The University of Arizona, Tucson, AZ, 85721, USA. [email protected]. (LGM) 708 W Roller Coaster Road, Tucson, AZ, 85704, USA. [email protected]. (JG) Departamento de Geociencias, Universidad Nacional. A. A. 14490, Bogota, [email protected]. (BH) Department of Geosciences, The University of Arizona, Tucson, AZ, 85721, USA. [email protected]. (MCM) Laborat6rio de Paleontologia, Museu de Ciencias e Tecnologia - PUCRS, Av. Ipiranga, 6681, 90619-900 Porto Alegre, RS, Brazil. [email protected]. (FW) Nationaal Natuurhistorisch Museum, PO Box 95 I 7, 2300 RA Leiden, The Netherlands, and Biology Faculty, University of Turku, Turku, [email protected]

Lundberg et al. - The Stage for Neotropical Fish Diversification: A History of Tropical South American Rivers

shifting courses of rivers, and repeated incursions and regressions of marine waters must have produced many vicariance events. The late Miocene assembly of the modern, west-east flowing Amazonas and Orinoco must have been major events of biotic merging and enrichment. The emergence of new lands in northern South America and lower Central America provided opportunities for extension of the Neotropical aquatic biota. Isolation of peripheral drainage systems south, west and north of the Parana, Amazonas, and Orinoco provided opportunity for allopatric divergence, and also was accompanied by much extirpation of once more widespread tropical fish species.

Resumo. Atualmente, 93% dos sistemas tluviais da America do Sul drenam para o Atliintico. 0 padrao da drenagem sulamericana foi moldado pelos seus persistentes escudos continentais guiano e brasileiro, pelos Andes emergentes ao longo das margens continentais oeste e norte, por uma bacia de antepafs a leste dos Andes e varios arcos estruturais. 0 contexto da tect6nica de placa da America do Sul foi estabelecido no infcio do Cretaceo (Aptiano, -112 Ma) com sua separac;:ao da Africa e a ahertura do Atliintico sul. 0 continente tern permanecido num estado de compressao oeste-leste do qua! os Andes sao o principal resultado. A hist6ria de 90 milhoes de anos dos Andes inclui multiplas fases de elevac;:ao tectonica que afctaram grandes segmentos das margens continental oeste e norte, bem como muitos eventos locais de elevac;:ao. Encurtamento e espessamento crustal elevaram as montanhas progressivamente de oeste para o leste. A concomitante carga tectonica nas montanhas e subsidencia (aumentada pela carga sedimentar) para o leste, adjacente e paralelos a pressao frontal, originou a bacia de antepafs. Quando a bacia de antepafs ainda nao estava totalmente preenchida de sedimentos. sua calha axial serviu para guiar os maiores rios para o none e sul, manter grandes lagos e receber varias transgressoes marinhas de variada extensao vindas do Caribe e/ou do Atlantico sul. 0 sistema do rio Parana apresentou uma hist6ria inicial de crescimento para o norte pela captura da bacia de um proto-Amazonas-Orinoco que previamente teve suas cabeceiras no Chile e Argentina. 0 divisor moderno entre as drenagens do Param1 e Amazonas foi estabelecido ha -30 Ma, com o infcio de um epis6dio tectonico e um perfodo de inclinac;:ao da oroclinal boliviana. Antes do Mioceno tardio, a bacia de antepafs drenou a vasta regiao da Amazonia ocidental, Orinoco ocidental e norte do Magdalena para o Caribe. 0 sistema do rio Magdalena formou-se ha -10 Ma com a elevac;:ao final da Cordilheira Oriental. A elevac;:ao continuada da Cordilheira de Merida e Cordilheira Oriental da Co16mbia, de -8.5 a 8 Ma, resultou no fechamento do portal caribenho do paleo-Amazonas-Orinoco oriental, e ha -8 Ma, o padrao atual de oeste para leste das drenagens dos rios Amazonas e Orinoco foi estabelecida A diversidade de peixes neotropicais tern uma longa hist6ria temporal que para alguns clados endemicos mais superiores se reporta ao Cretaceo. Alguns clados modernos a nfvel de genero, diferenciaram-se durante o Paleogeno, e a ictiofauna era essencialmente moderna no fim do Mioceno. Modelos de diversificac;:ao evolutiva e biogeografica da biota aquatica Neotropical que enfatizam fenomenos unicos OU do fim do Cenoz6ico sao incompletos. A longa e complexa hist6ria do cenario e sistema de rios da America do Sul devem ser considerados. A elcvac;:ao andina forneceu novos habitats aquaticos de planaltos, enquanto a subsidencia de bacias de antepafs permitiu por vezes o desenvolvimento de extensos habitats lacustres. A formac;:ao do divisor andino e de outras drenagens, mudanc;:as de cursos de rios, e repetidas incursoes e regress6es de aguas marinhas devem ter produzido muitos eventos vicariantes. A reuniao no fim do Mioceno do moderno fluxo oeste-leste do Amazonas e Orinoco deve ter sido um dos maiores eventos de combinac;:ao e enriquecimento bi6tico. A emergencia de novas terras no norte da America do Sul e sul da America Central proveu oportunidades para extensao da biota aqm1tica Neotropical. 0 isolamento de sistemas de drenagem perifericos do sul, oeste e norte do Parana, Amazonas e Orinoco deu oportunidades para divergencia alopatrica, e tambem foi acompanhado pela extinc;:ao local de mais de uma especie amplamente distribufda de peixes tropicais.

"A river is timeless - it will never have an end and it never had a well defined beginning. Different parts of a river system may have different ages" (Potter, 1997).

altered the contours of the land and, thereby, river courses and watershed limits. The tempo of drainage system evolution (-10 2 to 10 7 yr) is appropriately scaled to make it a powerful force in shaping the distribution and evolution of aquatic organisms. In the Neotropics, disruptions of drainage systems by formation of new watershed divides promoted vicariance, allopatric divergence and, thus, biotic diversification. The breaching of watershed divides and coalescence of drainage systems promoted merging and enrichment of biotas. The watering of emergent isthmian links and accreted terranes allowed extension of aquatic biotas. We synthesize geological and paleontological data bearing on the drainage evolution of South America. Our time frame is roughly the last 90 Myr (millions of years; a duration), that encompasses much of the age of diversification of modern Neotropical fishes (Lundberg, this volume). During this

"A river might better be thought of as having a heritage rather than an origin" (Leopold et al., 1964: 62).

Introduction Much of the diversification of the Neotropical freshwater fishes and other aquatic organisms took place in the dynamically changing rivers and watersheds of South America during Late Cretaceous and Cenozoic. These continental waters were at once the agents and the products of landscape evolution. Large-scale tectonic features and processes controlled and 14

Ph ylogeny and Classification of Neotropi cal Fishes. Part 1 - Fossil s and Geological Evidence

• Maracaibo rinoco Essequibo e=Casiquiare

San

Sao Francisco

Paraiba do Sul

Figure 1. The major river systems of South America. 15

Lundberg et al. - The Stage for Neotropical Fish Diversification: A History of Tropical South American Rivers (e.g. Eigenmann, 1909; Gery, 1969; Roberts, 1972). Future models for the evolutionary diversification of the Neotropical aquatic biota must consider multiple occurrences of vicariance plus drainage system coalescence and extension into new watersheds of emergent land. Our summary of the history of South American rivers uses a series of paleogeographic maps and associated text. To begin, we present base maps that lay out the major modern river systems and geomorphological features (Figs. 1-3). Figures 4-9 explain the general plate-tectonic and geodynamic processes and features that created and emerged from the Andes. Figure 10 gives two schema of South America at 105 and 90 Ma (megaanums or points in time), relatively early after its separation from Africa. Figures 11-20 present paleogeographic - paleodrainage reconstructions for a series of time periods between 73 Ma to Recent. We use M. J. Weitzman's map of modern South American rivers as our base map. It is useful to have the modern drainage pattern as a point of reference, but we do not intend, of course, to suggest that South America had its modern coastline throughout its history. Our paleogeographic maps emphasize the geology and drainage system development for tropical South America (north of ca. 35° S). Paleogeographic maps and discussions of drainage development for Late Cretaceous and Cenozoic basins south of 35°S are provided by Uliana & Biddle ( 1988), Marshall & Sempere (1993), and Legarreta & Uliana (1994). We follow the geologic time scale of Cande & Kent (1992, 1995) for Late Cretaceous and Cenozoic, and Haq et al. ( 1987) for Early and Middle Cretaceous.

period South America underwent three long term developments: 1) increasing isolation from Africa with continued opening of the Atlantic Ocean, 2) the complex history of Andean mountain building along its western and northern margin, and 3) the formation of its isthmian link to Central America that simultaneously separated the Pacific and Atlantic oceans. Of these we are particularly concerned with the second: formation of the Andes and its effects on river systems. Andean history is especially important to the evolution of rivers and their fishes for two reasons. The first follows from the fact that many extant monophyletic groups are represented in rivers that now separately flow off Andean slopes to the Atlantic, Caribbean and Pacific. For example, Carl Eigenmann, concerned with the relationship between the Magdalena and Orinoco-Amazonian faunas, wrote in 1920 (p. 21 ): "It is quite out of the question to transport all of these genera over the present barrier formed by the Cordilleras of Bogota." Eigenmann viewed the fish groups shared by these now isolated drainages as older than the high mountainous divides between them. Subsequent workers, armed with more precise phylogenetic hypotheses for more widely-ranging clades in northwestern South America, support the position that the uplift of the Andes subdivided ancestral biotic ranges (e.g. Vari & Weitzman, 1990; Retzer, 1994; Reis, this volume). Paleontological evidence directly substantiates this by dating several fish taxa as being older than the formation of the divide (Lundberg et al. 1986, 1988; Lundberg, 1997). The Andean vicariance view must equally apply to more austral groups, such as the Argentinian-Chilean diplomystid catfishes (Arratia, 1987; Azpelicueta, 1994 ), and cheirodontine characins (L. Malabarba, this volume). The development of high mountainous relief produced new hill-stream habitats that were successfully invaded by specialized clades such as astroblepid catfishes and parodontid characins. Also, we can envision how formation of the Bolivian Andes uplifted and isolated an originally lowland ancestral group of cyprinodontiforms that radiated into the extant Orestias of Lago Titicaca: a vertical "Noah's Ark" biogeographic event (sensu McKenna, 1973). Second, the Andes are important because, together with their foreland basin, their development has provided the most proximate and active controlling forces in the latest -90 Myr development of South American drainage pattern. Throughout their histories, rivers conformed to changing mountain slopes and foreland basin geometry. River courses were further influenced by stable continental features (cratonic shields and structural arches), fluctuating sea level and changing climate. Sediment eroded from the Andes provides both pattern and process in the development of the mountains, rivers, lakes and occasionally epicontinental seas. Andean tectonic history is extremely important for understanding biogeographic process and pattern. It is now known that the Andes were built by compressional tectonics during the last -90 Myr or even longer. It is, therefore, overly simplistic to view Andean vicariance as a singular event occurring with the Miocene uplift

South America's major tropical river systems River names and basin areas are taken from Zeisler & Ardizzone (1979). The Amazonas system (Fig. 1, green shading), including rio Tocantins, dominates drainage of the continent with an overall area of 7 ,050,000 km 2 that includes most of northern and central Brazil, and parts of Venezuela, Colombia, Ecuador, Peru, and Bolivia. The mouth of the Amazonas is in northeastern Brazil just south of the equator. Its discharge averages about 175,000 m 3/sec (Sioli, 1975), roughly 20% of freshwater input to the world's oceans. Major south bank tributaries all flowing off, or mostly off, the Brazilian Shield are the Tocantins-Araguaia, Xingu and Tapaj6s. The Madeira also receives drainage off the west and northwest part of the Brazilian Shield, but most of Madeira's waters come from the Peruvian and Bolivian Andes. Other main Amazonian affluents with Andean or near-Andean origins are the Purus, Jurua, Ucayali, Huallaga, Marafion, Pastaza, Napo, I~a (Brazil) or Putumayo (Colombia), Japura (Brazil) or Caqueta (Colombia), and the Vaupes plus Guanfa of Colombia that run into the upper rio Negro. The Negro and Branco, a major northern branch, drain much of the southwestern Guyana Shield. The rio Casiquiare (Fig. I, blue spot) is a 200 mile long distributary of the Orinoco in southern Venezuela that drains into the upper rio Negro, thus connecting the Arna16

Phylogeny and Classification of Neotropical Fishes. Part I - Fossils and Geological Evidence poque) drain the eastern margin of the Guyana Shield. South of the Amazonas the drier northeast coastal region of Brazil is drained by several small streams and the large rio Parnafba that runs off the Brazilian Shield. On the south side of the Atlantic bulge of Brazil is the mouth of the rio Sao Francisco. The Sao Francisco system has a drainage basin of about 4,500 km 2 situated west of the N-S trending highlands (Chapada da Diamantina and Serra do Espinhac,;o) near the eastern margin of the Brazilian Shield. South of the Sao Francisco mouth to the rfo de la Plata numerous rivers with relatively small catchment areas drain the east margin of the Brazilian Shield. The streams are especially short and steep along the southeastern Brazilian Serra do Mar. The Parana system (Fig. 1, purple shading), second largest on the continent, drains about 4,000,000 km 2 of Argentina, Bolivia, Brazil, Paraguay, and Uruguay. Its lowermost reach and mouth, rfo de la Plata, opens to the South Atlantic at about 35° S. Much of the area covered by the upper Parana and Paraguay is in the tropics and subtropics. The Paraguay is the main stream draining the Gran Chaco (Chaco Plain) and the Pantanal situated between the Andes and Brazilian Shield. Its two largest tributaries, Pilcomayo and Bermejo, head in the Central Andes. The Parana mainstcm drains a very large area of the southern Brazilian Shield. Along its lower course, the left bank of the Parana receives the Uruguay flowing off the southern tip of the shield, and entering on the right is the Salado that heads in the Andes and courses across the northern Pampas.

zonas and Orinoco systems. Other north bank Amazonas tributaries draining the southern Guyana Shield are the Uatuma, Trombetas, Paru, and Jari. Southwest of the Amazonas- system, isolated from the headwaters of the Madeira, and perched 3,810 m high on the Altiplano of Bolivia and Peru is the Lago Titicaca system. The main lake, a few smaller ones and their connecting rivers 2 form an endorheic basin, a little over 10,000 km in area. This region was occupied by many ancient lakes some of which in the Pleistocene were much larger than present day Lake Titicaca (Marshall & Scmpere, 1991 ). North of the Amazon, the Orinoco system (Fig. 1, yellow shading) of Venezuela and Colombia has a basin area of 830,000 km 2 . The mouths of the Orinoco emerge from its 20,000 km 2 delta in the Atlantic south of the islands of Trinidad and Tobago. Mainstream headwaters of rfo Orinoco are in southwestern Venezuela on the Guyana Shield and its major south bank tributaries that also flow off the shield are Caroni, Caura, and Ventuari. The larger Orinoco tributaries heading in the Eastern Andes (Eastern Cordillera and Merida Andes) are, from south to north: Guaviare, Vichada, Torno, Meta, Cinaruco, Capanaparo, Arauca, and Apure. The Pao and Guarico are two of larger north bank affluents draining the southern flanks of the V cnezuelan Coastal Cordillera. Several short rivers flow off the north faces of the Coastal Cordillera to the Caribbean. West of these, the Lago Maracaibo system includes the large lake (salinities of 2-7%) in Venezuela and short rivers flowing into it from the north side of the Merida Andes (Venezuela) and the east side of the Pcrija Andes (Venezuela and Colombia). The total drainage area of the Maracaibo system is -90,000 km 2 . The rfo Magdalena system (Fig. 1, light blue shading), including the Magdalena mainstem and the Cauca, drain 321,800 km 2 of central Colombia northward into the Caribbean Sea. The Magdalena is completely divided from the Orinoco and Amazonas by the high Eastern Cordillera of Colombia. The Magdalena formed its valley between the Eastern and Central Cordilleras. The Cauca flows in the valley between the Central and Western Cordilleras. Between the Western Cordillera of the Andes and Coastal Ranges, in the northwest corner of South America, the rfo Atrato flows north to the Caribbean from its low divide with the rfo San Juan that runs southwest to the Pacific. In easternmost Panama the rfo Tuyra, also flowing into the Pacific, is narrowly separated from the Atrato by the Scrrania de! Darien. South of the San Juan, short rivers and streams drain the westernmost Andes and narrow coastal plain. The largest of the rivers arc the Patia in Colombia and to the south the Guallabamba-Esmeraldas in Ecuador. Still farther along the increasingly arid west coast of Ecuador, Peru, and Chile freshwater streams are scarcely developed. Returning to the main rivers flowing to the Atlantic, along the coast of the Guianas the largest system is the Essequibo of Guyana and southeastcrnmost Venezuela. The Essequibo and others of the region (Corantijn, Suriname, Maroni, and Oia-

South America's continental shields, massifs, mountains and arches Shields and Massifs. The core of the continental South American Plate is formed by two immense areas of ancient Precambrian crystalline basement: the Guyana and Brazilian shields (Fig. 2). A cratonic shield is part of Earth's continental crust that is relatively stable and little deformed for a long time (10 8-10 9 yr). The Guyana Shield provides much of the surface of the Guianas, southern V cnezuela, southeastern Colombia, and Brazil north of the Amazon. The westernmost parts of this shield in central Colombia and northwestern Brazil are buried under mostly Cenozoic sediments deposited in a progressively eastward-migrating forcland basin. In the east, the Guyana Shield has been mostly emergent since Middle Precambrian, although during Triassic it was partly submergent and non-marine beds were deposited. The rio Negro flows diagonally across the southwestern corner of this shield and the Orinoco mainstem is guided by its northern edge. But most modern, and probably ancient, drainage from the shield has a radial pattern of large rivers flowing outward to the Atlantic, the lower Amazonas and the Orinoco (Fig. I). Bcmerguy & Sena Costa ( 1991) show how the courses of several Amazon tributaries on the shields might be controlled by directionally-oriented sets of faults and arches. 17

Lundberg et al. The Stage fo r Neotro pi cal Fi sh Di versifi cation: A History of T ro pical South Ameri can Ri vers

Arauca va U p e s r -

El Baul YA.JI.

Macarena Massif

Caravari Purus Monte Alegre

Iquitos Maranan Contaya

Serra do Fitzcaraldo

Sierras Pa"l)eanas Massif

Nordpatagon ico Massif

Shields & Massifs Arches

Deseado Massif

Andes and Magmatic Arcs

Figure 2. The maj or geologic and topographic features of South America: Continental shields, massifs, mountains and arches. 18

Phylogeny and Classification of Neotropical Fishes. Part 1 - Fossils and Geological Evidence

arinas-Apure East Venezuelan-Maturin Solimoes Middle Amazonas Lower Amazonas /Marajo

Mara non Ucayali

Madre de Dies - Beni

Jorge

Figure 3. The major geologic and topographic features of South America: Neotropical Neogene sedimentary basins. 19

Lundberg et al. - The Stage for Neotropical Fish Diversification: A History of Tropical South American Rivers Ecuador and Colombia were reached, as well as the further uplift of the Merida Andes in Venezuela (Wijminga, 1996). These compressional phases were approximately coeval with episodes of rapid convergence between the Nazca (oceanic) and South American (continental) plates (Pilger, 1984; PardoCasas & Molnar, 1987). The western margin of South America has been greatly modified by mountain building processes since -90 Ma (or earlier). Because these processes involve crustal compressional shortening and uplift, some parts of the margin have been decreased significantly in areal extent (Lamb & Hoke. 1997). The amount and style of shortening in the Andes has been largely influenced by the location and geometry of preexisting geologic features, including Paleozoic sedimentary wedges (e.g. the thin-skinned thrust belt of Bolivia), Precambrian crystalline basement (e.g. Sierras Pampeanas of northern Argentina), Mesozoic extensional basins (e.g. Santa Barbara thrust system of northern Argentina), and Mesozoic-Cenozoic accreted terranes (e.g. Western Cordilleras of Colombia and Ecuador). Reconstructions of the original geometry of the western margin (Schmitz, 1994; Pindell & Tab butt, 1995) indicate considerable horizontal shortening of continental crust from Late Cretaceous to Recent: 20-35 km in Ecuador and southern Patagonia, 250-300 km in parts of the Central Andes. In addition, pre-Neogene reconstructions indicate a total shortening in the northernmost Eastern Cordillera of Colombia in excess of 150 km, with 110 km of dextral shear and 25 km of shortening in the Merida Andes (Pindell & Tabbutt, 1995). Thus the amount of shortening is not uniform along the Andes. Structural Arches. Between the Andes and shields. structural arches occur that bound intracratonic basins and segments of the foreland basin. The term structural arch is used in a broad sense to include subsurface and low topographic features of various origin that are potential and real barriers of drainage systems. The exact location of some of the arches remains controversial due to the limited availability of subsurface data. The data suggest that some of the arches were formed and/or maintained by intraplatc compressional stresses. These stresses largely reactivated structural features that had originally developed in response to earlier plate movements and attendant stresses. The Iquitos, Serra do Moa and Serra do Divisor arches may be the results of lithospheric flexure due to tectonic loading of the Andes, although the last two may be considered the easternmost propagation of Andean thrust sheets. The stratigraphy of deposits in intervening sub-basins (Fig. 3) indicates that the Jutaf and Purus arches have been areas of positive relief since the Paleozoic, and the Iquitos and Gurupa arches first evolved during the Mesozoic. Data on some of these arches indicate that they are still being uplifted. For example, Iquitos arch has been uplifted since the beginning of the Quaternary, resulting in terraces 30 m above present river floodplains (Mertes et al., 1996). Other high (possibly to 100 m) fluvial terraces not bound to any arch are present all over Western Amazonia in the zone east of the foreland basins. Some arches (e.g. Yau-

The Brazilian Shield (Amazon Shield) provides the surface of most of central and southeast Brazil. Harrington ( 1962) distinguishes a Central Brazilian Shield and a Coastal Brazilian Shield separated by the NE-SW trending Parnafba, Sao Francisco and Parana basins. These ancient sedimentary basins have been largely non-marine since Triassic. As with the Guyana Shield, the westernmost margins of the Brazilian Shield are covered by mostly Cenozoic deposits, thrust and eroded fmm the emergent Andes to the west. The covered western edge of the Brazilian Shield in central Bolivia extends to a point below the eastern edge of the Altiplano (Lamb & Hoke, 1997, fig. 4). The modern rivers flowing off the shield are largely arrayed in a radial pattern. Between the Guyana and Brazilian shields is the Amazonas trough, a 6000 m sag in the crust that leads into a graben in the eastern half of the basin. This structural sag may have first developed in response to Paleozoic rifting and later was reactivated in the Triassic with the initial separation of South America and Africa. Although the Amazonas trough has been present since at least the Paleozoic, significant eastward flow of water commenced only in the late Miocene, -8 Ma (Curry et al., 1995; Mertes et al., 1996; see below). There are some smaller masses or massifs of emergent, Precambrian crystalline basement rocks that resist deformation. Massifs may be protruding bodies of basement rocks, consolidated during earlier orogenies, or younger plutonic bodies. In South America these include the Sierra Macarena in Colombia, the Sierras Pampeanas in northwestern Argentina, and in southern Argentina the Nordpatagonico (Somuncura) massif between the Colorado and San Jorge basins, and Deseado massif between the San Jorge and Magallanes basins. The Sierra Macarena is the significant northernmost of these massifs and rocks north of it have been bent eastward by movement of the Caribbean Plate, while the Deseado massif is the southernmost massif to the south of which the Andes have been bent by eastward movement of the Scotia Plate (Cobbold et al., 1996). Mountains. Red-shaded areas in Fig. 2 show the aerial extent of modern mountain ranges including the main ranges of the Andes and the coastal ranges of Venezuela, northwest Colombia and eastern Panama. The mountainous regions on the Guyana and Brazilian Shields are not labeled. Accretion of marine terranes on the northwestern margin of the continent (Ecuador to Venezuela) and uplift of the entire Andes from Trinidad to Patagonia were controlled by the direction and rate of convergence of the oceanic Nazca Plate (Coney & Evenchick, 1994 ). Compression related to plate convergence produced shortening and hence uplift of the western and northern edge of the South American Plate. Tectonic phases along much of the Andes (i.e. coeval along large segments of western margin where the Nazca Plate was being subducted) were initiated in the Central Andes at -89, - 73, -59, -43, -30 and -11 Ma (Marshall & Sempere, 1993; Sempere et al., 1997). For the entire northern Andes, possibly the biggest uplift took place between 5 and 3 Ma. During this time the high present elevations of the Eastern Cordilleras of

20

Phylogeny and Classification of Neotropical Fishes. Part I - Fossils and Geological Evidence

pes, El Baul, and Michicola) are simply the westernmost, largely subsurface extensions of shields now covered by foreland sediment largely derived from the Andes. Data for the structural arches were compiled principally from Bemerguy & Sena Costa (1991), Bigarella (1973), Mertes et al. (1996), Rasanen et al. (1987, 1990, 1992, 1995), and Hoorn (1993, 1994a,b,c ).

magmatic arc

intermontane

foreland

South America's Neotropical, neogene sedimentary basins Figure 4. West (left) to east (right) schematic section of the Andes showing geological features associated with mountain building and forcland basin development (modified after Fig. 1 in Marocco et al., 1995 and Fig. 2 in Horton & DeCcllcs, 1997).

The basins located along the Amazonas mainstream between the Guyana and Brazilian shields (i.e. Amazon, Solimoes, Middle Amazon, Lower Amazon, and Maraj6) arc intracratonic basins, bounded by shields and structural arches (Fig. 3). The Magdalena Valley (Colombia), Interandean Valley (Ecuador), and Altiplano (Peru, Bolivia) basins are intermontane basins situated between major Andean cordilleras. Each of these basins contain smaller sub-basins that formed by a variety of extensional, compressional, and strike-slip processes. The foreland basin parallel to, and east of, the N-S axis of the Andes is the result of regional flexural subsidence of the craton due to the weight of the tectonic load (thickened continental crust) of the Andes (Jordan, 1995; Horton & DeCelles, 1997). Segments of the foreland basin correspond to modern drainage basins.

Western

intermontane basin

Eastern cordillern

30 - 20 Ma

S.L.-

-S.L.

11

Ma

- S.L.

foreland basin

Figure 5. Eastward growth of the orogenic belt, foreland basin and forebulge. Modified after Fig. 7 in Jordan & Alonso (1987). S.L., sea level.

Andean mountain building, foreland basin evolution, and plate tectonics

underfilled of

Some fundamental features and processes of Andean mountain tectonics, foreland development and tectonics arc summarized in Figs. 4-9. The cross section through the Andes (Fig. 4) shows the position of the forearc basin within the arctrench gap west of the mountains. East of the Andes is the foreland. The forebulge is an upward crustal flexure on the eastern flank of the foreland basin produced in response to loading by the Andean orogenic belt. Note that foreland basin deposits can be incorporated into the orogenic belt and become parts of intermontane basins through eastward thrust propagation as occurred with the Magdalena Valley, lntermontane Valley, and Altiplano basin. Thus, with each thrust propagation, western watersheds flowing eastward off the Andes were potentially captured and incorporated into intermontanc basins that may have ultimately developed endorheic or possibly westward drainages. These were potential vicariance events for fresh water biotas. Three sequential cross sections (Fig. 5) of the Central Andes at -22° S show the eastward propagation of the orogenic belt and migration of the foreland basin and forebulge through time (see Figs. 14, 16 and 18 for paleogeographic reconstructions of these time periods).

'Oe\\

overfilled axial groove of foreland basin

Figure 6. The geometry and distribution of sediment in underfilled and overfilled foreland basins. Modified after Fig. 9.6 in Jordan (1995). Arrows indicate flow direction. 21

Lundberg et al. - The Stage for Neotropical Fish Diversification: A History of Tropical South American Rivers 120°

60°

I

I

Age (Ma)

Tectonic

Epoch/Period

Events

0 Pliocene major deformation in external Eastern Cordillera

10 Miocene

Equator - -

20

--Equator

major deformation in Eastern Cordillera

30

Oligocene relative tectonic quiescence

40 Eocene

so

I•

incipient deformation in western parts of foreland basin; initiation of orocline bending

••

• foreland basin 1n Altiplano and Eastern Cordillera initiaton of normal convergence;

60

Paleocene

ma·or tectonism alon

mar in

relative tectonic quiescence

shortening along margin and resumption of foreland subsidence

70 Late Cretaceous

80

Figure 7. The present relationship of the South American, Caribbean, Cocos, Nazca and Scotia plates. Based on map the "Earth's Dynamic Crust" by the National Geographic Society, August 1985.

12

ow

1

oow

8

relative tectonic quiescence shortening along margin and initiation of foreland basin

90 tectonic quiescence

100

Figure 9. Compressional tectonic events in the Central Andes from -90 Ma to Recent; black bars indicate compressional onset and duration. Modified after Sempere et al. ( 1997, fig. 12).

ow

I

expression and is a zone of sediment accumulation. Drainage in overfilled basins is typically transverse (perpendicular to the thrust front) as today in the Amazon basin (Fig. I). Different segments of the Andean foreland basin were underfilled and overfilled during different stages of tectonic evolution (see Fig. 9). The present relationship of the South American, Caribbean, Cocos, Nazca, and Scotia plates is shown in Fig. 7 The Cocos and Nazca plates were part of the larger Farallon Plate until their separation at -30 Ma (Pardo-Casas & Molnar, 1987). Relative motions of South American and Nazca plates since 68 Ma are plotted in Fig. 8. The positions of two points on the Nazca Plate, which formed at the time of anomaly 3031, are plotted with respect to South America at the times of various magnetic anomalies. Figure 9 summarizes the compressional tectonic phases in the Central Andes from Late Cretaceous to Recent (Sempcre et al., 1997, fig. 12). In this view of Andean tectonics, the times of initiation of compression are at -89, - 73, -59. -43, -30, -11. The plate tectonic setting of South America has not changed fundamentally since the opening of the South Atlantic in the Early Cretaceous (Albian). Since that time, the continent has been in a state of more or less E-W compression, attributed to ridge push from the Atlantic and East Pacific rise and by slab pull of the western Andean margin (Cobbold et al., 1996).

American Plate 205-

-·20S

40S-

-40S

(

Figure 8. Relative motions of South American and Nazca plates since 68 Ma. After Pardo-Casas & Molnar (1987, fig. 3). Ellipses represent uncertainties in the reconstructions. Figure 6 compares the geometry and distribution of sediment in underfilled and overfilled foreland basins. The underfilled basin forms a valley (basin axis adjacent and parallel to the thrust front) and receives sediment from both the thrust belt and forebulge. Basin subsidence permitted the accumulation of freshwater bodies including wetlands, lakes, and at times the ingression of marine waters when the basin surface subsided below relative sea level. Axial drainage (parallel to the thrust front) typically characterizes underfilled basins. In the overfilled basin, the forebulge has no topographic

22

Phylogeny and Classification of Neotropical Fishes. Part I - Fossils and Geological Evidence A.

Late

Aptian-Albian

(-112-105

& Tabbutt, 1995). The west coast of southern South America north to Ecuador was a magmatic Andean arc with emergent volcanic islands surrounded by marine seaways. During the Turonian (-90 Ma) rifting between South America and Africa had completed opening of the Atlantic (Fig. JOB). In the Central Andes compressional mountain building is evidenced by foreland basin deposits in the Central Andean domain (Sempere et al., 1997). Due to uplift of the western continental margin (the early Andes), paleoslopes and drainage directions (arrows) in the Magallanes and Neuqucn basins of southern South America changed from westward to eastward and into the Atlantic (Potter, 1997; Coney & Evenchick, 1994). Even earlier, since the earliest Cretaceous, the west border of northern South America in Colombia was a magmatic arc the early Central Cordillera, with a back-arc basin to the east and fore-arc basin on the west. Paleocurrent directions to the west and northwest (Rodriguez & Rojas, 1985) indicate sedimentation into the fore-arc basin on the Pacific side. Sedimentary rocks of BerriasianNalanginian (145-135 Ma) to Albian ( 112 Ma) age located today on the western flank of the Central Cordillera (Rodriguez & Rojas, 1985; Gomez et al., 1995) indicate a continental, Central Cordilleran volcanic arc complex with metamorphic/plutonic and active volcanic sources. By Albian time the back-arc basin was fully developed. Fluviatile deltaic and coastal sediments of BerriasianValanginian age have been documented on both sides (cast and west) of the back-arc basin. From the volcanic arc, early drainage of Hauterivian (-132 Ma) and older age, to the cast into the back-arc basin is also suggested by composition of several units (e.g. Murca Formation; Moreno, 1991 ). These include lithic arenites derived from metamorphic and plutonic-volcanic sources located on the magmatic arc. The deposits on the eastern side of the basin arc mostly mature quartz arenites derived, during Cenomanian (-97-94 Ma) and older Cretaceous, from a more stable source area on the Guyana Shield (Fabre, 1985; Guerrero & Sarmiento, 1996). All of these Early Cretaceous deposits onlap continental crust on both sides of a northerly elongated seaway. Thinning of the continental crust was accompanied by high angle normal faulting and subsidence. The dcpocentcr of the basin had an axis parallel to the magmatic arc, and was located along the present Eastern Cordillera and Magdalena Valley. Accordingly, drainage was directed from the east and west into the axis which finally connected north to the proto-Caribbean. Contrary to what happened during the middle and early Late Cretaceous in the Central Andes, there is no clear indication in Colombia of a compressional tectonic event leading to forcland basin development. Guerrero & Sarmiento ( 1996) proposed continuous subsidence of the back-arc basin and correlated the marine transgression at the CenomanianTuronian (- 94 Ma) boundary with a global (eustatic) sea level rise (Haq et al., 1987). This transgression reached the easternmost side of the basin in today's Llanos area and established fully marine conditions over most of Colombia.

Ma)

B. Turonian (-90 Ma)

Figure 10. A. South America and Africa at about the time of final separation of the continents in late Aptian-Albian (112-105 Ma, after Map 16 in Smith et al., 1994). B. South America and Africa in the Turonian (- 90 Ma) following complete separation (after Map 14 in Smith et al., 1994).

Continental separation, convergent tectonism and birth of the Andes (Figure 10) Figure 1OA shows South America and Africa about the time of final separation of the continents. Marine invertebrate fossils indicate a continuous, shallow South Atlantic seaway between Africa and South America by late Aptian (-112 Ma). By middle to late Albian (-105 Ma) epi- and mesa-pelagic oceanic habitats were in place (Bengtson & Koutsoukos, 1991; Koutsoukos, 1992; Maisey, in press). At this time, before significant uplift on the western edge of South America, drainage from the continental shields was likely predominately westward into the Pacific Ocean (e.g. White et al. 1995; arrows in our Fig. IOA). The onset of convergent tcctonism and foreland basin development also may be Albian in age, following continental separation in the equatorial Atlantic region (Coney & Evenchick, 1994; Pindell 23

Lundberg et al. - The Stage for Neotropical Fish Diversification: A History of Tropical South American Rivers

the widening Atlantic (Irion et al., 1995). Thus, western and eastern Amazonia were in separate drainage systems. This situation continued until late Miocene (-8 Ma), when the west-to-east transcontinental drainage seen today was established. The Marafion Portal (Guayaquil Gap of Nuttall, 1990; Andean Portal of Potter, 1997) was proposed by Katzer (1903) to be the site where the "Sanozama" river emptied into the Pacific. This portal was interpreted to have existed from Late Cretaceous to middle Miocene and permitted the ingression of marine waters into western Amazonia (Harrington, 1962; Grabert, 1983). It was specifically cited to explain the presence of foraminifera in the late Eocene-early Oligocene Pozo Formation in eastern Ecuador and Peru (Williams, 1949; Tschopp, 1953, fig. 9). Although the Andes are indeed low in this area, there is no compelling geological data to demonstrate that there was a large, drainage connection with the Pacific since before the Late Cretaceous (-73 Ma; Gayet et al., 1993). The Marafion Portal has been suggested as the site where marine taxa (e.g. potamotrygonid stingrays, manatees, dolphins) entered epicontinental seas and gave rise to fresh water forms (e.g. Brooks et al., 1981; Doming, 1982). Recent phylogenetic studies of the stingrays challenge this view and point to a Caribbean (northern) relationship for this group (Lovejoy, 1996). There was also a large northward flowing river into the southernmost end of the forcland basin between the western edge of the Sierras Pampeanas massif (to the east) and the emerging Andes to the west (Gayet et al., 1993). Thus, the headwaters of this basin included the westernmost part of the present day Parana system. A limited marine transgression from the South Atlantic entered the lower Parana basin and onlapped the eastern edge of the Sierras Pampeanas massif. There is no evidence to indicate that the transgressions from the north and south into the Andean foreland/back-arc basins in the area of the Sierras Pampeanas were ever connected (Gayet et al., 1993). The tectonic event initiated at - 73 Ma (Fig. 9) coincides with the base of the El Molino Formation in southern Bolivia and thus represents the compressional mechanism for initiation of foreland conditions at this time (Sempere et al., l 997). The relative motion of the Nazca Plate relative to the South American Plate was obliquely convergent at - 73 Ma (Fig. 8). The sequence of marine, brackish-water and continental strata seen during this interval in the Central Andes is closely matched in coeval foreland and back-arc basin sequences in northern Peru, Ecuador and Colombia, thus suggesting a similar and nearly synchronous tectonic transition. For example, in eastern Ecuador, the Maastrichtian-Paleocene Tena Formation contains early Maastrichtian marine levels mixed with continental and brackish-water strata (Sempere et al., 1997). Furthermore, the Late Cretaceous-early Paleocene underfilled foreland basin/back-arc of the Andes (Figs. 11, 13) is reminiscent of the Western Interior Seaway of North America (Sempere et al., 1997).

Paleogeography and Drainage Campanian to middle Maastrichtian, 83-67 Ma (Figure 11) From 73-67 Ma, extending southward from the Caribbean, there were elongated marine to restricted-marine, back-arc (Colombia) and foreland (further south) basins as far as northwestern Argentina, where the foreland basin onlapped the Sierras Pampeanas massif (Fig. 11 ). The marine influence became progressively less saline toward the southern intracontinental "cul-de-sac" part of the underfilled foreland basin. Ingressions of marine water were facilitated during times of sea-level highs and sufficient subsidence. Maximum inundation was achieved in the Central Andes at 71.5-70 Ma (Sempere et al., 1997). The Black Sea is a present day analogue of this marine to marine-influenced water body (Gayet et al., 1993). For the Central Andes, the timing of the marine transgression (represented by the Lower El Molino sequence in Bolivia) and basin development follows Sempere et al. (1997). Marine conditions continued to prevail in Colombia. Early Campanian (83 Ma) and early Maastrichtian (-74 Ma) sea level falls followed by sea level rises of late Campanian (-79 Ma) and late Maastrichtian (-71 Ma) age were correlated with eustatic sea level changes (Guerrero & Sarmiento, 1996; Haq et al., 1987), with no indications of the initiation of foreland basins by that time. Planktonic foraminifera and pollen in the base of the Guaduas Formation near Bogota record a marine transgression dated as middle (early late) Maastrichtian (Sarmiento, 1992; Martinez, 1995). Similarly, the correlative units of the lower part of the Guaduas, Umir and Cimarrona formations, the Umir and Cimarrona formations near Honda, contain planktonic foraminifera of middle and late Maastrichtian age (Tchegliakova, 1996). Thus, the marine flooding event of the base of the Guaduas Formation would correlate with the maximum flooding event of the Central Andes in Bolivia at - 71 Ma. Pindell & Tabbutt (1995) suggest that the Panamanian magmatic arc at the southern and western boundaries of the Caribbean Plate along the west coast of Ecuador, and the Aves Ridge at the northeastern boundary of the Caribbean Plate was positioned north of Colombia. Some terranes (Amaime) were accreted to the Western Cordillera of Colombia (Pindell & Tabbutt, 1995). It has been long thought (e.g. Katzer, 1903; Haseman, 1912) that a proto-Amazonas (named "Sanozama", Amazonas spelled backward, Almeida, 1974; or Solimoes, Bemerguy & Sena Costa, 1991) drained westward off an upland (Belterra surface, see below) somewhere between the present day areas of Manaus (Purus Arch) and Obidos (Monte Alegre Arch) (Fig. 2). Such drainage of unknown extent certainly flowed into the early foreland basin whether the basin contained transgressive marine waters, rivers or lakes. Similarly there must have been some drainage of easternmost Amazonia into 24

Ph ylogeny and Class ificati on of Neotro pical Fishes. Part 1 - Fossil s and Geological Evidence

Aves Ridge

Eastern Amazon

Maraiion Portal? ~.

Panama Arc Early Andes Present Thrust Front

Seaway

•• •

Marine Andes

and

magmatic

Rivers,

lakes

II

Shields

&

"'

Direction

&

arcs

wetlands

massifs of

flow

Figure 11. Late Cretaceous paleogeography and drainage from 83-67 Ma (Campanian to middle Maastrichti an) (after Gayet et al. 1993 , fi g. 12, and Pindell & Tabbutt, 1995, fig. 6). 25

Lundberg et al. The Stage for Neotropical Fish Diversification : A Hi story of Tropical South American Rivers

paleo - Amazon-Orinoco in foreland basin

Marine Andes

and

magmatic

& wetlands

Rivers,

lakes

Shields

& massifs

Direction

of

arcs

flow

Figure 12. Late Cretaceous to Early Tertiary paleogeography and drainage from 67-61 Ma (late Maastrichtian - early Paleocene). 26

Phylogeny and Classification of Neotropical Fishes. Part I - Fossils and Geological Evidence Complete regression of marine waters from the Central and northern Andes occurred at -59 Ma, near the DanianSelandian boundary. The first evidence of tectonism on the magmatic Andean arc leading to significant uplift of the Colombian Central Cordillera and development of a foreland basin to the east comes in the late Paleocene (Socha Inferior Alloformation [i.e. allostratigraphic units bounded by discontinuities and including laterally several lithostratigraphic units]; Guerrero & Sarmiento, 1996). This unit includes several formations close to and far from the sediment source area in the Central Cordillera. Sediments range from coarse conglomerates in proximal locations to fine sandstones in distal ones; they correspond mainly to fluviatile sedimentation with paleocurrent directions to the E, NE and N, including proximal alluvial fans and distal braided-stream systems. A regional erosional unconformity separates the base of the Socha Inferior Alloformation from older units. Uplift and erosion prior to the beginning of the late Paleocene (-59 Ma) sedimentation destroyed most of the underlying early Paleocene and late Maastrichtian sedimentary sequence to the east in the fore bulge (Fig. 4) but preserved it to the west in the axial groove of the foreland basin. During the early Eocene there was in Colombia a marine transgression due to either a global sea level rise, subsidence, or a shortage in sediment supply. The Socha Superior Alloformation (Guerrero & Sarmiento, 1996) includes several fine-grained contemporaneous units deposited in estuarine and fluviatile meandering systems that had their source area in the Central Cordillera Andean Arc. Important freshwater fish faunas for the 60-58 Ma interval are those of Tiupampa in Bolivia (Gayet, 1991) from the middle part of the middle Santa Lucfa Formation (59.0 Ma) and Corydoras revelatus (Reis, this volume; Lundberg, this volume) from the Mafz Gordo Formation (58.5-58.2 Ma) of NW Argentina (Marshall et al., 1997; Sempere et al., 1997). Both of these faunas were in the headwaters of the paleoAmazonas-Orinoco system. The upper Santa Lucfa Formation (-58.5 Ma) of central Bolivia and its stratigraphic equivalents in southern Bolivia (Impora) and northwest Argentina (Mafz Gordo) were deposited in a wetland to lacustrine environment (Sempere et al., 1997) in what Horton & DeCelles (1997) regard as the backbulge in a foreland basin system ("Lago Santa Lucfa," Fig. 14 ). A recent analogue of these units is possibly represented by the Pantanal wetland of Brazil (Fig. 20) that is located in the back-bulge of today's foreland basin (Horton & DeCelles, 1997). During the late Oligocene to early Miocene (30-20 Ma) a wetland/lacustrine environment in the back-bulge region is represented by the Petaca Formation of sub-Andean Bolivia ("Lago Petaca," Fig. 16) and in late Miocene (l l.8-10 Ma) by the Yecua Formation (Fig. 18) in the same area (Marshall et al., 1993). Thus, as the foreland propagated eastward during the Cenozoic it appears possible to identify the back-bulge depositional units and environments in four different time intervals and places.

Late Maastrichtian - early Paleocene, 67-61 Ma (Figure 12) Continental redbeds of the middle El Molino sequence in Bolivia (Sempere et al., 1997) and upper part of the Guaduas Formation in Colombia (Sarmiento, 1992), document northward regression of the marine seaway from the central Andes. Tracking this regression, a large river system developed with the mainstem located in the still underfilled axis of the foreland basin. Regression was not complete to the Caribbean, and some marine to brackish-waters may have persisted in parts of Colombia and western Venezuela (Sarmiento, 1992; Gayet et al., 1993; Pindell & Tabbutt, 1995, figs. 6, 7). It is only during the early Paleocene (and perhaps latest Maastrichtian) that fluviatile conditions are established in central Colombia as indicated by a meandering system of red beds in the upper part of the Guaduas Formation. Southward regression of the Parana seaway is less well-documented. The river systems established at this time may be regarded as ancestral to the western Amazonas-Orinoco (plus Magdalena and perhaps Maracaibo) and Parana systems. The north-flowing "paleo-Amazonas-Orinoco" at this time had a larger watershed and may have been about 25% longer than the present day Amazonas system. From -67 to -8 Ma (see below) this was the dominant pattern of South America's largest drainage system. The Parana has increased significantly in size since the Late Cretaceous by watershed capture of parts of the southwestern paleo-Amazonas-Orinoco system (Garrasino, 1988; Potter, 1997). A much restricted lower Parana system existed in the Late Cretaceous as evidenced by sediments in the Punta de! Este basin at the mouth of the present day rfo de La Plata (Potter, 1997, fig. 4 ). Aspects of the evolution of the middle and upper courses of the Parana River are discussed by Stevaux ( 1994 ).

Late early Paleocene, 61-60 Ma (Figure 13) Another marine transgression into the foreland basin as far south as the Central Andes is documented by the marineinfluenced strata of the Upper El Molino sequence in Bolivia (Sempere et al., 1997, Fig. 13). This transgression was apparently less extensive than the earlier one between - 73 and 67 Ma (Fig. 11 ). The early Paleocene transgression was the last into what today is Bolivia, until the Paranan transgression from the south Atlantic at -11 Ma (Fig. 18; see below). The transgression(s) into the southern part of Andean foreland basin (i.e. south and east of Sierras Pampeanas massif) are not calibrated as in the northern part of the basin, but fossil and tectonic evidence suggest they may have been contemporaneous (Gayet et al., 1993; Sempere et al., 1997).

Late Paleocene - middle Eocene, 60-43 Ma (Figure 14)

27

Lundberg et al. The Stage for Neotropical Fish Diversification : A History of Tropical South American Rivers

Seaway Marine

"

Andes and magmatic arcs Rivers, lakes & wetlands Shields & massifs Direction of flow

Figure 13. Early Tertiary paleogeography and drainage from 61-60 Ma (late early Paleocene). 28

Ph ylogeny and Class ificati on of Neotropi cal Fishes. Part I - Fossil s and Geological Evidence

Orinoco

paleo-Amazon-Orinoco in foreland basin Tiupampa fauna "Lago Maiz Gordo Formation

Marine Andes

and

magmatic

Rivers,

lakes

Shields

&

Direction

&

wetlands

massifs of

flow

Figure 14. Earl y Terti ary paleogeography and drain age from 60-43 Ma (late Paleocene - middl e Eocene). 29

arcs

Lundberg et al. - The Stage for Neotropical Fish Diversification: A History of Tropical South American Rivers

The tectonic event initiated at -59 Ma (Sempere et al., 1997 = Incaic I of Noble et al., 1990; see our Fig.9) coincides with achievement of nearly perpendicular convergence of the Nazca and South American plates at about anomaly 25 (Pilger, 1984; Pardo-Casas & Molnar, 1987) (Fig. 8). It marks the initiation of "classic" continental foreland sedimentation in Andean Bolivia (Sempere et al., 1997). Initiation of this tectonic episode marks the boundary between the Puca I Corocoro sequences in the Central Andes (Sempere et al., 1997) and coincides with the Zuni/Tejas megacycle sequence boundary in North America (Leighton & Kolata, 1991), suggesting that the Late Cretaceous-Cenozoic histories of the Pacific margins of both continents were controlled by related causes (Sempere et al., 1997). Paleocurrent data from rocks deposited during this time interval in Bolivia indicate that drainage was primarily to the N-NW (Sempere et al., 1997). Geological (Pitman et al., 1993) and paleontological (Marshall et al., 1997) data corroborate the existence of a land or island arc connection between South America and the Yucatan Peninsula (North America) during the 59 to 56 Ma interval. This emergent volcanic arc was established on the northeastern edge of the Caribbean Plate and formed by the proto-Greater Antilles and Aves Ridge. The Paleogene land bridge was enhanced by the tectonic episode initiated at -59 Ma and a significant marine lowstand between 58 to 56 Ma (Haq et al., 1987). However, since this land bridge was at a convergent margin, it was constantly subjected to tectonism and magmatic activity. Magmatic-tectonic quiescence and/or sea-level highstand periods would have promoted relative subsidence of part of the arc below sea-level. For these reasons, the land bridge "may therefore have frequently been impaired - passing from a pure corridor to a filter corridor to an impasse. Furthermore, species migrating from the north or the south may have reached some of the islands only to be cut off from behind or ahead and become isolated" (Pitman et al., 1993: 25). From the fossil record in both South and North America it is inferred that numerous land mammals dispersed in both directions during this time interval (Marshall et al., 1997), as did some land birds (Marshall, 1994 ). There is no fossil evidence of fishes, amphibians, or reptiles to corroborate the existence of such a connection. However, in Middle America the rich diversity and wide geographic range of certain modern fishes, amphibians, and reptiles with South American relationships have been cited as evidence for early land and freshwater connections, before the Pliocene Panamanian Isthmus (Savage, 1982; Bussing, 1985).

marine transgressions and also regressions with the renewal of fluviatile sedimentation, in response to tectonic pulses in the Central Cordillera. Geologic data from the Central Andes suggest a significant event in river system development when the boundary between the Amazonas and Parana shifted northward, and the Parami captured headwaters of the paleo-Amazonas-Orinoco system. The Sierras Pampeanas lost their influence as a barrier between the Amazonas and Parana systems, while the Michicola Arch (Fig. 2) began to assume influence as a new barrier. This is related to an eastward propagation of the Andean thrust front and formation of the Bolivian orocline as the principal geologic feature that defines the present day western boundary between the Amazonas and Parana systems (sec below). This time interval in the Central Andes is represented in part by the Mondragon and Bolivar formations in west-central Bolivia. The ages of these formations are not securely known, but are presumably Eocene to early Miocene (Sempere et al., 1997). Paleocurrent data from these forcland basin deposits suggest that the basin was underfilled and axially drained to the south (Sempere et al., 1997). These data are of special importance because paleocurrent data from older rock units in the same area (i.e. 73-55 Ma) show primary drainage to the NNW (Sempere et al., 1997). This change in paleocurrent direction is apparently related to an Eocene tectonic event that resulted in bending of the N-S axis of the central Andes (Bolivian orocline; Butler et al., 1995; Lamb & Hoke, 1997). This oroclinal bending and reversal of paleocurrent direction may be related to a tectonic event initiated at -43 Ma which coincides with an increase in plate convergence (Pardo-Casas & Molnar, 1987) and formation of the Hawaii-Emperor bend (Noble et al., 1990). During at least part of this time interval, large parts of the Peruvian and Ecuadorian Andean foreland were occupied by lakes to marginal marine waters ("Lago Pozo", Fig. 15). The Pozo Formation (late Eocene-early Oligocene) in eastern Peru consists of sandstones and tuffs that are covered with thin shales attributed to marine incursion (Williams, 1949). Equivalent deposits in eastern Ecuador (Chalcana Formation: Tschopp, 1953) contain a few species of marginal marine foraminifera such as Ammohaculites and Ammonia. These forams correlate with similar species from deposits in the Putamayo and Llanos basins to the north (Miller & EtayoSerna, 1972). This suggests that the marine influence reached eastern Ecuador and Peru from the north.

Middle Eocene - early Oligocene, 43-30 Ma

Late Oligocene - Early Miocene, 30-20 Ma

(Figure 15)

(Figure 16)

The middle Eocene (43 Ma; Incaic II of Nobel et al., 1990) event of Central Andean uplift and relative sea level fall, resulted in Colombia and Venezuela in the fluviatile deposition of several coarse-grained units (e.g. Mirador Formation). During the late Eocene and Oligocene, the Carbonera Formation and contemporaneous units record several minor

A major orogenic phase along much of the Andean chain was initiated at -30 Ma (Scmperc et al., 1990, 1994; Jaillard et al., 1995; Marocco et al., 1995). Although compressional tectonics had built a low-elevation proto-cordillcra since -90 Ma (Middle Cretaceous, Fig. I OB) along the western margin of South America, the first significant compressional tectono 30

--Phylogeny and Classification of Neotropical Fishes. Part I - Fossils and Geological Evidence

Eastern Orinoco

paleo-Amazon-Orinoco with "Lago Pozo"

II II II ~

,,,II

Marine Andes and magmatic arcs Rivers, lakes & wetlands Marine & fresh water flux Shields & massifs Direction of flow

Figure 15. Tertiary paleogeography and drainage from 43-30 Ma (middle Eocene - early Oligocene). 31

II Lundberg et al. The Stage for Neotropical Fish Diversification: A History of Tropical South American Rivers

't

Central

~America

}

Chapa re Buttress Parana with

Tremembe Formation

Buttress Marine Andes and magmatic arcs Rivers, lakes & wetlands . .Shields ~

& massifs Direction of flow

Figure 16. Middle Tertiary paleogeography and drainage from 30-20 Ma (late Oligocene - early Miocene). 32

Phylogeny and Classification of Neotropical Fishes. Part 1 - Fossils and Geological Evidence There is no geological evidence known to us that would support the suggestion (Parenti, 1984) that Orestias came to South America on a Jurassic accreted terrane. Judging from the presence of the gastropod Sheppardiconcha tuberculifera in the middle Miocene deposits of the Cuenca basin, there was probably a lowland connection with "Lago Pebas" (see below and Fig. 17) to the east. Fossil fishes of the Cuenca basin are not yet identified with sufficient precision to allow assessment of their biogeographic affinities, although most are lowland forms (e.g. Roberts, 1975; JGL pers. obs.). The Cuenca basin, therefore, may have drained into western Amazonia, and was not already isolated as indicated by Figs. 16 and 17. Marine transgressions from the Atlantic occurred in Brazil in easternmost Amazonia (Pirabas Formation, Para State), and in southern Argentina in the Colorado, San Jorge and Magallanes basins (Marshall & Sempere, 1993, fig. 12.1 1, and references therein). During high rates of compression of the Nacza Plate, deformation also occurred near the eastern margin of the South American Plate (Cobbold et al., 1996). The area of the present Tiete and Parafba do Sul rivers, eastern Sao Paulo Brazil, was occupied by an elongate rift depression. This corresponds to the continental rift of southeastern Brazil (Riccomini et al., 1987; Riccomini, 1989), whose origin and evolution were associated with the continued opening of the Atlantic Ocean (Almeida, 1976; Riccomini et al., 1987; Riccomini, 1989; Riccomini et al., 1991 ). The most continuous portion of this rift is between Sao Paulo (SP) and Volta Redonda (RJ), comprising -350 km and four geological basins: Sao Paulo. Taubatc, Resende, and Volta Redonda. These basins were dominated by alluvial fans and a braided fluvial system (Resende Formation) that graded to a lacustrine paleoenvironment (Tremembe Formation; Riccomini et al., 1987; Riccomini, 1989; Riccomini et al., 1991 ). Geological studies (Kiang et al., 1989; Riccomini, 1989) have confirmed the age of this formation as late Oligocene-early Miocene as suggested by fossil birds (Alvarenga, 1990), mammals (Soria & Alvarenga, 1989) and pollen (Lima et al., 1985). The lake system graded to a meandering fluvial system (Sao Paulo Formation) that dominated all of the rift valley. At that time, the upper rio Tiete drained to the sea in a composite drainage with the upper rio Parafba do Sul. The paleo-Parafba do Sul constituted a large meandering river, receiving those drainages. The Tremembe Formation fish fauna includes characiform species (e.g. Lignobrycon ligniticus, Megacheirodon unicus) closely related to recent taxa belonging to coastal drainages and Tietc headwaters (M. C. Malabarba, this volume). The common recent fish species shared by the Tiete headwaters and southeastern coastal drainages (Hollandichthys multifasciatus, Hyphessobrycon bifasciatus, Hyphessobrycon reticulatus, Pseudocorynopoma heterandria and Gymnotus pantherinus) corroborates the ancient connections between these two drainage systems (Langeani, 1989) Widespread marine settings (e.g. Roblecito Formation) were present -23 Ma in the Eastern Venezuelan basin as far

sedimentary episode along much of the Andes developed in late Oligocene and early Miocene time (30-20 Ma). Effects of this episode, including uplifts, are known from central Chile to Colombia (Sempere et al., 1990, 1994 ). The eastern part of the Merida Andes of Venezuela also began to be uplifted in late Oligocene time while the central and western parts were uplifted during the Miocene (Hoorn et al., 1995). Farther east there was significant development of Caribbean nappes of the Venezuelan Coastal Cordillera. The Bolivian orocline of the central Andes underwent further significant bending. The contact (Chapare Buttress) between the Andean thrust front and the subsurface edge of the Brazilian Shield along the northern edge of a pre-existing Paleozoic basin (Sempere et al., 1990) formed the new structural divide between the Amazonas and Parana systems (Figs. 16-18), although farther south the Michicola Arch dominates the present day divide (Fig. 20). This tectonic episode was a significant turning point in Andean history, and coincides with the breakup of the Farallon Plate into the Nazca and Cocos plates and an increase in plate convergence rate (Pilger, 1984; Pardo-Casas & Molnar, 1987; Sempere et al., 1990). Elevations in the Central Andes may have reached 3,000 m during this interval of time (Marshall & Sempere, 1993). Development of major topographic relief along the western margin of South America would have brought rain-shadow effects at least between 12 ° and 37°S. Paleontological data indicating the spread of grasslands in the Southern and Central Andes implies climatic change (Marshall & Sempere, 1993 and references therein; Lamb & Hoke, 1997). Extrapolating to aquatic habitats, it is possible that this also was the onset of the rain-shadow effect north of the Tropic of Capricorn that would have led to increasingly severe arid conditions on the western slopes of Chile and Peru (Marshall & Sempere, 1993). Intermontanc basins extended from Colombia to central Chile. As thrusting propagated significantly eastward, the once western edge of the low elevation foreland basin was captured in places and incorporated into the Andes, i.e. Altiplano basin (containing Lago Titicaca), and Ecuadorian Interandean Valley (containing the Cuenca basin; see Fig. 4 ). It is tempting to speculate that the aquatic biotas of the Lago Titicaca (extant) and the Cuenca basin (fossil) had their origins in the western Amazonian lowlands of late Paleogene to early Neogene. The extant mollusk groups endemic to Lago Titicaca are closely related to modern Argentinian freshwater groups. The fishes of modern Lago Titicaca are uninformative on biogeographic relationships of the biota. These are the widespread Andean catfish Trichomycterus, and the enigmatic cyprinodontinc endemic genus Orestias. Perhaps unexpectedly the sister group of Orestias is a distant northern hemisphere clade (Parenti, 1984; Costa, 1997). The biogeographic history of Orestias is obscure, but presumably evolved proximately from an ancestral group of adjacent lowlands. In the current tectonic framework of eastward propagation of the Andean thrust front, it is possible that a lowland ancestral group was uplifted with formation of the Altiplano basin. 33

Lundberg et al. - The Stage for Neotropical Fish Diversification: A History of Tropical South American Rivers (she used the term Solimoes/Pebas). Several sections of the Pebas Formation would span the middle Miocene (in Peru) and the middle to late Miocene (in Colombia). It is probable that in the term Solimoes/Pebas several Miocene lithostratigraphic units of different ages and origins are included along with several unrecognized unconformities. Some disagreement exists over the nature of the paleoenvironments represented by the Solimoes/Pebas system. However, in such a vast, well-watered, lowland we should expect a great range of aquatic situations from freshwater rivers, lakes and wetlands to marine-influenced water bodies. The Colombian Pebas sections include marine palynomorphs along with pollen and spores. Hoorn (1993) reported several marine incursions in Amazonia and the establishment of brackishwater conditions in the late middle Miocene and later supported by the presence of trace fossils such as Thalassinoides, marine palynomorphs, mangrove pollen, and sedimentological indications of tidal influence. This evidence suggests an environment of estuarine and coastal plains dominated by lagoons, meandering channels, and tidal flats. On the marine side, visualizing a Pebasian lowland water body probably much bigger than Lago Maracaibo, salinity would be controlled by the amount of river input and the size of the connection to the Caribbean. In such a situation, small eustatic fluctuations (I 0 m or less) of the sea level could change the situation very easily from coastal plains to lagoons, estuaries and shallow embayments. The term "Pebasian Sea" may be applied to the marine influenced water bodies during the 16 Ma interval represented in the Solimoes/Pebas deposits. Fish fossils from Solimoes/Pebas represent both strictly marine taxa such as myliobatiform rays, and also strictly freshwater taxa such as lungfishes, characiforms, and siluriforms (Monsch, in press; JGL pers. obs.). Obviously the fishes strongly support a wide range of aquatic situations. On the freshwater side, the Pebas area was fed by both Andean and shield-draining rivers; the term "Lago Pebas" can be applied to the freshwater lakes of the region. "Lago Pebas" should not be confused with the hypothetical Holocene "Lago Amazonas" of Frailey et al. (1988) and Campbell et al. ( 1985). Sediment geochemistry and strontium isotope ratios in fossil mollusk shells indicate a predominance of Andean input (Kronberg et al., 1989; Vonhof et al., in press). Hoorn et al. (1995) interpreted the Pebas system as fluvio-lacustrine, comparable to modern varzea environments, episodically reached by marine incursions. It is very likely that rivers and floodplain Jakes or savannahs existed in this expansive region. However, in the Pebas Formation, the aquatic invertebrate faunas (corbulid bivalves, hydrobiid gastropods, cypridid and cytheridid ostracods), isotopic evidence, characteristic finingup/coarsening up sedimentary cycles, as well as extensive lateral correlation of individual layers points also to a more permanent lacustrine environment. The Jake(s) must have been shallow, vegetated, deltaic systems with associated wetlands. The diverse endemic aquatic invertebrates of "Lago Pebas" are very different from the comparatively species-poor cosmopolitan type of faunas found in the major floodplains

west as the El Baul Arch (Audemard et al., 1985). The continuity of depositional settings east and west of the El Baul Arch is not well documented. However, at the same time the Llanos basin (possibly connected to the more westerly located Magdalena basin) was occupied by marine influenced lower coastal plain depositional settings (Ortega et al., 1987; Cooper et al., 1995). The environmental setting must have resembled the present day Lago Maracaibo, but on a far larger scale. At the same time, rivers draining the western Guyana Shield flowed northwestward towards the foreland and Llanos basins. During this time the Mariname Sand Unit was deposited in Colombian Amazonia. In this mostly fluvial deposit, the presence of lignites containing mangrove pollen indicates that marine influence must have reached briefly into northwestern Amazonia in late early Miocene (Burdigalian) times (Hoorn, 1994a).

Early and middle Miocene, 20-11.8 Ma (Figure 17) The major features and events of this time interval were the development of extensive lakes and inland seas in the foreland basin in western Amazonia and northward. These continued in the later Miocene as discussed in later sections. During this time interval the depocenter (deepest parts) of these water bodies shifted eastward with the foreland basin, and thus parts of the lake gradually occupied the western edge of the craton. Tectonically-controlled drainage reorganization of great biogeographic significance occurred in Colombia, Bolivia, and southeastern Brazil. Also, the accretion of oceanic terranes to the northwest corner of South America added a new land area. In Ecuador and Colombia, crustal thickening was accompanied by dextral wrenching in the Paleogene, whereas the direction of shortening veered towards the southeast during the Neogene, reflecting collision of the South American Plate with Central America (Cobbold et al., 1996). The stratigraphy, ages and paleoenvironments of the Neogene deposits in western Amazonia are complicated. Because there are no radiometric dates, their inferred ages are subject to some uncertainty; the age estimates are based on palynology (Hoorn, 1993, 1994 a, b, c ). The rocks included in the Miocene of Amazonia are part of at least four formations, the Solimoes in Brazil, the Pebas in Peru and Colombia, and the Mariname (see above) and Apaporis Sand units in Colombia. Hoorn (1993) studied a Brazilian well core from south of Leticia (Colombia) in which the lower 250 m of the approximately 980 m thick Solimoes Formation are palynologically dated as spanning early Miocene to early late Miocene, roughly 23 to 8 Ma. The Pebas Formation is documented only from stratigraphic sections no more than 25 m thick along several rivers including Amazonas and Caqueta. There is no record of its top or base and its total age range and thickness are is uncertain. Hoorn (1993) correlated the outcrops of the Pebas Formation in Colombia and Peru with part of the Solimoes 34

Ph ylogeny and Classification of Neotropical Fishes . Part 1 - Fossils and Geological Evidence

Central

~America

'If,

\.

.t

LaV~ fauna

Cuenca

"Lago Pebas" in paleo-Amazon-Orinoco

Marine Andes

,,,

and

magmatic

Rivers, lakes & wetlands Marine & fresh water flux Shields & massifs Direction

of

flow

Figure 17. Late Tertiary paleogeography and drainage from 20-11 .8 Ma (early - middle Miocene). 35

arcs

Lundberg et al. - The Stage for Neotropical Fish Diversification: A History of Tropical South American Rivers

and lakes of the present Amazonas system (Wesselingh, pers. obs.). The near absence of pulmonate snails and amphibious ampullarid snails, that are common in the varzea and are well known from the Pantanal wetlands, also points to a different depositional environment for "Lago Pebas". Pebasian mollusks and ostracods are reminiscent of Caspian Sea faunas: all the major groups (hydrobiid and neritid gastropods, dreissenid bivalves, cypridid and cytheridid ostracods) are present in both systems, that are furthermore dominated by endemic species-swarms of originally marine bivalve families (Corbulidae in the Pebas system vs. Cardidae in the Caspian Sea). "Lago Pebas" must have had some sort of overflow or semi-open northerly connection to the Llanos basin where marine influenced settings, including fossil foraminiferans, occurred from -16-10.5 Ma. Between 13.5- I 2.9 Ma, the under filled central and northern Andean foreland basin continued to drain via rivers and interconnected lakes northward to a Llanos basin - Caribbean outlet. The area of the present Magdalena Valley was dominated by a meandering to braided river system with an E-SE transport direction (Guerrero, 1993, 1997; Hoorn et al., 1995) (Fig. 17). This system drained into the Llanos basin, that at the time was occupied by marine or marine influenced settings (Leon Formation; Cooper et al., 1995). The middle Miocene age (13.5 to 11.5 Ma) La V enta aquatic fauna in the present Magdalena Valley of Colombia documents a large river system containing numerous fishes, with living relatives found today in the Amazonas and Orinoco (Lepidosiren, Arapaima, Colossoma, Brachyplatystoma. and Phractocephalus; Lundberg, 1997). Coincident or subsequent to the later tectonic formation and isolation of the Magdalena Valley, many elements of its former aquatic biota were extirpated (Lundberg, 1997). Between 12.9 and 11.8 Ma, the Colombian Eastern Cordillera started developing. At this time, a meandering fluvial system continued in the Magdalena region with flow to the NNE in addition to a still predominant flow direction to the ESE. The main rivers and interconnected foreland-basin lakes in the underfilled Andean foreland basin continued to drain the present western Amazonas basin, western part of the present Orinoco basin and present Magdalena Valley into the Caribbean in the area of the Maracaibo and Falcon basins (Hoorn, 1993; Hoorn et al., 1995; Kulke, 1995; Diaz de Gamero, 1996; Guerrero, 1997). The watershed of this system extended south as far as the Madre de Dios-Beni basins of northeastern Bolivia, north of the Chapare Buttress. Restricted marine transgressions entered the Maracaibo, Llanos, and Eastern Venezuelan (Eastern Orinoco) and Maraj6 (Eastern Amazon) basins and northeastern state of Para (Arai et al., 1989; Rossetti et al., 1989, 1990). In the northwesternmost corner of South America, the beginning (16.1-15.1 Ma) and the accelerated uplift (12.911.8 Ma) of the Panamanian Isthmus during the middle Miocene (Duque, 199 I a) provided new uplifted and accreted land areas that presumably would have acquired aquatic biotas including freshwater fishes that dispersed from older rivers to the east in the Western Andes or beyond. According to Duque

(I 990b ), the Choco Block including the main valleys of the rfos San Juan and Atrato, was accreted to the northwest corner of South America during and after the middle Miocene. Continued uplift of the Bolivian Eastern Cordillera at -15 Ma tectonically dammed the Altiplano drainage and subsequent infill of sediment produced the low-relief topography observed today (Vandervoort et al., 1995). In southeastern Brazil, the continental rift located in the middle of the South American Plate suffered intraplate compressional stresses. The resultant displacements caused uplift of the Aruja Structural High (Riccomini, 1989), which eventually separated the Tiete and Parafba do Sul drainages. To the west of the Aruja High, a new fluvial meandering system was established (ltaquaquecetuba Formation) with an E-W drainage direction comparable with the present Tiete River (Coimbra et al., 1983). To the east of the Aruja High, the Parafba do Sul drainage reorganization found an uneven relief leading to an erosional period, after which a new fluvial meandering system was established (Pindamonhangaba Formation; Riccomini et al., 1990) during a time of tectonic quiescense in the Pliocene-Pleistocene, probably in a warm and wet environment.

Late Miocene, 11.8-10.0 Ma (Figure 18) The signature event of this time interval was extensive marine transgression into the low-lying basins of South America. Beginning -11 Ma, resumption of nearly perpendicular movement of the Nazca Plate against the western edge of the South American Plate initiated a tectonic episode (Quechua of Steinmann, 1929, 1930) along the Central Andes of Bolivia (Marshall & Sempere, 1993; Marshall et al., 1993 ). Crustal shortening, hence uplift, resulted in tectonic loading in the mountain belt and subsidence (enhanced by sediment loading) in the foreland basin. During this time there was an increase of Andean derived sediment into foreland basins (Potter, 1994 ). Because of a lag time before erosion could provide enough sediment to compensate subsidence in the foreland basin, its axis, running adjacent and parallel to the thrust front, subsided without being completely filled by alluvial deposits (Marshall et al., 1993 ). Flexural subsidence permitted the ingression of marine waters into the underfilled axis which was, at least at times and in places, tens of meters below sea level. A marine highstand for the latest Serravallian (Haq et al., 1987) would have augmented this tectonically induced transgressive event, but the transgression event was not induced by a eustatic component (Marshall & Lundberg, 1996) In the southern (Parana) and northern (Pebas to Llanos) parts of the Andean foreland basin, the first units to be deposited in relation to this tectosedimentary episode show evidence of marine influence. This is referenced in a diverse terminology: "Paranan Sea" or "Seaway," "Pebasian Sea" or "Seaway" (= in part "East Andean Sea" of Haseman, 1912; = "Iquitosian Sea" of Steinmann, 1930: 357). Transgressions 36

Phylogeny and Classification of Neotropical Fishes. Part 1 - Fossils and Geological Evidence

al., 1993). Farther north, waters became less saline, g1vmg way to lagoons and lakes (Marshall et al., 1993; Marshall & Lundberg, 1996) which onlapped the Chapare Buttress (Sempere et al., 1990). A freshwater setting for the northern "cul-de-sac" of the Yecua Formation is confirmed by the presence of a fossil gymnotiform electric fish that would have been tied to freshwater due to osmoregulatory and electrosensory physiology (Gayet & Meunier, 1991). The W-NW lateral equivalent of the Yecua Formation in the northern Subandean zone is the continental Quendeque Formation (Marshall & Sempere, .1991 ). The Amazonas - Parana divide: The southernmost sediments assigned to the Pebasian sea occur -10° S in the State of Acre, Brazil (Rasanen et al., 1995) and in the Madre de Dios basin, Peru (R. Marocco, pers. comm.), whereas the northernmost sediments assigned to the Paranan sea occur -17° S NNW of Santa Cruz, Bolivia (Marshall et al., 1993 ). Although the extent of saline waters is debatable, the depositional environment of the northern extent of the Yecua Formation (Marshall et al., 1993) and paleocurrent data for Pebasian sediments (Hoorn, 1993) suggest there was no interconnection of these water bodies between -10° and -17° S. This was the northernmost documented extent of the Amazonas-Parana basin boundary, which was established when the foreland was underfilled. Subsequently, during overfilling, the boundary of the basins shifted farther south to its present location at the Michicola Arch (Figs.1, 2, 20). Thus, this sequence of events documents capture of headwaters of the Parana system by the Amazonas system during the last 10 Myr. The present drainage divide between the Amazonas and Parana basins (-20° S) is the narrowest point between the Andes and the Brazilian Shield. The topography resulting from areas resisting foreland subsidence and post-late Miocene deposition in the foreland basin monitored the present location of the watersheds (Iriondo, 1993). The Lower (Eastern) Amazonas - Upper (Western) Amazonas divide: There is no geologic evidence for a direct connection between the Atlantic Ocean and the Miocene Pebasian basin. In fact, Bemerguy & Sena Costa ( 1991) suggest that the Purus arch was a drainage barrier separating the Eastern and Western Amazon from the Late Paleozoic until late Miocene or Pliocene. Miocene age marine deposits are unknown between the Purus Arch west ofManaus (Bigarella, 1973) and the westernmost extension (-200 km landward) of the Pirabas Formation at the mouth of the Amazonas (Hoorn, 1993; Fig. I). During the middle late Miocene transgressions into "Lago Pebas", an upland area is hypothesized to have existed between the Purus and the Monte Alegre arches in central Amazonia where there is evidence of a complex of hard lateritic weathering horizons and erosional surfaces, i.e. Belterra surface(s) (Sombroek, 1966; Irion et al., 1995). The age(s) of the Bclterra surface(s) is estimated on clay mineralogy sequence to be 10 Ma or tens of millions of years older (Irion et al., 1995). This upland could have been the watershed divide for river systems draining west and east in the main Amazonas valley (Haseman, 1912; Domining, 1982; Frailey et al., 1988; Hoorn, 1993).

also occurred into Eastern Venezuelan (Eastern Orinoco) and Maraj6 (Eastern Amazon) basins. The Paranan seaway had a broad connection to the South Atlantic; the Pebasian seaway had a restricted connection between "Lago Pebas" and the Caribbean. There is no evidence of Pebasian-Paranan interconnection crossing the continent (Marshall & Lundberg, 1996). The basins were bounded to the west by the Andean Quechua thrust front and to the east by the Guyana and Brazilian shields and associated structural arches. As the Andes were built progressively from west to east (Marshall & Sempere, 1993), the western edges of the Paranan and Pebasian basins were deformed and uplifted by subsequent thrust deformation. The Solimoes/Pebas system records both marine incursions and increasing sediment input from Andean rivers, leading toward overfilling of the foreland basin in western Amazonia. Foraminifera, barnacles, pollen of mangroves, marine or brackish gastropod taxa, as well as strontium isotope evidence and sedimentology, indicate marginal marine settings with salinities to about 6 ppm (Vonhof et al., in press). Marine influences reached the lake from the Caribbean through either (1) the Eastern Venezuelan basin over the westernmost edge of the Guyana Shield, or (2) from the Maracaibo area or Eastern Venezuelan basin through the Llanos basin. The middle to late Miocene Apoporis Sand Unit of Hoorn ( 1994a), from Colombian Amazonia, contains flu vial deposits from the Guyana Shield and a few lignitic levels with foraminifcran fossils and abundant pollen of mangrove vegetation. This indicates that marine incursions did not enter western Amazonia exclusively through the foreland area. The western margin of "Lago Pebas" graded into a lower fluvial floodplain draining the adjacent Andes. At 11.8 Ma, paleocurrent directions in the Magdalena Valley shifted to the west in a meandering to anastomosing fluvial system (Guerrero, 1993, 1997). This shift indicates the existence of an important new sediment source area to the east, confirming the appearance of the Colombian-Venezuelan Eastern Cordillera as a continuous range. This new range was high enough to permanently divide the former foreland basin into the Magdalena and Llanos basins (Guerrero, 1993, 1997; Hoorn et al., 1995). After a short period of erosion and no sedimentation ( 11.5-10.1 Ma), deposition of conglomerates in a braided fluvial system with N-NE transport directions began at -10.1 Ma. These oldest relics of modern rfo Magdalena were deposited at the same time as an episode of volcanism and uplift of the Central Cordillera and Eastern Cordillera (Hoorn et al., 1995; Guerrero, 1997). At the same time marine influence retreated from the Llanos basin, that after -10.5 Ma became entirely fluvial controlled (Cooper et al., 1995). Paranan sea sediments of late Miocene age have yielded numerous benthonic foraminifera that indicate shallow to littoral environments (Boltovskoy, 1991; Marshall et al., 1993). In the Bolivian part of the Paranan sea (Yecua Formation), marine invertebrates are known only south of 18° S. This is the area where Rasanen et al. (1995) report "tidal sediments", which derive from the Paranan basin (Marshall et 37

Lundberg et al. The Stage for Neotropical Fish Diversifica tion: A Hi story of Tropical South American Ri vers

Central

'·\.

~America Rio Magdalena

~

Belterra surface

Pebasian and "Lago

Paranan Sea

•• •• ,,,• Ii

Buttress Marine Andes Rivers,

magmatic

lakes

&

Direction

of

flow

arcs

wetlands

Marine & fresh water Shields & massifs

Figure 18. Late Tertiary paleogeography and drainage from 11.8-10.0 Ma (late Miocene).

38

and

flux

Ph yloge ny and Classifi cation of Neotropi cal Fi shes. Part 1 - Fossil s and Geological Evidence

Central

~America

'If,

l

).

Urumaco ~ una

Macarena Massif

Buttress Marine Andes

,,

and

magmatic

Rivers,

lakes

Shields

&

Direction

wetlands

massifs of

Figure 19. Late Tertiary paleogeography and drainage from I 0-8 .0 Ma (late Miocene). 39

& flow

arcs '

Lundberg et al. - The Stage for Neotropical Fish Diversification: A History of Tropical South American Rivers day. These animals document a direct connection with the Orinoco and Amazonas systems at this time or just before. "Lago Pebas" remained, although confined to Brazilian Amazonia and perhaps with a period of endorheic drainage. Sediment carried by Andean rivers filled "Lago Pebas" in western Amazonia until the final breakthrough of the Amazonas river towards its modern course, -8 Ma (see below).

The Eastern Orinoco - Western Orinoco divide: The Eastern Venezuela basin is delimited to the west by the El Baul Arch and its connection with the Guyana Shield (Lorente, 1986). Palynological studies of Upper Tertiary sediments along the southern flank of the Orinoco basin show shallow marine conditions with a strong fluvial influence during the late middle and early late Miocene (Freites Formation, Lorente, 1986). This marine inundation did not transgress the El Baul Arch in the west of the basin. Large fluvial systems (perhaps to be considered precursors of rfos Caroni, Caura and Ventuari) draining the Guyana Shield were located in the south and southwest part of the basin. During the late Miocene (Lorente, 1986) regression occurred in an easterly direction. The Orinoco - Amazonas divide: The present divide between the upper Amazonas and upper Orinoco river systems occurs on the Vaupes Arch, which extends from the Guyana Shield west to the vicinity of Serrania de la Macarena (Hoorn, 1993; Hoorn et al., 1995; Kulke, 1995; Diaz de Gamero, 1996). This is today the narrowest point between the Andes and the Guyana Shield. Prior to the Quechua thrusting episode, the distance between the Vaupes Arch and the Macarenas was greater, because the latter was lower and located farther to the west. The Macarenas developed as a positive feature as a result of uplift of the Eastern Cordillera in the middle to late Miocene. Foreland basement response to thrust propagation from the west beginning -11 Ma elevated the Macarenas and Vaupes Arch structures, and brought them into closer proximity. Coeval overfilling of the foreland subsequently resulted in a relative high area that is now the watershed divide between the Amazonas and Orinoco systems. The divide between the Amazonas and Orinoco systems is still incomplete on the Guyana Shield where the rfo Casiquiare connects the rio Negro (Amazonas system) with the upper rfo Orinoco (Fig. 1). With emplacement of this drainage divide, northwestern Amazonia joined the Amazonas watershed as delimited today.

Late Miocene - Recent, 8 Ma-present (Figure 20) During this latest time interval the present-day pattern of river systems and drainage divides on the South American continent was achieved. Pleistocene sea-level fluctuation altered habitats in the lower reaches of the Amazonas and perhaps other coastal rivers and the Isthmian link with Central America was completed. Toward the end of the late Miocene (-8 Ma), the Maracaibo/Falcon outlet of the Orinoco was closed by continued uplift and union of the central and western parts of the Merida Andes, Sierra de Perija and Eastern Cordillera (Hoorn, 1993; Hoorn et al., 1995; Kulke, 1995; Diaz de Gamero, 1996). The present altitudes of those ranges were attained by further uplift between 5 and 2 Ma (Wijminga, 1996). A period of endorheic drainage may have occurred in the Andean foreland basin in Colombia and Venezuela. Coeval overfilling of this basin resulted in overflow and subsequent breaching of the El Baul Arch by the western Orinoco. This established the modern west-to-east transcontinental drainage of the modern Orinoco. The Orinoco delta appears in the Maturin basin of NE Venezuela at the end of the late Miocene. Coeval overfilling of the Andean foreland basin in western Amazonia resulted in overflow and subsequent breaching of the Purus Arch (Bemerguy & Sena Costa, 1991 ), or other divide, between the Eastern and Western Amazon. This established the modern west-to-east transcontinental drainage of the modern Amazon. For both the Amazonas and Orinoco these events coincide with the change from carbonate platform to siliciclastic sedimentation on the Atlantic shelf near the river mouths (Rod, 1981; Hoorn, 1993; Hoorn et al., 1995; Kulke, 1995; Diaz de Gamero, 1996). At the mouth of the Amazon, this change is seen between the carbonate platform Marajo and Amapa· formations (Paleocene - late Miocene) and the late Miocene - Pleistocene siliciclastic Pirarucu Formation (Schaller & Vasconcelos, 1971 ). In addition, a leg of the Ocean Drilling Program on the Ceara Rise found that a major change in geochemistry took place between -8.5-8 Ma indicating the establishment of the modern Amazonas system (Curry et al., 1995). Also correlated with the evolution of the modern west-toeast Atlantic drainage of the Amazonas and Orinoco, the Caribbean in the Maracaibo/Falcon basin areas was abandoned as an outlet for very large, sediment-laden rivers. Collins (1996) and Collins et al. (1996) identified changes in benthic foraminifera and corals indicative of increasing carbonate substrates in the Caribbean -8-7 Ma; this is about

Late Miocene, 10.0-8 Ma (Figure 19) The main event in this time interval is termination of marine incursions into the Pebasian system and Llanos basin. Concomitantly another large river system extended with a major outlet draining into the Caribbean via the Maracaibo and Falcon basins (Hoorn, 1993; Hoorn et al., 1995; Kulke, 1995; Diaz de Gamero, 1996). This outlet is documented in the Falcon basin by the late Miocene age Urumaco Formation (Diaz de Gamero & Linares, 1989; Lorente, 1986) which is divided into three members. The lower two members are marine deposits associated with the Pebasian transgressions. The upper member represents a river delta that has yielded strictly freshwater taxa, including a large pimelodid catfish (Phractocephalus), a mata-mata turtle ( Chelus), and an iniid dolphin (Lundberg et al., 1988; Sanchez-Villagra et al. 1995), identical to those in the Amazonas and Orinoco systems to40

Phylogeny and Classification of Neotropical Fishes. Part l - Fossils and Geological Evidence same time that Diaz de Gamero (1996) dates the Orinoco River shifting to its easterly flowing course into the Atlantic, thereby removing a powerful source of freshwater and organic rich sediments from the Caribbean. Although rivers can be dated by sediments in their deltas and fans, deltas may never have been deposited or were dispersed due to transcurrents as at the mouths of the Amazonas and Magdalena, or their record consumed at a convergent margin as may have occurred at the Marafion Portal (Potter, 1997). Overfilling of the Orinoco and Parana basins resulted in the trunk streams being displaced away from the main sediment influx sources of the rising Andes, and the Orinoco and Parana rivers came to lie adjacent and parallel to the exposed Guyana and Brazilian Shield boundaries respectively. The present-day western boundaries between the AmazonasOrinoco and Amazonas-Parana systems occur at the closest points between the Andes and Guyana Shield and Andes and Brazilian Shield, respectively. They also correspond to structural arches (Vaupes and Michicola, respectively). During glacial lowstands, the lower part of the Amazonas system, including the lower reaches of main tributaries at least to rio Negro, incised through erosion. Irion et al. (1995) reconstructed a large, narrow freshwater lake in the lower Amazonas in the late Pliocene or early Pleistocene that they believed formed during a sea-level high stand (20-25 m above ms!). The 20-25 m highstand of Irion et al. (1995) is based on raised terraces from the southern Brazilian coast that could be tectonic artifacts. Nevertheless, at the end of major glacial periods when the global climate warmed and glacial ice melted, sea level rose to impound the lower eroded part of the Amazonas valley with freshwater. At least one large, narrow freshwater lake likely developed from the Amazonas mouth to west of Manaus (65° W) with a length of about 2,500 river km from the river mouth and a width not exceeding 100 km (Irion et al., 1995). This lake was not filled immediately by the sediments from the Amazon. The "ria" lakes in central-eastern Amazonia are also remnants of this huge lake that have not been infilled yet. This huge lake is not the "Lago Amazonas" of Frailey et al. (1988) and Campbell et al. (1985) that supposedly covered much of inland Amazonia in the Holocene but which has not been substantiated by geological data (Tuomisto et al., 1992). It is possible that impounded lakes formed in the lower reaches of other major rivers with great discharge (Orinoco and Parana), whereas rivers with lower discharge may have experienced marine incursion. Emplacement and closure of the Panamanian isthmus was a protracted series of events that occurred since -15 Ma as the South American Plate began to move in a more northwesterly direction and collide with Central America (Duque, l 990b; Coates & Obando, 1996; Cobbold et al., 1996; Collins, 1996). Formation of the complete marine barrier between the East Pacific and Caribbean happened between 3.1 and 2.8 Ma, but with a possible younger breach during the late Pliocene. Formation of a complete land bridge with probable initiation of freshwater dispersal routes between northwestern South America and lower Central America took place by 2.5 Ma

(Marshall & Semperc, 1993). Perhaps the clearest examples of the potential freshwater routes, and one that is supported by strong biotic similarity, is between the rfos Atrato and Tuyra at the contact between South America and the Isthmus.

The Diversification of South American fishes in relation to drainage evolution In the literature on diversification and biogeography of Neotropical fishes (and other groups) there is a prevalent but, we believe, misleading tendency to oversimplify scenarios for estimating the age and causes of diversification (see also Weitzman & Weitzman, 1982; Vari, 1988). The focus is too often on single events or phenomena. In particular, many authors have emphasized the "final" (usually meaning Miocene to Pleistocene) uplift of the Andes as the quintessential event in cis/transAndean vicariance and in the formation of the Amazonas watershed (e.g. Eigenmann, 1909; Gery, 1969; Roberts, 1972; Brooks et al., 1981; Vari, 1988). Such emphasis preceded or overlooks knowledge of the far deeper history of: (1) Neotropical fishes, even at fairly fine taxonomic levels (Weitzman & Weitzman, 1982; Vari, 1988; Lundberg, 1997 and this volume); (2) the Andes that have been developing for at least -90 Myr; and (3) immense, lowland watersheds that have existed continuously in western Amazonia and northward for at least the last -67 Myr. As another example of a narrow focus, Gery ( 1984) and Frailey et al. ( 1988) proposed, by comparison with the faunas of large and ancient lakes, that the development of large lakes in Amazonia during the Tertiary or Quaternary triggered the explosive radiation of Ncotropical fishes. This notion also overlooks the great age of Neotropical fishes, it does not specify a speciation mechanism per sc, and there is no paleontological evidence for diversification of fishes in known or suspected lake deposits. Freshwater fishes aside, however, the molluscs of Miocene "Lago Pebas" did undergo a significant radiation that is currently being documented (Wesselingh, 1993). The Pleistocene refuge - allopatric divergence model (e.g. Prance, 1982) is probably the most widely-known and invoked singular mechanism for explaining biological diversity in South America. However appealing the Pleistocene refuge theory may be in principle, it is difficult to apply it to freshwater fishes (Weitzman & Weitzman, 1982), and, again, given the deep temporal framework for fish diversification, the model could only apply to the most terminal cladogenetic events within the majority of Neotropical fish cladcs. How is any long and complex history of rivers relevant to understanding the diversification of Neotropical fishes, the geographic distribution of fish clades, and the composition of regional biotas or areas of endcmism? Largely it will be up to individual systematists and biogeographers working with their particular groups and regions to apply and test our or any historical framework. For everyone, however, an essential first question is what fish clades were present during the last -90 Myr? 41

Lund berg et al. T he Stage fo r Neo tro pi cal Fish Di versifica ti on: A History of T ropi cal South Ameri can Ri vers

'If,

l

Maracaibo

Basi~·

Panamanian Isthmus

El Baul Arch breached create

Orinoco Arch breached create Amazon

Vaupes Arch

Impounded lakes lower Amazon times during Pleistocene

Foreland basins get overfilled Pantanal Michicola Arch

Buttress Marine Andes

~

and

magmatic

Rivers,

lakes

Shields

&

Direction

& flow

Figure 20. Late Tertiary-Qu atern ary paleogeography and drainage from 8 Ma (l ate Mi ocene) to Recent. 42

wetlands

massifs of

arcs

Phylogeny and Classification of Neotropical Fishes. Part 1 - Fossils and Geological Evidence flux of fluvial and lacustrine habitats. When brackish and marine waters spread inland, salinity-intolerant freshwater fishes could have been allopatrically fragmented in peripheral rivers systems. The Miocene extension of marine waters again provided brackish or salt-water organisms with access to the interior. As noted above, in contrast to the molluscs, the Tertiary lacustrine and inland sea realms of western Amazonia yield no evidence of "explosive" evolution of unusual fishes. A more discrete, tectonically-driven vicariance event took place from middle to late Miocene with elevation of the Eastern Andes in Colombia and the Caribbean Coastal Cordillera. This was of great biogeographic importance because it formed new drainage divides that cut off the Magdalena and Maracaibo basins from the Orinoco and from one another. Those new drainage systems contain endemic species that probably originated after vicariant isolation. Important also is the extirpation from both the Magdalena and Maracaibo regions of fishes that continue to live in the Amazonas or Orinoco (Lundberg, 1997). Other cases of extirpation of Neotropical fishes from areas peripheral to the tropical, lowland core of South America are in Argentina (Arratia & Cione, 1996), Chile (Rubilar, 1994 ), and the Intemontane Cuenca Basin of Ecuador (Roberts, 1975). Near the end of the Miocene (-8 Ma), the assembly of the present-day west-to-east flowing Amazonas and Orinoco systems resulted in two major western-eastern biotic mergers, and presumably fauna! enrichment. The results of these have not been examined in any detail as far as we know, but establishment of sympatry among relatively close species within clades of lowland fishes is an expectation and such patterns have been noticed (e.g. Mago-Leccia et al., 1985; Vari, 1988; Retzer, 1996; Rapp Py-Daniel, 1997; Toledo-Piza, 1997; Reis, this volume). Coincident, or nearly so, with the foregoing was the Orinoco-Amazonas vicariance event. A significant conclusion is that Pliocene and Pleistocene Earth history events in South America had nothing to do with creating the higher taxonomic diversity of Neotropical fishes. As noted before and by others (Weitzman & Weitzman, 1982), the Pleistocene-refuge theory apparently has little to do with freshwater fishes even on a small scale. Nevertheless, following some Late Cenozoic vicariance or dispersal, allopatric divergence and speciation of Neotropical fishes started and probably continues. Clear or likely examples include: formation of Pleistocene Lago Valencia that contains four endemic species (Mago-Leccia, 1970); isolation of rivers during Pleistocene sea-level high stands (Weitzman et al., 1988); stream capture or dispersal-based habitation by fishes of brand new drainage systems on the emergent Panamanian isthmus (Bussing, 1985) and Caribbean and Pacific slopes of northern South America. If nothing else, we hope that this review provides a small step forward in illustrating for biologists the long and complex history of South American river systems in relation to the continent's geomorphological evolution. Such complexity was suggested by Weitzman & Weitzman (1982) in their discuss10n of the biogcography and diversification of freshwater

Interestingly, the more we discover in the fossil record and the more we resolve phylogenetic relationships of fishes, the very much older our estimates of taxic origin get (Lundberg, this volume; Gayet & Meunier, this volume; M. C. Malabarba, this volume; Arratia & Cione, 1996). We know that by Late Cretaceous some familiar Neotropical groups had originated and differentiated (e.g. lepidosirenids, some characiforms and siluriforms, Arapaima-like osteoglossomorphs). It is reasonable to expect that some diversification of Neotropical higher-ranked clades (e.g. the otophysan orders) occurred before complete separation of Africa and South America (Lundberg, 1993). The deepest diversification events of many larger clades of Neotropical fishes probably happened before our history of rivers begins in detail, and certainly well before the latest episodes of Andean uplift and organization of modern drainage pattern. Reconstructing the earlier history of rivers will be a daunting task. Although few in number, fossil fishes of Paleocene through Oligocene age clearly indicate that the evolution of a wide variety modern Neotropical fishes was well underway in the Early Tertiary. Paleogene fossils of pimelodid and callichthyid catfishes, cheirodontidinc and curimatid characiforms, and percichthyids are readily assigned to modern generic-level taxa. These suggest considerable diversification and differentiation within their enclosing clades. Thus, the latest Cretaceous and Early Cenozoic history of aquatic habitats should be highly relevant to understanding the diversification of some modern fish clades. During this time, extensive marine regression was accompanied by the spread of continental waters, and the evolution of the Andes provided the landscape for very large river systems in the foreland basin, i.e. the north-flowing "paleo-Amazonas-Orinoco" and the south-flowing Parana. Andean growth and uplift would have also provided opportunities for Early Tertiary cis/trans- and intra-Andean vicariance, as well as the development of new, high-gradient habitats in the uplands. Perhaps some of the marine-derivative clades of Neotropical freshwater fishes, such as potamotrygonid rays or sciaenids, were established during these transgression-regression phases, but it will be difficult to say which without better benchmarks for the ages of these clades. During the Miocene a great diversity of small clades ranked as genera, species groups and even some species of modern Neotropical fishes appear in the fossil record (Lundberg, this volume). Placement of many of these within their phylogenetic frameworks allows the inference of much greater fish diversity at that time but not yet known directly in the fossil record. By the late Miocene, therefore, the Neotropical fish fauna was essentially modern across wide taxonomic and ecological scopes. Thus the late Paleogene through Miocene evolution of Neotropical landscape and drainage could have been among the most important in influencing diversification of fishes. It was during this interval in the foreland basin that lacustrine habitats were obviously, if not increasingly, extensive. Marine incursions from both north and south were felt far into the continent's interior, causing 43

Lundberg et al. - The Stage for Neotropical Fish Diversification: A History of Tropical South American Rivers South America. Pp. 9-72 in: G. Arratia (ed.), Contributions of southern South America to vertebrate paleontology. F. Pfeil. Mtinchen. Azpelicueta, M. M. 1994. Three East-Andean species of Diplomvstes (Siluriformes: Diplomystidae). Ichthyol. Explor. Freshwaters, 5(3): 223-240. Audemard, F., I. Atzpiritxaga, P. Baumann, A. !sea & M. Latreille. 1985. Marco geologico de! Terciario de la faja petrolifera del Orinoco, Venezuela. VI Congreso Geologico Venezolano, 70109. Bemerguy, R. L. & J.B. Sena Costa. 1991. Considerai;:oes sobre a evolui;:ao do sistema de drenagem da Amazonia e sua relai;:ao com o arcaboui;:o tectonico-estrutural. Mus. Paraense Emilio Goeldi, Ser. Cicncias da Terra. 3: 75-97. Bengtson, P. & E. A. M. Koutsoukos. 1991. Ammonite and forminiferal dating of the first marine connection between the central and south Atlantic. 1er Collogue Stratigraphic et Paleogeographie des bassins sedimentaires ouest-Africans, 11 e Coll. Africain de Micropaleont. 1991, Abstracts. Bigarella, J. J. 1973. Geology of the Amazon and Parnafba basins. Pp. 26-86 in: A. E. M. Nairn & F. G. Stehli (eds.), The Ocean Basins and Margins, The South Atlantic. Plenum Press, New York. Vol. I. Boltovskoy, E. 1991. Ihering's hypothesis in the light of foraminiferological data. Lethaia, 24: 191-197. Brooks, D. R., T. B. Thorson & M. A. Mayers. 1981. Freshwater stingrays (Potamotrygonidae) and their helminth parasites: testing hypotheses of evolution and coevolution. Pp. 147-175 in: V. A. Funk. & D. R. Brooks (eds.), Advances in Cladistics. New York Botanical Garden. New York. Bussing, W. A. 1985. Patterns of distribution of the Central American ichthyofauna. Pp. 453-473 in: F. G. Stehli & S. D. Webb (eds.), The Great American Interchange. Plenum Pub!. Corp. New York. Butler, R. F., D. R. Richards, T. Sempere & L. G. Marshall. 1995. Paleomagnetic determinations of vertical-azis tectonic rotations from Late Cretaceous and Paleocene strata of Bolivia. Geology, 23: 799-802. Campbell, K, E, Jr., C. D. Frailey & L. J. Arellano. 1985. The geology of the rfo Beni: further evidence for Holocene flooding in Amazonia. Contrib. Sci., 364: 1-18. Cande, S. C. & D. V. Kent. 1992. A new geomagnetic polarity time scale for the Late Cretaceous and Cenozoic. J. Geophys. Res., 97: 13917-13951. Cande, S. C. & D. V. Kent. 1995. Revised calibration of the geomagnetic polarity timescale for the Late Cretaceous and Cenozoic. J. Geophys. Res., 100: 6093-6095. Coates, A. G. & J. A. Obando. 1996. The geologic evolution of the Central American isthmus. Pp. 21-56 in: J.B. C. Jackson, A. F. Budd & A. G. Coates (eds.), Evolution and Environment in Tropical America. University of Chicago Press. Cobbold, P. R., P. Szatmari, C. Lima & E. A. Rossello. 1996. Cenozoic deformation across South America: continent-wide data and analogue models. Trosieme Symposium International sur la Geodynamique Andine, Saint-Malo, p. 21-24. Coimbra, A. M., Riccomini, C. & Melo, M. S. 1983. A Formai;ao Itaquaquecetuba: evidencias de tectonismo no quaternario paulista. Simp6sio Regional de Geologia, 4, Sao Paulo, Atas, p. 253-266. Collins, L. S. 1996. Environmental changes in Caribbean shallow waters relative to closing of the Tropical American Seaway. Pp. 130-167 in: J. B. C. Jackson, A. F. Budd & A. G. Coates (eds.).

fishes in the Neotropics. Of course, the "complete" story of the rivers, lakes and other aquatic habitats will never be known but Earth scientists and biogeographers no doubt will continue to generate new knowledge, refined models and predictive hypotheses. Those interested in the evolutionary diversification and biogeography of the Neotropical aquatic biota must be aware of the facts and the uncertainties surrounding the history of watersheds because these were the stage for biotic evolution. We fully agree with Vari (1988: 370) that in light of South America's rich history, informative "geomorphological events are potentially very numerous and could allow us to date a number of vicariance events within a phyletic scheme" but that it will be difficult to retrieve the "vast majority" of these correlated events. Difficult, yes, but well worth the search because this integrative historical approach considers both pattern and process. Furthermore, in addition to identifying the singular events that caused vicariance, an effort needs to be made toward identifying geomorphological events that promoted other biogeographically important processes such as biotic merging and enrichment, extension, and extirpation.

Acknowledgments We extend thanks to the many people who provided a tremendous variety of information, leads to the vast literature on the geological history of South America, and critical insights in the life and Earth sciences. In particular, we are most grateful to R. F. Butler, C. Hoorn, P. Potter, M. E. Rasanen, R. Reis, D. Richards, D. Rosetti, J. Salo, G. Sarmiento and T. Sempere. We thank M. J. Weitzman for permission and encouragement to use her South American drainage map, and S. Merideth for her digital artwork. During the final stages of revising this manuscript we greatly benefited from listening and interacting with faculty and students participating in Dr. S. Beck's Andes seminar in the Department of Geosciences, University of Arizona. We assume full responsibility for any errors of fact and for all fanciful or frivolous interpretations.

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