Biogeographical and evolutionary considerations of Mauritia

REVIEW OF

PALAEOBOTANY AND

PALYNOLOGY E L S E V I E R Review of Palaeobotany and Palynology 100 (1998) 109 122

Biogeographical and evolutionary considerations of Mauritia (Arecaceae), based on palynological evidence

Valent i Rul l *

MARAVEN, Exploration Dep., Geological Serv. (Biostratigraphy), PO Box 829, Caracas, IOIO-A, Venezuela

Received 27 May 1997; received in revised form 21 October 1997; accepted 10 November 1997

Abstract

The available palynological evidence allows reconstruction of the principal trends in the historical biogeography and evolution of the neotropical palm genus Mauritia. Its pollen is known from the Palaeocene0 and has been widespread throughout the entire neotropical region, during the Tertiary. From the Late Miocene until the Pleistocene, populations have been progressively fragmented by the Andean orogeny, thus promoting local extinction in some smaller basins and allopatric speciation in others. During the Pleistocene, Mauritia survived the repeated dry and cool periods characteristic for glaciations either by restricting its distribution to assumed Amazon forest 'refugia' or by forming large-scale, heterogeneous forest communities. After the last glaciation, palynological studies document a broad dispersal process of Mauritia, leading to its present range. However, Holocene climatic shifts, together with human disturbance, created a heterogeneous distribution pattern with spatial and temporal differences within this process. Therefore, biogeographical inferences based on present-day observations can only account for the most recent changes and are not suitable for generalizations. The history of Mauritia derived from the fossil record suggests opportunities for both vicariance and allopatric speciation, and reveals dispersal patterns through time. The presence of Mauritia in sediments can also be used as a reliable indicator of warm tropical lowland environments flooded by fresh (occasionally oligohalyne) waters. Several hypothesis about its biogeographical history and its palaeoecological implications are offered that can be tested with future studies. © 1998 Elsevier Science B.V. All rights reserved.

Keywords." Mauritia; palynology; biogeography; evolution; Neotropics

1. Introduction

The purpose o f this paper is to discuss evolutionary and biogeographical aspects o f the palm genus Mauritia in light o f palynological fossil evidence, and to propose several hypotheses regarding its biogeographical and ecological history. The discus- sion addresses three main time and space domains

* E-mail: [email protected]

0034-6667/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII: 0034-6667(97)00060-2

(Delcour t et al., 1983): megascale (plate tectonics and evolution); macroscale (Pleistocene glaciations); and microscale (minor climate shifts and human disturbance). In general, the more recent the fossil evidence, the more detailed the inferences on the past, but small-scale events are not exclusive o f recent times. This is a bias introduced by methodological factors, but short- term processes such as for example ecological succession must have occurred at any time (Margalef , 1986; Rull

110 Ic Rull / Review of Palaeobolany and Palynology 100 (1998) 109 122

and Vegas-Vilarrfibia, 1991). Conversely, small- scale events frequently obey the rules of forcing agents of higher hierarchy, which determine the major trends and rhythms of their behaviour (Rull, 1991a). Therefore, the three domains considered in the present work are and have been present and overlapping through time.

The review begins with the present geographical distribution of Mauritia and the main hypotheses derived from it. Emphasis is placed on the northern and eastern parts of its present range, which have been subjected to intense geological and environmental changes, and from which most of the evidence comes. Then, the palynological evidence is reviewed according to the three domains mentioned. They are arranged in a time-descendent order and relate to the main geological and environmental events which could have been significant for the evolution of Mauritia. Finally, the more reliable conclusions derived from the analysed evidence are presented, and suggestions are made for further studies for a better understanding of Mauritia evolution and biogeography. From a taxonomic point of view, the classification of Mauritia and its species is beyond the objectives of the present work.

2. The modern distribution of Mauritia

At present, Mauritia palms are mostly restricted to the lowlands of the Amazon and Orinoco basins between 10 '~ latitudes on each side of the Equator (Fig. 1). Mauritia is absent from the native flora of Central America and the Caribbean islands, although it is sometimes cultivated (Grayum, pers. commun., 1996). There are about three species in Trinidad, Colombia, Ecuador, Peru, Venezuela, Guyana, Surinam, French Guiana and Brazil (Uhl and Dransfield, 1987). They grow in several vegetation types, ranging from coastal swamps to inland gallery forests (Lindeman, 1954; Muller, 1959; Huber, 1986; Gonzfilez, 1987; Tissot et al., 1988), and at elevations up to about 1000m (Huber, 1994). Among these, Mauritia f iexuosa L.f. is the most frequent and abundant, as component of mixed forests or in pure stands locally

called morichales (Colombia and Venezuela), vere- das or buritizales (Brazil), and aguajales (Peru). According to Gonz/tlez (1987), a tropical warm and humid climate with more than 1000 mm of annual precipitation, and permanently flooded soils are essential for its development. Muuritia does not grow in saline waters (Lindeman, 1954), its coastal representatives growing in back-mangrove zonal belts flooded by fresh or oligohalyne waters (Muller, 1959). These particular ecological requirements and its present geographical distribution (Fig. 1) has lead some workers to consider the Amazon Basin as the centre of genetic differentiation and of an assumed dispersion of this genus towards the peripheral lowlands of the basin (Gonzfilez, 1987). Van der Hammen (1957) proposed the Roraima region as the 'centre of origin" for several palms, including Mauritia. This region has been considered an important centre of origin for many plants (Pielou, 1979), and neotropical vertebrates (Miiller, 1973). These hypotheses derive from the idea that biogeographical ranges are the result of expansions from an assumed 'centre of origin' (in fact a 'centre of diversity'). Other authors think that the present geographical range of a given taxon could have been reached without any dispersal at all (Pielou, 1979). The first group ('dispersalists') believes that speciation follows the dispersal process, while the second ('panbiogeographers') considers that allopatric speciation, after the splitting of a former continuous distribution, is almost the only method of formation of new species (Croizat et al., 1974). The present distribution of M. macrocla&t, the only Mauritia species on the western side of the Andes (Hernfindez et al., 1992a,b), seems consistent with the 'panbiogeographical' point of view. Indeed, this species grows separated from the main distribution area by the Andean range, in the Colombian Choc6 area, which is an important centre of taxonomic endemism and has been considered a refuge for taxa of the humid lowlands (Hernfindez et al., 1992a,b). However, Mauritia is absent from other seemingly suitable adjacent areas, as for example the warm, humid and flooded lowlands of the Magdalena and Maracaibo basins, among others (Fig. 1). In Venezuela, there are some interesting patterns of distribution. Although

V. Rull / Review of Palaeobotany and Palynology 100 (1998) 109-122 111

Pacific

(i!i>7 . . . . . " . ,

Caribbean :INIDAD

~~~~-- ~ Atlantic

Solimoes /-

C , : : : .~ .......... 7 • , . j

'.,. ~

Amazon Basin

BRAZIL

10°N

~ Guiana Shield

Miocene marine connections

I~ Ternary records of M. franc/scoi

- - Tertiary Mauritiidites absent

Present boundary of M. flexuosa

~ Mauritia ma~lada

Fig. 1. Map of northern South America showing the present and past distribution of Mauritia, and the sites mentioned in the text. 1 = Falcon Basin; 2 = Mararacibo Basin; 3 = Catatumbo Delta; 4 = Magdalena Basin. The absence of Mauritia in the easternmost coasts of Brazil is based on Regali et al. (1974). Note that the total range of the genus coincides with the range of M. flexuosa, except for the isolated M. macroelada in the Choch6 region.

an accurate mapping of the range of Mauritia is not yet available, the palm is mainly distributed in the southern part of the Orinoco watershed and its delta, as well as in a small area in the southwestern Llanos (Fig. 2). Climate is a limiting factor for its occurrence along most of the Caribbean coasts (including northern Maracaibo lake and the Falc6n region), while elevation is a critical factor in more continental areas. These factors, however, can not explain the absence of Mauritia from most of the Llanos area (Fig. 2). The Orinoco river is very close to the northernmost edge of the geographical range of the palm, which roughly coincides with the northern edge of the Guiana Shield (Gibbs and Barron, 1983). The more distant occur- rences of Mauritia along the Orinoco course are around its delta and in the E1 Bat~l region (Fig. 2). A peculiarity of this area is the unique occurrence of igneous Paleozoic rocks outcropping in this flat savanna lowland on Plio-Pleistocene unconsoli- dated sediments (Gonz~ilez de Juana et al., 1980; Feo-Codecido et al., 1984). This fact, together

with the absence of Mauritia from the western and northernmost part of the Llanos, would suggest some geochemical constraint related to these igneous rocks, but this point has not been studied, and is not supported by the extensive occurrence of the genus in other areas devoid of igneous rocks. Alternatively, this might represent a relictual distribution pattern, prior to the establishment of the present course for the Orinoco river, or an interme- diate step in the hypothetical northward expansion of Mauritia populations.

In summary, according to the present knowl- edge, Maurit ia is absent from in several areas, even though the major environmental requirements for its occurrence are fulfilled. The more important ones are the Colombian Magdalena basin, and the Venezuelan Maracaibo basin (notably the flooded lowlands of the Cata tumbo delta, Fig. 2), as well as extensions into the Llanos. In the past, however, the geographical distribution of Mauritia has been different, and a historical analysis is essential to understand the present patterns.

112 V. Rull / Review (?/'Palaeobotany and Palynology 100 (1998) 109 122

Caribbean

~O

B / 1

Or inoco 5-~'~Delta

Colombia t

<1000 mm/year I (Veillon, 1989)

>1000 m a.s.I.

Guiana Shield

~ ~-~ M. flexuosa (GonzSlez, 1987)

• M. #exuosa (Vel~squez, 1994) O M. martiana

Phytocoenoses with Mauritia (Huber & Alarcbn, 1988) 1- open savannas (non-flooded)

- gallery forests ombrophUous forests and palm swamps

- shrubby savannas herb swamps

6- flooded ombrophilous forests 7- grass savannas

\ \

\

"~' .... OYY ; ;~ ; ; ,-~,III[ .........

1 1 1 1 1 1 1 1 1 / I / i / i I ~ I . " i ill/

i i i i i ~ i i I i i i i i I i i i i i ( i i i i i i i / i i i i i i i ~ i i i i / i i i i i i I i i , i i i i i i / i i ..... ,~ ....... ~ . , , ; ;.J~, . . . . . . . .

. i i i i i i i i / i i i i i i i i i i / •

. . . . . . . . . . . . Brazil

1 / 1 1 1 / / 1 1 1 1 1 1 1 1 1 I i

~ 1 / I / i i i i / i / i / / i / I/// / i i / / 1 / / I /

/ i / I / I I / I / / / I r ~ - - i / / ~ / / / / / / / / ~ / / / / /

i i i i i I I / ~ / / / / i

, i / / i / ,

/ / / / / . / / / /

i / / / / / / /

\ // " 1 1 / 1 1

/ r/ / / / / / ~ / /

/ I / / / i i / / i i 1 / / / / / / / / / / /

I i i i

Fig. 2. Map of the northernmost edge of the present-day distribution of Mauritia in Venezuela (Trinidad not included). CT= Catatumbo Delta; EB= El Baul region; GS=Gran Sabana. (Van der Hammen and Burger, 1966; Huber and Alarc6n, 1988; Veillon, 1989; Vehisquez, 1994).

3. The fossil pollen record of Mauritia

The earliest record of Mauritia pollen extends back to the Palaeocene. It appears near the Cretaceous/Tertiary (K/T) boundary (about 65 million years before present, or m.y.B.P.), and is represented by the fossil pollen form-species Mauritiidites J~'anciscoi ( Muller, 1970, 1980 ). This was originally described by Van der Hammen (1956) as Monocolpites franciscoi, but later changed to the genus Mauritiidites, by Van Hoeken-Klinkenberg (1964). The Mauritia pollen can also appear in the literature as Mauritiidites

franciscoi group, Mauritiidites sp., Mauritiidites, Mauritia-type, and Mauritia. It is widespread and common throughout the Tertiary (65 to 1.7 m.y. B.P.) in northern South American basins of Brazil/Colombia/Peru (Hoorn, 1993, 1994a,b), Colombia (Van der Hammen, 1956, 1957; Van der Hammen and Garcia de Mutis, 1966; Gonz~lez, 1967; Schuler and Doubinger, 1970; DuelSas, 1980; Duefias and Castro, 1981; Wijninga and Kuhry, 1990, 1993; Wijninga, 1996a,b), Guyana (Van der Hammen and Wijmstra, 1964; Van der Hammen and Burger, 1966; Leidelmeyer, 1966; Wijmstra, 1969, 1971 ), and Venezuela (Kuyl et al.,


1955; Fasola et al., 1985; Lorente, 1986; Muller et al., 1987; Boesi et al., 1988; Monroy and Van Erve, 1988; Colmenares and Ter/m, 1993; Rull, 1997). Evolutionary change is revealed by morpho- logic differences within M. franciscoi, which allows a subdivision into three varieties (Van der Hammen and Garcia de Mutis, 1966). Especially noteworthy is the change in aperture from the monocolpate (with a single, elongated aperture) forms characteristic of M. franciscoi, to the porate or ulcerate (with a single pore or ulcus) ones of the modern Mauritia flexuosa and other species. According to Sowunmi (1968), the present pores within Mauritia species represent shortened colpi. Unfortunately, the study of the fossil pollen genus Mauritiidites has not been sufficient to formally distinguish among different form-species (extinct or not), that would correspond to Mauritia species, in order to follow particular distribution patterns and evolutionary trends. This type of analysis would be important. However, it is widely accepted that M. franciscoi pollen has been produced by species of the genus Mauritia (Lorente, 1986), and that its morphological varieties probably represent real species. The only two living genera that produce similar pollen are Lepidocaryum and Mauritiella (Thanikaimoni, 1970; Erdtman, 1972; Muller, 1980; Ferguson, 1986), both of the same tribe as Mauritia (Lepidocaryoideae), and with similar general ecological requirements and distribution. Furthermore, Lepidocaryum has been considered a 'small Mauritia' (Braun and Delascio, 1987), and Mauritiella is included within the genus Mauritia by many plant taxonomists. Combining both plant taxonomy and pollen morphology, these three genera have been considered to belong to one single natural genus by Thanikaimoni (1970).

Concerning the taphonomy of Mauritia pollen, it has only local dispersal potential; that is, its presence in sediments is almost restricted to where this palm is growing. This low dispersal capacity has been found in studies on modern pollen sedi- mentation involving morichales from both coastal swamps (Muller, 1959) and inland fluvial valleys (Rull, 1991b, 1996b). Similarly, M. franciscoi has been found in Tertiary sediments accumulated in alluvial plains and coastal plain swamps, and is only rarely present in marine sediments (Muller,

1959; Lorente, 1986). Therefore, it can assumed that the presence of this pollen indicates the presence of Mauritia communities, at the time of deposition. In addition, both the absolute and the relative amounts of Mauritia pollen in sediments vary according to the abundance of the parent species in the community (Muller, 1959; Rull, 1991b), therefore, its pollen signal is a reliable indicator of its population density.

4. The Tertiary and the Andean orogeny

It is interesting to note the Tertiary occurrence of Mauritia pollen in areas where this taxon no longer occurs (Fig. 1). Besides the influence of climate and topography, the recent Andean uplift also played an important role producing isolation of lowland biota. Recent palynological findings support this view. According to the available geological evidence, the first orogenic pulses in the northern Andes began in the Late Oligocene (Hoorn et al., 1995), but they did not create especially high elevations, and the lowland connections of Caribbean and Pacific fluvial basins with the Amazon Basin seem to have persisted until a Late Miocene uplift, 10 Ma ago (Van der Hammen, 1988, 1989, pers commun.). The Colombian Andes did not attain present elevations until the beginning of the Pliocene, between 5 and 3 Ma (Van der Hammen, 1974, pers. commun.), and the Venezuelan Andes not until the Pleistocene (Giegengack, 1984). The orogenic events were the result of interactions between the Caribbean and Pacific plates with the South American Plate.

In the Late Oligocene/Early Miocene, two marine connections existed in the present western Amazon Basin, a Pacific one in the approximate location of the present Ecuadorian Andes, and a Caribbean one to the north at the present Maracaibo lake (Fig. 1; Hoorn, 1993; Hoorn et al., 1995). The main drainage patterns from the Guiana Shield were oriented to the north (the paleo-Orinoco River) and the west. During this time, lowland fluvial ecosystems dominated by palm swamps with Mauritia and tropical forests occupied the Amazon Basin, spiked with coastal mangroves with Rhizophora during episodic marine

114 V. Rull / Review of Palaeobotany and Palynology 100 (1998) 109 122

incursions (Hoorn, 1993, 1994a). These marine connections, together with the continuity of continental fluvial lowlands provided a suitable and continuous habitat for inland and coastal populations of Mauritia, which helps to explain the widespread occurrence and the abundance of M. franciscoi in the Miocene sediments of northern South America. The Pacific marine connection was broken in the Middle Miocene by the Andean uplift, and the drainage patterns began to shift towards the east, as in the present. A concomitant strong decrease of M. franciscoi can be observed in the Middle/Late Miocene pollen diagrams (Hoorn, 1993, 1994a,b), probably as the result of the disappearance of many lowland terrains, especially the coastal ones, and their substitution by mountain landscapes (Fig. 3). The persistence of inland Amazonian lowlands, however, prevented the total disappearance of Mauritia. The continuing uplift of the Andes also resulted in the progressive isolation of the Venezuelan and Colombian minor basins (Magdalena, Maracaibo, etc.) from the Amazon and Orinoco basins, and the disappearance of the Caribbean marine connection. This took place between the Late Miocene and the Pliocene, and was paralleled by a progressive disappearance of M. Ji'anciscoi. In the Magdalena Basin, this occurred in the Early/Middle Pliocene (Duefias and Castro, 1981; Wijninga, 1996a,b), whereas in the Maracaibo and Falc6n basins it must have occurred during the Pleistocene, because M. franciscoi was present in

the Pliocene (Lorente, 1986), but Mauritia is absent at present. This sequence of disappearance of M. franciscoi clearly parallels the Andean orogeny, suggesting that the progressive elimination of lowland and marine connections, as well as the isolation of minor basins by the emerging mountain barriers may have played an important role in its disappearance (Fig. 3). The ecological mech- anisms of this disappearance can only be assumed. On one hand, the new mountainous topography itself is not suitable habitat for Mauritia, but it also could have induced unsuitable environmental conditions (drier climates, lacking topography for flooding) in isolated lower basins. On the other hand, Mauritia did not disappear rapidly after isolation, and Pleistocene climatic oscillations could have produced the local elimination of its populations in these basins. Re-colonization from the main area of distribution--the Amazon and Orinoco basins--was prevented by mountain barriers. The local extinctions can also be viewed under the framework of the 'island biogeography' theory (MacArthur and Wilson, 1963, 1967), considering each isolated basin as an 'island' and the main distribution area the 'continent'. According to this scheme, the final number of species of an island is a balance between the rate of immigration of taxa from continental areas and the rate of extinction of those already there. An important aspect of this theory is that the number of species is a function of the area. The smaller the island, the smaller the population of each species it can

i S MB (VEN) ~~ '~ MDB (COL)

i

f

ISOLATION OF BASINS MDB MB

Pacific Caribbean CLOSSURE OF MARINE CONNECTIONS ~ . ~{. ~Lp t

¥ EARLY I MIDDLE i LATE EARLY LATE PLEISTO

. . . . . . , . . . . M,, C E . . . . , . . . . PL,O CENE CE.E my BP 20 15 10 5 0

Fig. 3. Tertiary time scale and occurrence patterns of Mauritia in relation with the main events connected to the Andean orogeny. MB= Maracaibo Basin; MDB= Magdalena Basin; S B - S o l i m o e s Basin; P l = C l o s u r e of Panama Isthmus.


support, and the greater the risk of extinction (Pielou, 1979). In our case, immigration of lowland elements from the 'continental' area was severely restricted, hence extinction must have been dominant. Mauritia seems to have been one of the victims in all these areas except for one-- the Colombian Choc6 area--where it still survives as an isolated species (Fig. 1). This argues in favour of allopatric speciation, after disconnection from the main area of distribution owing to the Andean uplift. The subsequent emergence of the Isthmus of Panama in the Early Pliocene provided a dispersal pathway for many taxa both to the north and south (Duque-Caro, 1990; Graham, 1992). However, in spite of favourable altitude and climate conditions (Graham, 1992), Mauritia did not cross this bridge as demonstrated by its present distribution and its absence from the Tertiary fossil record from Central America and Caribbean regions (Graham and Jarzen, 1969; Graham, 1976, 1985, 1987a,b, 1988a,b, 1989a,b,c,d, 1991a,b, pers. commun., 1996). There is no explanation sug- gested by fossil evidence for its restriction to South America, but by the time of the emergence of the Isthmus, the Magdalena Basin was already isolated (Fig. 3 ), and the Mauritia populations were probably shrinking, rather than expanding. Further- more, the region is close to the northern edge of the distribution of Mauritia, which would diminish its adaptive capacity and its competitive ability against other taxa better adapted to these latitudes.

Another consequence of the Andean orogeny is the shift in the course of the Orinoco River towards the east. This was initially proposed by Rod ( 1981 ) for Eocene and Oligocene times as a hypothesis to be tested. However, more recently available evidence indicates that this shift occurred later, between the Miocene and the Pleistocene. According to the reconstruction of Diaz de Gamero (1996), the proto-Orinoco river flowed towards the present Maracaibo Lake until the Oligocene. Later (Early Miocene), its delta was probably located in the Falc6n Basin, and the subsequent incipient uplift of the M6rida Andes during the Middle Miocene caused a significant shift towards the east. Finally, the Orinoco reached its present course in the Pleistocene, based on a reliable record of its potential delta in the eastern

Llanos during the Pliocene. Therefore, the occur- rences of Mauritia in the E1 Bafil region and the eastern Llanos could be relics of a former more widespread distribution prior to the establishment of the present course of the Orinoco River. If so, its absence from other adjacent areas with similar conditions, as for example the northern and western Llanos, remains to be explained.

5. The Pleistocene glaciations

During the Pleistocene (the last 1.7 m.y.) at least 30 glacial/interglacial cycles have been recorded worldwide (Mudie and Harland, 1996), causing profound biogeographical reorganizations. For example, the Last Glacial Maximum (LGM) occurred at about 18 ka (CLIMAP, 1976) and since then the temperate glaciated areas have been colonized by the modern vegetation through sequential invasions. Many communities were not entirely established in their present state until the last few hundred years. In the Neotropics, the Pleistocene glacial cycles have been documented by long pollen records in mountain areas (Hooghiemstra and Ran, 1994). Here, the glacial/interglacial alternation produced vertical displacements of mountain vegetation belts of 1000 m or more which strongly affected the evolution of high altitude ecosystems such as the p~r- amos and the tepuian summits producing alternating connection and isolation of their biota (Vuilleumier, 1979; Rull, 1991b, 1996b). In the lowlands, where Mauritia lives, the glacial/ interglacial sequence has been paralleled by drier/ wetter climatic alternation (Damuth and Fairbridge, 1970; Garner, 1975; Schubert, 1988). A review of the available information for the Amazon Basin (Van der Hammen and Absy, 1994) shows the consistent occurrence of two dry periods, one between 60 and 40 ka, and another between 22 and 14 ka; the last one being especially intense and coinciding with the maximum of the last glaciation. The longest pollen profile for the Amazon Basin extends back to about 60 ka (Absy et al., 1991), and records the occurrence of savannas during the dry periods and rain forests in the wetter ones. These results can be extrapolated to


former glacial phases, thus supporting the ~refuge' hypothesis, that involves the cyclic contraction of tropical forests into glacial 'refuges' surrounded by savannas and, in the subsequent interglacial, re-expansion in response to greater precipitation (Haffer, 1969, 1982).

According to Van der Hammen and Absy (1994), a reduction of 500 to 1000 mm in annual precipitation from the present regime is necessary to explain the occurrence of glacial-age savannas. Assuming a decrease of this magnitude, the existence of a major 'refuge' in the western Amazon Basin, and several others of medium size spread along the rest of the region, has been proposed by these authors. Other refuge areas based on the present vegetation patterns have been proposed (review in Prance, 1982). Mauritia has been an important component of the Amazon forests and has persisted during both wet and dry phases, although its abundance was notably reduced during the more dry periods (Absy and Van der Hammen, 1976). Thus, the distribution area of Mauritia would have been alternatively reduced and fragmented (dry glacials) and expanded and interconnected (humid interglacials) during the Pleistocene. Van der Hammen and Absy (1994) also suggest that the diversity of the present Amazon flora is lower than during the Miocene~ due to extinction during dry episodes. This could have been balanced, however, by comparatively low extinction rates and more intense speciation in the forest 'refuge' areas~ which also helps to explain the persistence of Mauritia through time.

In the same context, an interesting situation is that of the northernmost edge of distribution of this species beyond the Orinoco course (Fig. 2). In this region, the occurrence of fossil dunes of Late Pleistocene age (Roa, 1979) indicates the existence of desert climates unfavourable for Mauritia during the last glacial. This climate is assumed to have prevailed across the whole Llanos, including the Colombian areas (Schubert, 1988). Therefore, it is unlikely that Mauritia grew on these terrains, which must have been colonized by this palm the last glacial from its main distribution area to the south, or from small forest or morichal 'refuges'. If so, another possible explanation emerges for the absence of Mauritia in areas where

the ecological requirements are theoretically fulfilled: Mauritia simply had not yet arrived. In contrast to those basins isolated by the Tertiary Andean orogeny, these lowlands could be eventu- ally re-colonized by Mauritia. The apparent con- tradiction between this hypothesis and that of relictual distribution north of the Orinoco can not be solved without further studies. One important question is whether during arid phases the Orinoco existed and flowed through the same course as today.

The existence of refuges for the Amazon forests during the dry glacial periods, however, has been criticized by defenders of heterogeneous patterns, lacking sharp discontinuities (Connor, 1986; Colinvaux, 1987). In addition, they argue that the existence and location of the assumed 'refuges' is based on the modern diversity and endemism patterns and that palaeobotanical evidence for them is lacking. Some palaeoecological studies in areas formerly considered 'refugia' have supported this statement, by showing that diversity and endemism are not necessarily related to refuges (e.g. Bradbury et al., 1981 ; Schubert, 1988 ).

Whether the 'refuge' hypothesis is a realistic concept or not, two points can be made. One is that in spite of their intensity and duration, the Pleistocene climate changes did not eliminate Mauritia from the Amazon Basin. This seemingly conflicts with the assumption that climate changes did contribute to its local extinction in northwest- ern small basins (see above). However, two important differences exist, namely the size of the ~islands' (notably larger in the Amazon Basin) and the extent of the disconnection (in the case of the small basins) from the main distribution area as the potential source for new propagates. The second is that the existence of unfavourable areas for Mauritia (treeless savannas and deserts) during the glacial periods seems unquestionable; hence a certain degree of postglacial dispersal is necessary to explain the present-day occurrence of a single species (M. flexuosa) throughout the entire Amazon and Orinoco basins (Fig. 1 ). It is possible that Mauritia has progressively colonized formerly dry areas while they became gradually more humid, in a similar way than many trees of the northern hemisphere advanced northwards

ld Rull / Review of Palaeobotany and Palynology lO0 (1998) 109-122 117

following the progressive ice retreat (for example, Davis, 1976, 1981). More studies are necessary to document an expansion process of this nature.

6. Holocene climatic shifts and fires

The Holocene is characterized by climate shifts of lower intensity than the glacial ones, and by the impact of human activities on ecosystems. In the neotropical region, the available Holocene palynological data available on Mauritia indicate that the taxon has been consistently present, but its abundance has changed owing to environmental shifts. In the Atlantic Guianan coasts a marine regression accompanied by a progradation of the coast line towards the sea, occurred around 5 ka resulting in an increase of back-mangrove communities, of which Mauritia is a common component (Van der Hammen, 1963; Wijmstra, 1971; Tissot et al., 1988). Further inland, in the Amazon Basin, the changing percentages of Mauritia pollen in the diagrams are considered indicating successive expansion or shrinking of its stands in response to increased or reduced flooding (Wijmstra and Van der Hammen, 1966; Absy, 1985; Colinvaux, 1987; Colinvaux et al., 1988; Liu and Colinvaux, 1988). For example, a major decline in Mauritia pollen was recorded in the Ecuadorian Amazon Basin owing to the creation of non flooded terra firme forest, that replaced the former wet palm forest, owing to a drier climate, some 800 years ago (Colinvaux et al., 1988). This decrease in moisture was also reported in the Venezuelan Gran Sabana (Rull, 1991b) and thus was extensive. In the same region, the repeated alternation of humid and dry phases favoured the expansion of Mauritia stands or morichales, which lead to progressive reduction of the lowland forests (Rull, 1991b).

The Holocene climate oscillations reported in northern South America, however, shows notable temporal and geographical heterogeneity (Markgraf and Bradbury, 1982; Rull, 1996a); therefore, a single general expansion or reduction trend for Mauritia during this time seems unlikely. On the other hand, not all variations in the Mauritia pollen amounts should be interpreted in terms of climate. For example, in the Peruvian

Amazon Basin high rates of lateral migration of the river courses have been reported, owing to active tectonic tilting linked to the Andean uplift (Salo et al., 1986). In these areas, many abandoned river courses and meanders are colonized by the so-called aguajales, or pure stands of Mauritia flexuosa (Puhakka and Kalliola, 1993). Finally, the expansion of Mauritia also can be favoured by the disturbance of original vegetation, if flooding is guaranteed. Indeed, in one particular site of the Gran Sabana (Uru6), instead of decreasing, Mauritia increased spectacularly during the last millennium in the absence of significant climate changes (Fig. 4). Independent evidence showed that this site had been burnt since before about 1.4 ka, and the former mixed gallery forest was eliminated and replaced by secondary vegetation and treeless savanna. A subsequent humidity increase produced the growth of a morichal instead of a potential mixed forest recovery (Rull, 1992).

Although the palaeovegetational evidence is not sufficient for a conclusive assessment, it is remark- able that the first Holocene record of Mauritia in the Gran Sabana dates from about 5 ka, but that it did not attain the pollen abundance typical for the present morichales after about 2 ka (Fig. 4; Rull, 1991b, 1992). The values of Mauritia pollen before this date resemble those of mixed forests, in which Mauritia is a minor component. The removal of these forests by an extensive arid phase, followed by a humidity increase, allowed the establishment of the morichales. Hence, these communities seem to be of recent origin in this area, an interesting aspect, considering that this region, the Roraima region, is the assumed 'centre of origin' for Mauritia, and almost the centre of its present distribution (Fig. 1 ). These palynological data suggest that Mauritia could had been dispersed formerly through an expansion of those forests, and that the morichales developed later as secondary colonizing communities after dry oscillations or disturbance. This would support the view of GonzS.lez (1987), who considers M. flexuosa a colonizer species. The same author proposed a successional process based on the observation of present spatial patterns of the communities with M. flexuosa, culminating in a hypothetical stabi- lized evergreen swamp forest. This Clementsian


5

Eo e -

"~ 3 A 0 0 o 2

1

0

I

' t _ ° . , .ATE] -P t . . . . 1 ° " .or . . . . . . . . . . L -P.esent 1

i

i ST/\, = i _J--_ = - J ' / j V ~ , . . . . ~ \

i - - ~ - , i - - i - - i ~ i i i

6 5 4 3 2 1 0

ka BP

Fig. 4. Influx index (grains per unit area per time) of the pollen of Mauritia in three sites from the Gran Sabana (Venezuela) during the last 6 ka. ST= Santa Teresa; D V= Divina Pastora; UR = Uru6. Humidity trends were deduced from lake-level fluctuations; see Rull (1991b, 1992) for details and data.

(Clements, 1916) approach has not been supported so far (Rull and Vegas-Vilarrfibia, 1991; Rull, 1992), but palynological studies available to date are not conclusive. Similarly, more studies on the arrival and increase of Mauritia compared to other present-day morichal areas are needed.

7. General conclusions and some proposals

Explaining biogeographical patterns of Mauritia through time is not a simple matter, because diverse forcing factors acting at different spatial and temporal scales need to be considered. Mauritia seems to be an old element of the Neotropical flora, and to trace its history it is necessary to consider megascale, macroscale and microscale factors (sensu Delcourt et al., 1983). Andean orogeny linked to plate tectonics, sea-level oscillations, glaciations, climatic oscillations and human activities have resulted in varying geographical patterns for the fairly constant habitat of this palm. Therefore, any extrapolation of the biogeography of Mauritia based on present observations would probably account only for the most recent trends.

Globally speaking, the modern distribution of Mauritia seems to have been the result of the

reduction and splitting of its former geographical range, rather than expansion. However, the hypothesis of a recent postglacial expansion trend seems plausible in the light of the available evidence. Hence, both dispersal and fragmentation, followed by allopatric speciation, are probably needed to explain the evolutionary and biogeographical trends of Mauritia. With the available evidence, evolutionary changes are difficult to trace, but a careful morphological characterization of the different Tertiary and Pleistocene pollen types, their geographical extension and strati- graphic range could be very useful in this respect. So far, allopatric speciation owing to the Andean uplift is strongly supported by palynological studies, whereas a hypothetical Tertiary evolution of Mauritia during an assumed general spreading has not yet been recorded in the fossil record.

On the other hand, the habitat consistency recorded through time by Mauritia suggests that it is an important indicator of past conditions in the lowland Neotropics, in a somewhat inverse sense of other elements such as for example Alnus or Quercus, which occurrence has been used as indication of the presence of high elevations (Van der Hammen, 1988). The presence of Mauritia pollen can be considered a reliable indicator of flooded, tropical lowlands, either along the coast

v. Rull / Review of Palaeobotany and Palynology 100 (1998) 109 122 119

(in c o m b i n a t i o n with m a n g r o v e pol len, as for example Rhizophora) or a long rivers and small wate r currents (co-occur r ing with pol len o f G r a m i n e a e or in land forest trees). Similar ly , since the t o d a y Mauri t ia species are d i s t r ibu ted on ei ther sides o f the e q u a t o r to 10 ° la t i tude , their fossil pol len could be used as a re l iable pa l aeo l a t i t ude indica tor . The c o n c o m i t a n t d i s appea rance o f Maurit i idi tes and the first occurrence o f Alnus in the no r the rn A n d e s suppor t s this view. In this sense, the b ios t r a t ig raph ic significance o f the last occurrence o f Maurit i idi tes should be eva lua ted , with special care to its t iming with respect to the A n d e a n o rogeny

Based on the ava i lab le evidence, the b iogeo- g raph ica l t rends o f Mauri t ia can only be roughly t raced, and a n u m b e r o f aspects a b o u t its h i s tory remain still unclear . Accu ra t e m a p p i n g o f the present d i s t r ibu t ion o f the Mauri t ia species is essential . In add i t ion , i m p o r t a n t aspects w o r t h y o f pa lyno log ica l inves t iga t ion are: (1) m o r p h o l o g i c a l d i f ferent ia t ion o f several Ter t i a ry form-species wi th t a x o n o m i c and evo lu t iona ry significance; (2) a m o r e de ta i led recons t ruc t ion o f the d i s appea rance o f Mauri t ia pol len f rom nor thwes te rn A n d e a n basins, dur ing the Tert iary; (3) the influence o f g lac ia l / in terg lac ia l (dr ier /wet ter ) cycles on Mauri t ia popu la t ions ; (4) the hypo the t i ca l sequence o f r e -co lon iza t ion (as well as o f eventual significant increases) o f the A m a z o n and Or inoco lowlands by Mauri t ia after the last g lacia t ion; (5) the de ta i led recons t ruc t ion o f the successional t rends in communi t i e s in which Mauri t ia occurs or cou ld occur theoret ica l ly ; and (6) the influence o f c l imat ic and h u m a n d i s tu rbance on the t axon ' s d i s t r ibu t ion .

F ina l ly , it is also wor th men t ion ing tha t bo th the h is tor ica l t rends and the p r o p o s e d fur ther s tudies on Mauri t ia might be val id for o the r t axa with s imilar env i ronmen ta l requirements . Ecologica l p h e n o m e n a take place at the commun i ty level, and c o n c o m i t a n t local ext inct ions, spec ia t ion processes, and ecological r eorgan iza - t ions in o the r l owland elements are expected.

Acknowledgements

The a u t h o r is very grateful to A l a n G r a h a m and T h o m a s van der H a m m e n for cri t ical revision, and

i m p o r t a n t c o m m e n t s and suggest ions. The com- ments o f two u n k n o w n referees were also helpful to improve the manuscr ip t . Mike G r a y u m pro- v ided useful b iogeograph ic in fo rmat ion . The edi- t ion facilit ies were p rov ided by M A R A V E N .

References

Absy, M.L., 1985. Palynology of Amazonia: The history of the forests as revealed by the palynological record. In: Prance, G.T., Lovejoy, T.E. (Eds.), Key environments Amazonia. Pergamon Press, Oxford, pp. 72-82.

Absy, M.L., Van der Hammen, T., 1976. Some paleoecological data from Rondonia, southern part of the Amazon Basin. Acta Amazon. 6 (3), 293 299.

Absy, M.L., Cleef, A.M., Fournier, M., Martin, L., Servant, M., Sfeddine, A., Ferreira da Silva, M.F., Soubies, F., Suguio, K., Turq, B., Van der Hammen, T., 1991. Mise en 6vidence de quatre phases d'ouverture de la for6t dense dans le sud-est de l'Amazonie au cours des 60.000 derni~res ann6es. C. R. Acad. Sci. Paris Ser. II 312, 673 678.

Boesi, T., Galea, F., Rojas, G., Lorente, M.A., Durfi, n, I., Velasquez, M., 1988. Estudio estratigrS.fico del flanco noran- dino en el sector Lobatera-El Vigia. Memorias Ill Simposio Bolivariano, Exploraci6n de las Cuencas Subandinas, pp. 2-41.

Bradbury, J.P., Leyden, B., Salgado-Labouriau, M.L., Lewis Jr., W.M., Schubert, C., Nenford, M.W., Frey, D.G., White- head, D.R., Weibezahn, F.H., 1981. Late Quaternary environmental history of Lake Valencia, Venezuela. Science 214, 1299 1305.

Braun, A., Delascio, F., 1987. Palmas aut6ctonas de Venezuela y de los paises adyacentes. Martinez, Caracas. 156 pp.

Clements, F.E., 1916. Plant succession, an analysis of the development of vegetation. Carnegie Inst., Washington.

CLIMAP, 1976. The surface of Ice-Age Earth. Science 191, 1131 1137.

Colinvaux, P.A., 1987. Amazon diversity in the light of the paleoecological record. Quat. Sci. Rev. 6, 93 114.

Colinvaux, P.A., Frost, M., Frost, I., Liu, K.B., Steinitz- Kannan, M., 1988. Three pollen diagrams of forest disturbance in the Western Amazon Basin. Rev. Palaeobot. Palynol. 55, 73 81.

Colmenares, O.A., Terfin, L., 1993. A biostratigraphic study of Paleogene sequences in southwestern Venezuela. Palynology 17, 67-89.

Connor, E.F., 1986. The role of Pleistocene forest refugia in the evolution and biogeography of tropical biota. Trends Ecol. Evol. 1 (6), 165-168.

Croizat, L., Nelson, G.J., Rosen, D.E., 1974. Centers of origin and related concepts. Syst. Zool. 23, 25 287.

Damuth, J.E., Fairbridge, R.W., 1970. Equatorial atlantic deep- sea arkosic sands and Ice-Age aridity in Tropical South America. Geol. Soc. Am. Bull. 81, 189 206.

120 V. Rull / Review ~[ Palaeobotany and Palynology 1 O0 (1998) 109-122

Davis, M.B., 1976. Pleistocene biogeography of temperate deciduous forests. Geosci. Man 13, 13-26.

Davis, M.B., 1981. Quaternary history and the stability of forest communities. In: West, D.C., Shugart, H.H., Botkin, D.B. (Eds.), Forest Succession Concepts and Applications. Springer, New York, NY, pp. 132-153.

Delcourt, H.R., Delcourt, P.A., Webb lII, T., 1983. Dynamic plant ecology: The spectrum of vegetational change in space and time. Quat. Sci. Rev. 1, 153 175.

Diaz de Gamero, M.L., 1996. The changing course of the Orinoco river during the Neogene: a review. Palaeogeogr., Palaeoclimatol., Palaeoecol. 123, 385-402.

Duefias, H., 1980. Palynology of Oligocene-Miocene strata of borehole Q-E-22, Planeta Rica, Northern Colombia. Rev. Palaeobot. Palynol. 30, 313 328.

Duefias, H., Castro, G., 1981. Asociacion palinol6gica de la Formaci6n Mesa en la region de Falan Tolima Colombia. Geol. Norandina 3, 27-36.

Duque-Caro, H., 1990. Neogene stratigraphy, paleoceanog- raphy and paleobiogeography in northwest South America and the evolution of the Panama Seaway. Palaeogeogr., Palaeoclimatol., Palaeoecol. 77, 203 234.

Erdtman, G., 1972. Pollen Morphology and Plant Taxonomy. Angiosperms. Harrier, New York, NY, 553 pp.

Fasola, A., Giffuni, G., Crespo, S., Paredes, I., Euribe, A., 1985. Estudios bioestratigr~.ficos del intervalo Cretaceo Superior (Maestrichtiense) a Mioceno Inferior en el norte del Estado Gu~,rico, Venezuela. Memorias VI Congreso Geol6gico Venezolano, pp. 587 645.

Feo-Codecido, G., Smith Jr.~ F.D., Aboud, N., DiGiacomo, E., 1984. Basement and Paleozoic rocks of the Venezuelan Llanos basins. Geol. Soc. Am. Mere. 162, 175 187.

Ferguson, I.K., 1986. Observations on the variations in pollen morphology of Palmae and its significance. Can. J. Bot. 64, 3079-3090.

Garner, H.F., 1975. Rain forests, deserts and evolution. An. Acad. Brazil. Cienc. 47, 127 133.

Gibbs, A.K., Barron, Ch.N., 1983. The Guiana Shield reviewed. Episodes 2, 7-14.

Giegengack, R., 1984. Late Cenozoic tectonic environments of the Central Venezuelan Andes. Geol. Soc. Am. 162, 343 364.

Gonzb_lez, A.E., 1967. A palynological study on the upper Los Cuervos and Mirador formations (lower and middle Eocene; Tibu area, Colombia). Brill, Leiden, 68 pp.

Gonza, lez, V., 1987. Los morichales de 1o Llanos orientales. Un enfoque ecol6gico. CORPOVEN, Caracas, 35 pp.

Gonz~lez de Juana, C., lturralde, J.M., Picard, X., 1980. Geo- logia de Venezuela y de sus cuencas petroliferas (vol. l, II ). FONINVES, Caracas, 1031 pp.

Graham, A., 1976. Studies in neotropical paleobotany. II. The Miocene communities of Veracruz Mexico. Ann. Mo. Bot. Gard. 66, 572 576.

Graham, A., 1985. Studies in neotropical paleobotany. IV. The Eocene communities of Panama. Ann. Mo. Bot. Gard. 72, 504 534.

Graham, A., 1987a. Miocene communities and paleoenvironments of Southern Costa Rica. Am. J. Bot. 74, 1501 1518.

Graham, A., 1987b. Tropical American Tertiary floras and paleoenvironments: Mexico, Costa Rica and Panama. Am. J. Bot. 74, 1519 1531.

Graham, A., 1988a. Studies in neotropical paleobotany. V. The lower Miocene communities of Panama the Culebra Formation. Ann. Mo. Bot. Gard. 75, 1440 1466.

Graham, A., 1988b. Studies in neotropical paleobotany. VI. The lower Miocene communities of Panama the Cucaracha Formation. An. Mo. Bot. Gard. 75, 1467 1479.

Graham, A., 1989a. Paleofloristic and paleoclimatic changes in the Tertiary of northern Latin America. Rev. Palaeobot. Palynol. 60, 283 293.

Graham, A., 1989b. Late Tertiary paleolatitudes and vegetation zonation in Mexico and central America. Acta Bot. Neerl. 38 (4), 417 424.

Graham, A., 1989c. Lower Miocene floras and biogeography of central America. J. Geogr. Soc. Jamaica 25, 8 15.

Graham, A., 1989d. Studies in neotropical paleobotany. VII. The Lower Miocene communities of Panama the La Boca Formation. An. Mo. Bot. Gard. 76, 50 66.

Graham, A., 1991a. Studies in neotropical botany, VIII. The Pliocene communities of Panama - Introduction and ferns, gymnosperms and angiosperms (monocots). Ann. Mo. Bot. Gard. 78, 190 200.

Graham, A., 1991b. Studies in neotropical botany, X. The Plio- cene communities of Panama - Composition, numerical rep- resentations, and palaeocommunity palaeoenvironmental reconstructions. Ann. Mo. Bot. Gard. 78, 465 475.

Graham, A., 1992. Utilization of the isthmian land bridge during the Cenozoic paleobotanical evidence for timing, and the selective influence of altitudes and climate. Rev. Palaeobot. Palynol. 72, 119 128.

Graham, A., Jarzen, D.M., 1969. Studies in neotropical paleobotany. I. The Oligocene communities of Puerto Rico. An. Mo. Bot. Gard. 56, 308-.357.

Hailer, J., 1969. Speciation in Amazonian forest birds. Science 165, 131 137.

Haffer, J., 1982. General aspects of the refuge theory. In: Prance, G.T. (Ed.), Biological Diversification in the Tropics. Columbia Univ. Press, New York, NY, pp. 6-24.

Hern~,ndez, J., Hurtado, A., Ortiz, R., Walschburger, T., 1992a. Centros de endemismo en Colombia. In: Halffter, G. (Ed.), La Diversidad Biologica de Iberoam6rica. Inst. Ecologia, A.C., M6xico.

Hern~mdez, J., Walschburger, T., Ortiz, R., Hurtado, A., 1992b. Origen y distribucion de la biota suramericana y colombiana. In: Halffter, G. (Ed.), La Diversidad Biologica de Iberoam- 6rica. Inst. Ecologia, A.C., M6xico.

Hooghiemstra, H., Ran, E.T.H., 1994. Late Pliocene Pleisto- cene high resolution pollen sequence of Colombia: an over- view of climatic change. Quat. Int. 21, 63-80.

Hoorn, C., 1993. Marine incursions and the influence of Andean tectonics on the Miocene depositional history of Northwest- ern Amazonia: results of a palynostratigraphic study. Palaeo- geogr.~ Palaeoclimatol., Palaeoecol. 105, 267 309.

Hoorn, C., 1994a. Fluvial palaeoenvironments in the intracra- tonic Amazon Basin (Early Miocene to early Middle Mio-


cene Colombia). Palaeogeogr., Palaeoclimatol., Palaeoecol. 109, 1-55.

Hoorn, C., 1994b. An environmental reconstruction of the palaeo-Amazon River system (Middle-Late Miocene NW Amazonia). Palaeogeogr., Palaeoclimatol., Palaeoecol. 112, 187-238.

Hoorn, C., Guerrero, J., Sarmiento, G.A., Lorente, M.A., 1995. Andean tectonics as a cause for changing drainage patterns in Miocene northern South America. Geology 23 (3), 237 240.

Huber, O., 1986. La vegetaci6n de la Cuenca del Rio Caroni. Interciencia 11 (6), 301 310.

Huber, O., 1994. Sabanas y herbazales de la Gran Sabana. In: Dezzeo, M. (Ed.), Ecologia de la altiplanicie de la Gran Sabana (Guayana Venezolana). I. Investigaciones sobre la dinamica bosque-sabana en el sector SE: subcuencas de los rios Yuruani Arabop6 y Alto Kuken~m. Sci. Guayanae 4, 106 115.

Huber, O., Alarc6n, C., 1988. Mapa de vegetaci6n de Venezuela (esc. 1:2.000.000). MARNR, Caracas.

Kuyl, O.S., Muller, J., Waterbolk, H.Th., 1955. The application of palynology to oil geology with reference to western Vene- zuela. Geol. Mijnbouw 3 (17), 49 76.

Leidelmeyer, P., 1966. The Paleocene and Lower Eocene pollen flora of Guyana. Leidse Geol. Meded. 38, 49 70.

Lindeman, J.C., 1954. The vegetation of the coastal region of Surinam. Van Eedenfonds, Amsterdam, 135 pp.

Liu, K.B., Colinvaux, P.A., 1988. A 5200-year history of Amazonian rain forest. J. Biogeogr. 15, 231 248.

Lorente, M.A., 1986. Palynology and palynofacies of the Upper Tertiary in Venezuela. Diss. Bot. 99, 222

MacArthur, R.H., Wilson, E.O., 1963. An equilibrium theory of insular zoogeography. Evolution 17, 373-387.

MacArthur, R.H., Wilson, E.O., 1967. The theory of Island Biogeography. Princeton Univ. Press, Princeton, 203 pp.

Margalef, R., 1986. Sucesi6n y evoluci6n: su proyecci6n biogeogr~tfica. Paleontol. Evol. 20, 7 26.

Markgraf, V., Bradbury, J.P., 1982. Holocene climatic history of South America. Striae 16, 40 45.

Monroy, Z., Van Erve, A., 1988. Revisi6n palinoestratigrfi, fica del Cret~icico y Terciario de Apure (Venezuela surocciden- tal). Memorias lIl Simposio Bolivariano, Exploraci6n Petrolera de las Cuencas Subandinas, pp. 143-167.

Mudie, P.J., Harland, R., 1996. Aquatic Quaternary. In: Jan- sonius, J., McGragro, D.C. (Eds.), Palynology, Principles and Applications. AASP Foundation, Salt Lake City, pp. 843 877.

Muller, J., 1959. Palynology of recent Orinoco delta and shelf sediments. Micropalaeontology 5, 1-32.

Muller, J., 1970. Palynological evidence on early differentiation of angiosperms. Biol. Rev. 45, 417-450.

Muller, J., 1980. Fossil pollen records of extant angiosperms. Bot. Rev. 47( l ) , 1 142.

Muller, J., Di Giacomo, E., Van Erve, A.W., 1987. A palynological zonation for the Cretaceous, Tertiary, and Quaternary of Northern South America. AASP Contrib. Ser. 19, 7-76.

Mt~ller, P., 1973. The dispersal centres of terrestrial vertebrates in the neotropical realm. Biogeographica 2 (10), 52-198.

Pielou, E.C., 1979. Biogeography. Wiley, New York, NY, 351

PP. Prance, G.T. (Ed.), 1982. Biological Diversification in the Trop-

ics. Columbia Univ. Press, New York, NY. Puhakka, M., Kalliola, R., 1993. La vegetaci6n en ~.reas de

inundacion en la selva baja de la Amazonia peruana. In: Kalliola, R., Puhakka, M., Danjoy, W. (Eds.), Amazonia perunana-vegetaci6n ht~meda tropical en el llano subandino. PAUT and ONERN, Jyv/iskyl~,, pp. 113-138.

Regali, M., Uesugui, N., Santos, A., 1974. Palinologia dos sedi- mentos meso-cenoz6icos do Brasil (I, II ). Bol. T6c. PETRO- BRAS 17(3), 177 191,263 301.

Roa, P., 1979. Estudio de los medanos de los Llanos centrales de Venezuela: evidencias de un clima des6rtico. Acta Biol. Venezuel. 10, 19 49.

Rod, E., 1981. Notes on the shifting course of the ancient Rio Orinoco from Late Cretaceous to Oligocene time. GEOS 26, 54 56.

Rull, V., 1991a. Quaternary palaeoecology and ecological theory. ORSIS5,91 111.

Rull, V., 1991b. Contribuci6n a la paleoecolgia de Pantepui y la Gran Sabana (Guayana Venezolana): clima, biogeografia y ecologia. Sci. Guayanae 2, 133

Rull, V., 1992. Successional patterns of the Gran Sabana (south- eastern Venezuela) vegetation during the last 5000 years, and its responses to climatic fluctuations and fire. J. Biogeogr. 19 (3), 228-239.

Rull, V., 1996a. Late Pleistocene and Holocene climates of Venezuela. Quat. Int. 31, 85-94.

Rull, V., 1996b. Holocene vegetational succession on the Guaiquinima and Chimanttk massifs (SE-Venezuela). lnterciencia 21 ( 1 ), 7 20.

Rull, V., 1997. Sequence analysis of western Venezuelan Cretaceous to Oligocene sediments using Palynology: chrono-paleoenvironmental and paleovegetational approaches. Palynology 21, 79-90.

Rull, V., Vegas-Vilarrflbia, T., 1991. A paleolimnological example of modulated succession from Southeast Venezuela, and its significance in Pleistocene Palaeoecology. Curr. Res. Pleis- tocene 8, 81 83.

Salo, Y., Kalliola, R., H~kkinen, 1., M~kinen, Y., Niemel~, P., Puhakka, M., Coley, P., 1986. River dynamics and the diversity of Amazon lowland forest. Nature 322, 254 258.

Schubert, C., 1988. Climatic changes during the Last Glacial Maximum in Northern South America and the Caribbean: a review, lnterciencia 13 (3), 128 137.

Schuler, M., Doubinger, J., 1970. Observations palynologiques dans le bassin d'Amaga (Colombie). Pollen Spores Xll (3), 429-450.

Sowunmi, M.A., 1968. Pollen morphology in the Palmae, with special reference to trends in aperture development. Rev. Palaeobot. Palynol. 7, 45 53.

Thanikaimoni, G., 1970. Pollen morphology, classification and phylogeny of Palmae. Adansonia Ser. 2 10 (3), 347 365.

Tissot, C., Djuwansah, M.R., Marius, C., 1988. Evolution de

122 IC Rull / Review c~['Palaeobotany and Palynology 100 (1998) 109 122

la mangrove en Guyane au cours de l'Holocene. Etude paly- nologique. Inst. Fr. Pondichdry Tray. Sect. Sci. Tech. XXV, 125 137.

Uhl, N.W., Dransfield, J., 1987. Genera Palmarum. Allen Press, 600 pp.

Van der Hammen, T., 1956. Description of some genera and species of fossil pollen and spores. Bol. Geol. 2:3, 111 117.

Van der Hammen, T., 1957. Climatic periodicity and evolution of South American Maestrichtian and Tertiary floras. Bol. Geol. 2, 49 91.

Van der Hammen, T., 1963. A palynological study on the Qua- ternary of British Guiana. Leidse Geol. Meded. 29, 125 180.

Van der Hammen, T., 1974. The Pleistocene changes of vegetation and climate in tropical South America. J. Biogeogr. 1,3 26.

Van der Hammen, T., 1988. South America. In: Huntley, B., Webb, T., Ill (Eds.), Vegetation History. Kluwer, Dordrecht, pp. 307-337.

Van der Hammen, T., 1989. History of the montane forests of the northern Andes. Plant Syst. Evol. 162, 109 114.

Van der Hammen, T., Absy, M.L., 1994. Amazonia during the last glacial. Palaeogeogr., Palaeoclimatol., Palaeoecol. 109, 247 261.

Van der Hammen, T., Burger, D., 1966. Pollen flora and age of the Takutu Formation (Guyana). Leidse Geol. Meded. 38, 173 180.

Van der Hammen, T., Garcia de Mutis, C., 1966. The Paleocene pollen flora of Colombia. Leidse Geol. Meded. 35, 105 116.

Van der Hammen, T., Wijmstra, T.A., 1964. A palynological study on the tertiary and Upper Cretaceous of British Guiana. Leidse Geol. Meded. 30, 183 241.

Van Hoeken-Klinkenberg, P.M.J., 1964. A palynological inves-

tigation of some Upper Cretaceous sediments from Nigeria. Pollen Spores 6, 209-231.

Veillon, J.-P., 1989. Los bosques naturales de Venezuela. Parte 1. El Medio Ambiente. Inst. Silvicultura, ULA, Mdrida, 118 pp.

Vel~isquez, J., 1994. Plantas acufiticas vasculares de Venezuela. CDCH-UCV, Caracas, 992 pp.

Vuilleumier, F., 1979. Comparacion y evoluci6n de las comuni- dades de aves de pfiramo y puna. In: Salgado-Labouriau, M.L. (Ed.), E1 Medio Ambiente Pfiramo. CEA-IVIC, Caracas.

Wijmstra, T.A., 1969. Palynology of the Alliance well. Geol. Mijnbouw 48 (2), 125 133.

Wijmstra, T.A., 1971. The palynology of the Guiana coastal basin. De Kempenaer, Oegstgeest, 62 pp.

Wijninga, V.M., 1996a. Paleobotany and palynology of neogene sediments from the high plain of Bogot~i (Colombia). Evolu- tion of the Andean flora from a paleoecological perspective. Pounsen and Looijen, Wageningen, 370 pp.

Wijninga, V.M., 1996b. Palynology and Paleobotany of the Early Pliocene section Rio Frio 17 (Cordillera Oriental, Colombia): biostratigraphical and chronostratigraphical implications. Rev. Palaeobot. Palynol. 92 (3 4), 329 350.

Wijninga, V.M., Kuhry, P., 1990. A Pliocene flora from the Subachoque Valley (Cordillera Oriental, Colombia. Rev. Palaeobot. Palynol. 62, 249 290.

Wijninga, V.M., Kuhry, P., 1993. Late Pliocene palaeoecology of the Guasca Valley (Cordillera Oriental, Colombia). Rev. Palaeobot. Palynol. 78, 69 127.

Wijmstra, T.A., Van der Hammen, T., 1966. Palynological data on the history of tropical savannas in the Northern South America. Leidse Geol. Meded. 38, 71-90.

Documents

Biogeographical and evolutionary considerations of Mauritia