REVIEW OF PALAEOBOTANY AND PALYNOLOGY ELSEVIER Review of Palaeobotany and Palynology 100 (1998) 109 122

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

Valenti Rull * MARAVEN, Exploration Dep., Geological Serv. (Biostratigraphy), PO Box 829, Caracas, IOIO-A, 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 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 (Delcourt et al., 1983): megascale (plate tectonics and evolution); macroscale (Pleistocene glacia- The purpose of this paper is to discuss evolution- tions); and microscale (minor climate shifts and ary and biogeographical aspects of the palm genus human disturbance). In general, the more recent Mauritia in light of palynological fossil evidence, the fossil evidence, the more detailed the inferences and to propose several hypotheses regarding its on the past, but small-scale events are not exclusive biogeographical and ecological history. The discus- of recent times. This is a bias introduced by sion addresses three main time and space domains methodological factors, but short-term processes such as for example ecological succession must * E-mail: [email protected] have occurred at any time (Margalef, 1986; Rull

0034-6667/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII: 0034-6667(97)00060-2 110 Ic Rull / Review of Palaeobolany and Palynology 100 (1998) 109 122 and Vegas-Vilarrfibia, 1991). Conversely, small- called morichales ( and Venezuela), vere- scale events frequently obey the rules of forcing das or buritizales (), and aguajales (). agents of higher hierarchy, which determine the According to Gonz/tlez (1987), a tropical warm major trends and rhythms of their behaviour (Rull, and humid climate with more than 1000 mm of 1991a). Therefore, the three domains considered annual precipitation, and permanently flooded in the present work are and have been present and soils are essential for its development. Muuritia overlapping through time. does not grow in saline waters (Lindeman, 1954), The review begins with the present geographical its coastal representatives growing in back-man- distribution of Mauritia and the main hypotheses grove zonal belts flooded by fresh or oligohalyne derived from it. Emphasis is placed on the northern waters (Muller, 1959). These particular ecological and eastern parts of its present range, which have requirements and its present geographical distribu- been subjected to intense geological and environ- tion (Fig. 1) has lead some workers to consider mental changes, and from which most of the the as the centre of genetic differen- evidence comes. Then, the palynological evidence tiation and of an assumed dispersion of this genus is reviewed according to the three domains men- towards the peripheral lowlands of the basin tioned. They are arranged in a time-descendent (Gonzfilez, 1987). Van der Hammen (1957) pro- order and relate to the main geological and envi- posed the Roraima region as the 'centre of origin" ronmental events which could have been significant for several palms, including Mauritia. This region for the evolution of Mauritia. Finally, the more has been considered an important centre of origin reliable conclusions derived from the analysed for many (Pielou, 1979), and neotropical evidence are presented, and suggestions are made vertebrates (Miiller, 1973). These hypotheses for further studies for a better understanding of derive from the idea that biogeographical ranges Mauritia evolution and biogeography. From a are the result of expansions from an assumed taxonomic point of view, the classification of 'centre of origin' (in fact a 'centre of diversity'). Mauritia and its species is beyond the objectives Other authors think that the present geographical of the present work. 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 2. The modern distribution of Mauritia ('panbiogeographers') considers that allopatric speciation, after the splitting of a former con- At present, Mauritia palms are mostly restricted tinuous distribution, is almost the only method of to the lowlands of the Amazon and Orinoco basins formation of new species (Croizat et al., 1974). between 10 '~ latitudes on each side of the Equator The present distribution of M. macrocla&t, the (Fig. 1). Mauritia is absent from the native flora only Mauritia species on the western side of the of Central America and the islands, Andes (Hernfindez et al., 1992a,b), seems consis- although it is sometimes cultivated (Grayum, pers. tent with the 'panbiogeographical' point of view. commun., 1996). There are about three species in Indeed, this species grows separated from the main Trinidad, Colombia, Ecuador, Peru, Venezuela, distribution area by the Andean range, in the , Surinam, French Guiana and Brazil (Uhl Colombian Choc6 area, which is an important and Dransfield, 1987). They grow in several vege- centre of taxonomic endemism and has been con- tation types, ranging from coastal swamps to sidered a refuge for taxa of the humid lowlands inland gallery forests (Lindeman, 1954; Muller, (Hernfindez et al., 1992a,b). However, Mauritia is 1959; Huber, 1986; Gonzfilez, 1987; Tissot et al., absent from other seemingly suitable adjacent 1988), and at elevations up to about 1000m areas, as for example the warm, humid and flooded (Huber, 1994). Among these, Mauritia fiexuosa lowlands of the Magdalena and Maracaibo basins, L.f. is the most frequent and abundant, as compo- among others (Fig. 1). In Venezuela, there are nent of mixed forests or in pure stands locally some interesting patterns of distribution. Although V. Rull / Review of Palaeobotany and Palynology 100 (1998) 109-122 111

Caribbean ~ GuianaShield :INIDAD 10°N Miocene marine connections I~ Ternaryrecords ~~~~-- ~ Atlantic of M. franc/scoi -- Tertiary Mauritiidites absent

Pacific Present boundary of M. flexuosa

~ Mauritiama~lada (i!i>7 . ...." ., Solimoes /- Amazon Basin

BRAZIL

C,:::• , . .~...... j7 '.,. ~

Fig. 1. Map of northern 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 with the absence of Mauritia from the western and not yet available, the palm is mainly distributed in northernmost part of the Llanos, would suggest the southern part of the Orinoco watershed and some geochemical constraint related to these igne- its delta, as well as in a small area in the southwest- ous rocks, but this point has not been studied, and ern Llanos (Fig. 2). Climate is a limiting factor is not supported by the extensive occurrence of for its occurrence along most of the Caribbean the genus in other areas devoid of igneous rocks. coasts (including northern Maracaibo lake and the Alternatively, this might represent a relictual distri- Falc6n region), while elevation is a critical factor bution pattern, prior to the establishment of the in more continental areas. These factors, however, present course for the Orinoco river, or an interme- can not explain the absence of Mauritia from most diate step in the hypothetical northward expansion of the Llanos area (Fig. 2). The Orinoco river is of Mauritia populations. very close to the northernmost edge of the geo- In summary, according to the present knowl- graphical range of the palm, which roughly coin- edge, Mauritia is absent from in several areas, cides with the northern edge of the Guiana Shield even though the major environmental requirements (Gibbs and Barron, 1983). The more distant occur- for its occurrence are fulfilled. The more important rences of Mauritia along the Orinoco course are ones are the Colombian Magdalena basin, and the around its delta and in the E1 Bat~l region (Fig. 2). Venezuelan Maracaibo basin (notably the flooded A peculiarity of this area is the unique occurrence lowlands of the Catatumbo delta, Fig. 2), as well of igneous Paleozoic rocks outcropping in this flat as extensions into the Llanos. In the past, however, savanna lowland on Plio-Pleistocene unconsoli- the geographical distribution of Mauritia has been dated sediments (Gonz~ilez de Juana et al., 1980; different, and a historical analysis is essential to Feo-Codecido et al., 1984). This fact, together understand the present patterns. 112 V. Rull / Review (?/'Palaeobotany and Palynology 100 (1998) 109 122

Caribbean ~O

Orinoco 5-~'~Delta B / 1

Colombia t 1/111/ /111111111 Ii 1/I/ iiii/i/i/ /i/ ~ I/// /ii//1//I /

/ i / I/ I I / I / / / Ir~-- i / <1000 mm/year /~////////~///// I (Veillon, 1989) i iiiiII/~////i

>1000 m a.s.I. ,i//i/, Guiana Shield / ////. / ///

i / /// \ /// \ \ // "11/11 \ ~ ~-~ M. flexuosa (GonzSlez, 1987) / r/ / / / / / ~/ /

/I/// ii// i i1 • M. #exuosa (Vel~squez, 1994) ///// //// / / O M. martiana I i Phytocoenoses with Mauritia "~' .... OYY ; ;~ ; ; ,-~,III[...... i (Huber & Alarcbn, 1988) i 111111111/I/i/iI~I." i ill/ 1- open savannas (non-flooded) iiiii~iiIiiiiiIi iiii(iiiiiii/ii - gallery forests iiiii~iiii/iiiiiiIii, iiiiii/ii ombrophUous forests and palm swamps ..... ,~ ...... ~ . , , ; ;.J~, ...... - shrubby savannas herb swamps 6- flooded ombrophilous forests .iiiiiiii/iiiiiiiiii/ • 7- grass savannas ...... Brazil

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 franciscoi group, Mauritiidites sp., Mauritiidites, Mauritia-type, and Mauritia. It is widespread and The earliest record of Mauritia pollen extends common throughout the Tertiary (65 to 1.7 m.y. back to the Palaeocene. It appears near the B.P.) in northern South American basins of Cretaceous/Tertiary (K/T) boundary (about 65 Brazil/Colombia/Peru (Hoorn, 1993, 1994a,b), million years before present, or m.y.B.P.), and is Colombia (Van der Hammen, 1956, 1957; Van der represented by the fossil pollen form-species Hammen and Garcia de Mutis, 1966; Gonz~lez, Mauritiidites J~'anciscoi ( Muller, 1970, 1980 ). This 1967; Schuler and Doubinger, 1970; DuelSas, was originally described by Van der Hammen 1980; Duefias and Castro, 1981; Wijninga and (1956) as Monocolpites franciscoi, but later Kuhry, 1990, 1993; Wijninga, 1996a,b), Guyana changed to the genus Mauritiidites, by Van (Van der Hammen and Wijmstra, 1964; Van der Hoeken-Klinkenberg (1964). The Mauritia pollen Hammen and Burger, 1966; Leidelmeyer, 1966; can also appear in the literature as Mauritiidites Wijmstra, 1969, 1971 ), and Venezuela (Kuyl et al., V. Rull / Review of Palaeobotany and Palynology 100 (1998) 109-122 113

1955; Fasola et al., 1985; Lorente, 1986; Muller 1959; Lorente, 1986). Therefore, it can assumed et al., 1987; Boesi et al., 1988; Monroy and Van that the presence of this pollen indicates the pres- Erve, 1988; Colmenares and Ter/m, 1993; Rull, ence of Mauritia communities, at the time of 1997). Evolutionary change is revealed by morpho- deposition. In addition, both the absolute and the logic differences within M. franciscoi, which allows relative amounts of Mauritia pollen in sediments a subdivision into three varieties (Van der vary according to the abundance of the parent Hammen and Garcia de Mutis, 1966). Especially species in the community (Muller, 1959; Rull, noteworthy is the change in aperture from the 1991b), therefore, its pollen signal is a reliable monocolpate (with a single, elongated aperture) indicator of its population density. forms characteristic of M. franciscoi, to the porate or ulcerate (with a single pore or ulcus) ones of the modern and other species. 4. The Tertiary and the Andean orogeny According to Sowunmi (1968), the present pores within Mauritia species represent shortened colpi. It is interesting to note the Tertiary occurrence Unfortunately, the study of the fossil pollen genus of Mauritia pollen in areas where this taxon no Mauritiidites has not been sufficient to formally longer occurs (Fig. 1). Besides the influence of distinguish among different form-species (extinct climate and topography, the recent Andean uplift or not), that would correspond to Mauritia species, also played an important role producing isolation in order to follow particular distribution patterns of lowland biota. Recent palynological findings and evolutionary trends. This type of analysis support this view. According to the available geo- would be important. However, it is widely accepted logical evidence, the first orogenic pulses in the that M. franciscoi pollen has been produced by northern Andes began in the Late Oligocene species of the genus Mauritia (Lorente, 1986), and (Hoorn et al., 1995), but they did not create that its morphological varieties probably represent especially high elevations, and the lowland connec- real species. The only two living genera that tions of Caribbean and Pacific fluvial basins produce similar pollen are and with the Amazon Basin seem to have persisted (Thanikaimoni, 1970; Erdtman, 1972; until a Late Miocene uplift, 10 Ma ago (Van Muller, 1980; Ferguson, 1986), both of the same der Hammen, 1988, 1989, pers commun.). The tribe as Mauritia (Lepidocaryoideae), and with similar general ecological requirements and distri- Colombian Andes did not attain present elevations bution. Furthermore, Lepidocaryum has been con- until the beginning of the Pliocene, between 5 and sidered a 'small Mauritia' (Braun and Delascio, 3 Ma (Van der Hammen, 1974, pers. commun.), 1987), and Mauritiella is included within the genus and the Venezuelan Andes not until the Pleistocene Mauritia by many taxonomists. Combining (Giegengack, 1984). The orogenic events were the both plant and pollen morphology, result of interactions between the Caribbean and these three genera have been considered to belong Pacific plates with the South American Plate. to one single natural genus by Thanikaimoni In the Late Oligocene/Early Miocene, two (1970). marine connections existed in the present western Concerning the taphonomy of Mauritia pollen, Amazon Basin, a Pacific one in the approximate it has only local dispersal potential; that is, its location of the present Ecuadorian Andes, and a presence in sediments is almost restricted to where Caribbean one to the north at the present this palm is growing. This low dispersal capacity Maracaibo lake (Fig. 1; Hoorn, 1993; Hoorn et al., has been found in studies on modern pollen sedi- 1995). The main drainage patterns from the mentation involving morichales from both coastal Guiana Shield were oriented to the north (the swamps (Muller, 1959) and inland fluvial valleys paleo-Orinoco River) and the west. During this (Rull, 1991b, 1996b). Similarly, M. franciscoi has time, lowland fluvial ecosystems dominated by been found in Tertiary sediments accumulated in palm swamps with Mauritia and tropical forests alluvial plains and coastal plain swamps, and is occupied the Amazon Basin, spiked with coastal only rarely present in marine sediments (Muller, mangroves with Rhizophora during episodic marine 114 V. Rull / Review of Palaeobotany and Palynology 100 (1998) 109 122 incursions (Hoorn, 1993, 1994a). These marine the Pliocene (Lorente, 1986), but Mauritia is connections, together with the continuity of conti- absent at present. This sequence of disappearance nental fluvial lowlands provided a suitable and of M. franciscoi clearly parallels the Andean orog- continuous habitat for inland and coastal popula- eny, suggesting that the progressive elimination of tions of Mauritia, which helps to explain the lowland and marine connections, as well as the widespread occurrence and the abundance of M. isolation of minor basins by the emerging moun- franciscoi in the Miocene sediments of northern tain barriers may have played an important role South America. The Pacific marine connection was in its disappearance (Fig. 3). The ecological mech- broken in the Middle Miocene by the Andean anisms of this disappearance can only be assumed. uplift, and the drainage patterns began to shift On one hand, the new mountainous topography towards the east, as in the present. A concomitant itself is not suitable habitat for Mauritia, but it strong decrease of M. franciscoi can be observed also could have induced unsuitable environmental in the Middle/Late Miocene pollen diagrams conditions (drier climates, lacking topography for (Hoorn, 1993, 1994a,b), probably as the result of flooding) in isolated lower basins. On the other the disappearance of many lowland terrains, hand, Mauritia did not disappear rapidly after especially the coastal ones, and their substitution isolation, and Pleistocene climatic oscillations by mountain landscapes (Fig. 3). The persistence could have produced the local elimination of its of inland Amazonian lowlands, however, pre- populations in these basins. Re-colonization from vented the total disappearance of Mauritia. The the main area of distribution--the Amazon and continuing uplift of the Andes also resulted in the Orinoco basins--was prevented by mountain bar- progressive isolation of the Venezuelan and riers. The local extinctions can also be viewed Colombian minor basins (Magdalena, Maracaibo, under the framework of the 'island biogeography' etc.) from the Amazon and Orinoco basins, and theory (MacArthur and Wilson, 1963, 1967), con- the disappearance of the Caribbean marine connec- sidering each isolated basin as an 'island' and the tion. This took place between the Late Miocene main distribution area the 'continent'. According and the Pliocene, and was paralleled by a to this scheme, the final number of species of an progressive disappearance of M. Ji'anciscoi. In island is a balance between the rate of immigration the Magdalena Basin, this occurred in the of taxa from continental areas and the rate of Early/Middle Pliocene (Duefias and Castro, 1981; extinction of those already there. An important Wijninga, 1996a,b), whereas in the Maracaibo and aspect of this theory is that the number of species Falc6n basins it must have occurred during the is a function of the area. The smaller the island, Pleistocene, because M. franciscoi was present in the smaller the population of each species it can

~~iS 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,OCENE 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; SB-Solimoes Basin; Pl=Closure of Panama Isthmus. V. Rull / Review of Palaeobotany and Palynology 100 (1998) 109-122 115 support, and the greater the risk of extinction Llanos during the Pliocene. Therefore, the occur- (Pielou, 1979). In our case, immigration of low- rences of Mauritia in the E1 Bafil region and the land elements from the 'continental' area was eastern Llanos could be relics of a former more severely restricted, hence extinction must have been widespread distribution prior to the establishment dominant. Mauritia seems to have been one of the of the present course of the Orinoco River. If so, victims in all these areas except for one--the its absence from other adjacent areas with similar Colombian Choc6 area--where it still survives as conditions, as for example the northern and west- an isolated species (Fig. 1). This argues in favour ern Llanos, remains to be explained. of allopatric speciation, after disconnection from the main area of distribution owing to the Andean uplift. The subsequent emergence of the Isthmus 5. The Pleistocene glaciations of Panama in the Early Pliocene provided a dis- persal pathway for many taxa both to the north During the Pleistocene (the last 1.7 m.y.) at least and south (Duque-Caro, 1990; Graham, 1992). 30 glacial/interglacial cycles have been recorded However, in spite of favourable altitude and cli- worldwide (Mudie and Harland, 1996), causing mate conditions (Graham, 1992), Mauritia did not profound biogeographical reorganizations. For cross this bridge as demonstrated by its present example, the Last Glacial Maximum (LGM) distribution and its absence from the Tertiary fossil occurred at about 18 ka (CLIMAP, 1976) and record from Central America and Caribbean since then the temperate glaciated areas have been regions (Graham and Jarzen, 1969; Graham, 1976, colonized by the modern vegetation through 1985, 1987a,b, 1988a,b, 1989a,b,c,d, 1991a,b, pers. sequential invasions. Many communities were not commun., 1996). There is no explanation sug- entirely established in their present state until the gested by fossil evidence for its restriction to South last few hundred years. In the Neotropics, the America, but by the time of the emergence of the Pleistocene glacial cycles have been documented Isthmus, the Magdalena Basin was already isolated by long pollen records in mountain areas (Fig. 3 ), and the Mauritia populations were proba- (Hooghiemstra and Ran, 1994). Here, the bly shrinking, rather than expanding. Further- glacial/interglacial alternation produced vertical more, the region is close to the northern edge of displacements of mountain vegetation belts of the distribution of Mauritia, which would diminish 1000 m or more which strongly affected the evolu- its adaptive capacity and its competitive ability tion of high altitude ecosystems such as the p~r- against other taxa better adapted to these latitudes. amos and the tepuian summits producing Another consequence of the Andean orogeny is alternating connection and isolation of their biota the shift in the course of the Orinoco River towards (Vuilleumier, 1979; Rull, 1991b, 1996b). In the the east. This was initially proposed by Rod ( 1981 ) lowlands, where Mauritia lives, the glacial/ for Eocene and Oligocene times as a hypothesis to interglacial sequence has been paralleled by drier/ be tested. However, more recently available evi- wetter climatic alternation (Damuth and dence indicates that this shift occurred later, Fairbridge, 1970; Garner, 1975; Schubert, 1988). between the Miocene and the Pleistocene. A review of the available information for the According to the reconstruction of Diaz de Amazon Basin (Van der Hammen and Absy, 1994) Gamero (1996), the proto-Orinoco river flowed shows the consistent occurrence of two dry periods, towards the present Maracaibo Lake until the one between 60 and 40 ka, and another between Oligocene. Later (Early Miocene), its delta was 22 and 14 ka; the last one being especially intense probably located in the Falc6n Basin, and the and coinciding with the maximum of the last subsequent incipient uplift of the M6rida Andes glaciation. The longest pollen profile for the during the Middle Miocene caused a significant Amazon Basin extends back to about 60 ka (Absy shift towards the east. Finally, the Orinoco reached et al., 1991), and records the occurrence of savan- its present course in the Pleistocene, based on a nas during the dry periods and rain forests in the reliable record of its potential delta in the eastern wetter ones. These results can be extrapolated to 116 V. Rull / Review of Palaeobotany and Palynology 100 (1998) 109 122 former glacial phases, thus supporting the ~refuge' the ecological requirements are theoretically ful- hypothesis, that involves the cyclic contraction of filled: Mauritia simply had not yet arrived. In tropical forests into glacial 'refuges' surrounded contrast to those basins isolated by the Tertiary by savannas and, in the subsequent interglacial, Andean orogeny, these lowlands could be eventu- re-expansion in response to greater precipitation ally re-colonized by Mauritia. The apparent con- (Haffer, 1969, 1982). tradiction between this hypothesis and that of According to Van der Hammen and Absy relictual distribution north of the Orinoco can not (1994), a reduction of 500 to 1000 mm in annual be solved without further studies. One important precipitation from the present regime is necessary question is whether during arid phases the Orinoco to explain the occurrence of glacial-age savannas. existed and flowed through the same course as Assuming a decrease of this magnitude, the exis- today. tence of a major 'refuge' in the western Amazon The existence of refuges for the Amazon forests Basin, and several others of medium size spread during the dry glacial periods, however, has been along the rest of the region, has been proposed by criticized by defenders of heterogeneous patterns, these authors. Other refuge areas based on the lacking sharp discontinuities (Connor, 1986; present vegetation patterns have been proposed Colinvaux, 1987). In addition, they argue that the (review in Prance, 1982). Mauritia has been an existence and location of the assumed 'refuges' is important component of the Amazon forests and based on the modern diversity and endemism has persisted during both wet and dry phases, patterns and that palaeobotanical evidence for although its abundance was notably reduced them is lacking. Some palaeoecological studies in during the more dry periods (Absy and Van der areas formerly considered 'refugia' have supported Hammen, 1976). Thus, the distribution area of this statement, by showing that diversity and ende- Mauritia would have been alternatively reduced mism are not necessarily related to refuges (e.g. and fragmented (dry glacials) and expanded and Bradbury et al., 1981 ; Schubert, 1988 ). interconnected (humid interglacials) during the Whether the 'refuge' hypothesis is a realistic Pleistocene. Van der Hammen and Absy (1994) concept or not, two points can be made. One is also suggest that the diversity of the present that in spite of their intensity and duration, the Amazon flora is lower than during the Miocene~ Pleistocene climate changes did not eliminate due to extinction during dry episodes. This could Mauritia from the Amazon Basin. This seemingly have been balanced, however, by comparatively conflicts with the assumption that climate changes low extinction rates and more intense speciation did contribute to its local extinction in northwest- in the forest 'refuge' areas~ which also helps to ern small basins (see above). However, two impor- explain the persistence of Mauritia through time. tant differences exist, namely the size of the In the same context, an interesting situation is ~islands' (notably larger in the Amazon Basin) and that of the northernmost edge of distribution of the extent of the disconnection (in the case of the this species beyond the Orinoco course (Fig. 2). small basins) from the main distribution area as In this region, the occurrence of fossil dunes the potential source for new propagates. The of Late Pleistocene age (Roa, 1979) indicates second is that the existence of unfavourable areas the existence of desert climates unfavourable for for Mauritia (treeless savannas and deserts) during Mauritia during the last glacial. This climate is the glacial periods seems unquestionable; hence a assumed to have prevailed across the whole Llanos, certain degree of postglacial dispersal is necessary including the Colombian areas (Schubert, 1988). to explain the present-day occurrence of a single Therefore, it is unlikely that Mauritia grew on species (M. flexuosa) throughout the entire these terrains, which must have been colonized by Amazon and Orinoco basins (Fig. 1 ). It is possible this palm the last glacial from its main distribution that Mauritia has progressively colonized formerly area to the south, or from small forest or morichal dry areas while they became gradually more 'refuges'. If so, another possible explanation humid, in a similar way than many of emerges for the absence of Mauritia in areas where the northern hemisphere advanced northwards ld Rull / Review of Palaeobotany and Palynology lO0 (1998) 109-122 117 following the progressive ice retreat (for example, Amazon Basin high rates of lateral migration of Davis, 1976, 1981). More studies are necessary to the river courses have been reported, owing to document an expansion process of this nature. 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 6. Holocene climatic shifts and fires so-called aguajales, or pure stands of Mauritia flexuosa (Puhakka and Kalliola, 1993). Finally, The Holocene is characterized by climate shifts the expansion of Mauritia also can be favoured by of lower intensity than the glacial ones, and by the the disturbance of original vegetation, if flooding impact of human activities on ecosystems. In the is guaranteed. Indeed, in one particular site of the neotropical region, the available Holocene palyno- Gran Sabana (Uru6), instead of decreasing, logical data available on Mauritia indicate that the Mauritia increased spectacularly during the last taxon has been consistently present, but its abun- millennium in the absence of significant climate dance has changed owing to environmental shifts. changes (Fig. 4). Independent evidence showed In the Atlantic Guianan coasts a marine regression that this site had been burnt since before about accompanied by a progradation of the coast line 1.4 ka, and the former mixed gallery forest was towards the sea, occurred around 5 ka resulting in eliminated and replaced by secondary vegetation an increase of back-mangrove communities, of and treeless savanna. A subsequent humidity which Mauritia is a common component (Van der increase produced the growth of a morichal instead Hammen, 1963; Wijmstra, 1971; Tissot et al., of a potential mixed forest recovery (Rull, 1992). 1988). Further inland, in the Amazon Basin, the Although the palaeovegetational evidence is not changing percentages of Mauritia pollen in the sufficient for a conclusive assessment, it is remark- diagrams are considered indicating successive able that the first Holocene record of Mauritia in expansion or shrinking of its stands in response to the Gran Sabana dates from about 5 ka, but that increased or reduced flooding (Wijmstra and Van it did not attain the pollen abundance typical for der Hammen, 1966; Absy, 1985; Colinvaux, 1987; the present morichales after about 2 ka (Fig. 4; Colinvaux et al., 1988; Liu and Colinvaux, 1988). Rull, 1991b, 1992). The values of Mauritia pollen For example, a major decline in Mauritia pollen before this date resemble those of mixed forests, was recorded in the Ecuadorian Amazon Basin in which Mauritia is a minor component. The owing to the creation of non flooded terra firme removal of these forests by an extensive arid phase, forest, that replaced the former wet palm forest, followed by a humidity increase, allowed the estab- owing to a drier climate, some 800 years ago lishment of the morichales. Hence, these communi- (Colinvaux et al., 1988). This decrease in moisture ties seem to be of recent origin in this area, an was also reported in the Venezuelan Gran Sabana interesting aspect, considering that this region, the (Rull, 1991b) and thus was extensive. In the same Roraima region, is the assumed 'centre of origin' region, the repeated alternation of humid and dry for Mauritia, and almost the centre of its present phases favoured the expansion of Mauritia stands distribution (Fig. 1 ). These palynological data sug- or morichales, which lead to progressive reduction gest that Mauritia could had been dispersed for- of the lowland forests (Rull, 1991b). merly through an expansion of those forests, and The Holocene climate oscillations reported in that the morichales developed later as secondary northern South America, however, shows notable colonizing communities after dry oscillations or temporal and geographical heterogeneity disturbance. This would support the view of (Markgraf and Bradbury, 1982; Rull, 1996a); GonzS.lez (1987), who considers M. flexuosa a therefore, a single general expansion or reduction colonizer species. The same author proposed a trend for Mauritia during this time seems unlikely. successional process based on the observation of On the other hand, not all variations in the present spatial patterns of the communities with Mauritia pollen amounts should be interpreted in M. flexuosa, culminating in a hypothetical stabi- terms of climate. For example, in the Peruvian lized evergreen swamp forest. This Clementsian 118 V. Rull / Review of Palaeobotany and Palynology 100 (1998) 109 122

I 5

_ °.,.ATE] -P .... t 1 °" .or...... L -P.esent 1 Eo e- ' t "~ 3

A 0 0 i o 2 i ST/\, = 1 i _J--_ =-J'/jV~, .... ~\

0 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 reduction and splitting of its former geographical so far (Rull and Vegas-Vilarrfibia, 1991; Rull, range, rather than expansion. However, the 1992), but palynological studies available to date hypothesis of a recent postglacial expansion trend are not conclusive. Similarly, more studies on the seems plausible in the light of the available evi- arrival and increase of Mauritia compared to other dence. Hence, both dispersal and fragmentation, present-day morichal areas are needed. followed by allopatric speciation, are probably needed to explain the evolutionary and biogeo- graphical trends of Mauritia. With the available 7. General conclusions and some proposals evidence, evolutionary changes are difficult to trace, but a careful morphological characterization Explaining biogeographical patterns of Mauritia of the different Tertiary and Pleistocene pollen through time is not a simple matter, because types, their geographical extension and strati- diverse forcing factors acting at different spatial graphic range could be very useful in this respect. and temporal scales need to be considered. So far, allopatric speciation owing to the Andean Mauritia seems to be an old element of the uplift is strongly supported by palynological Neotropical flora, and to trace its history it is studies, whereas a hypothetical Tertiary evolution necessary to consider megascale, macroscale and of Mauritia during an assumed general spreading microscale factors (sensu Delcourt et al., 1983). has not yet been recorded in the fossil record. Andean orogeny linked to plate tectonics, sea-level On the other hand, the habitat consistency oscillations, glaciations, climatic oscillations and recorded through time by Mauritia suggests that human activities have resulted in varying geo- it is an important indicator of past conditions in graphical patterns for the fairly constant habitat the lowland Neotropics, in a somewhat inverse of this palm. Therefore, any extrapolation of the sense of other elements such as for example Alnus biogeography of Mauritia based on present obser- or Quercus, which occurrence has been used as vations would probably account only for the most indication of the presence of high elevations (Van recent trends. der Hammen, 1988). The presence of Mauritia Globally speaking, the modern distribution of pollen can be considered a reliable indicator of Mauritia seems to have been the result of the flooded, tropical lowlands, either along the coast v. Rull / Review of Palaeobotany and Palynology 100 (1998) 109 122 119

(in combination with mangrove pollen, as for important comments and suggestions. The com- example Rhizophora) or along rivers and small ments of two unknown referees were also helpful water currents (co-occurring with pollen of to improve the manuscript. Mike Grayum pro- Gramineae or inland forest trees). Similarly, since vided useful biogeographic information. The edi- the today Mauritia species are distributed on either tion facilities were provided by MARAVEN. sides of the equator to 10 ° latitude, their fossil pollen could be used as a reliable palaeolatitude indicator. The concomitant disappearance of Mauritiidites and the first occurrence of Alnus in References the northern Andes supports this view. In this sense, the biostratigraphic significance of the last Absy, M.L., 1985. Palynology of Amazonia: The history of the occurrence of Mauritiidites should be evaluated, forests as revealed by the palynological record. In: Prance, G.T., Lovejoy, T.E. (Eds.), Key environments Amazonia. with special care to its timing with respect to the Pergamon Press, Oxford, pp. 72-82. Andean orogeny Absy, M.L., Van der Hammen, T., 1976. Some paleoecological Based on the available evidence, the biogeo- data from Rondonia, southern part of the Amazon Basin. graphical trends of Mauritia can only be roughly Acta Amazon. 6 (3), 293 299. traced, and a number of aspects about its history Absy, M.L., Cleef, A.M., Fournier, M., Martin, L., Servant, remain still unclear. Accurate mapping of the M., Sfeddine, A., Ferreira da Silva, M.F., Soubies, F., Suguio, K., Turq, B., Van der Hammen, T., 1991. Mise en present distribution of the Mauritia species is 6vidence de quatre phases d'ouverture de la for6t dense dans essential. In addition, important aspects worthy of le sud-est de l'Amazonie au cours des 60.000 derni~res palynological investigation are: (1) morphological ann6es. C. R. Acad. Sci. Paris Ser. II 312, 673 678. differentiation of several Tertiary form-species with Boesi, T., Galea, F., Rojas, G., Lorente, M.A., Durfi,n, I., taxonomic and evolutionary significance; (2) a Velasquez, M., 1988. Estudio estratigrS.fico del flanco noran- more detailed reconstruction of the disappearance dino en el sector Lobatera-El Vigia. Memorias Ill Simposio of Mauritia pollen from northwestern Andean Bolivariano, Exploraci6n de las Cuencas Subandinas, pp. 2-41. basins, during the Tertiary; (3) the influence Bradbury, J.P., Leyden, B., Salgado-Labouriau, M.L., Lewis of glacial/interglacial (drier/wetter) cycles on Jr., W.M., Schubert, C., Nenford, M.W., Frey, D.G., White- Mauritia populations; (4) the hypothetical head, D.R., Weibezahn, F.H., 1981. Late Quaternary envi- sequence of re-colonization (as well as of eventual ronmental history of Lake Valencia, Venezuela. Science 214, significant increases) of the Amazon and Orinoco 1299 1305. lowlands by Mauritia after the last glaciation; (5) Braun, A., Delascio, F., 1987. Palmas aut6ctonas de Venezuela y de los paises adyacentes. Martinez, Caracas. 156 pp. the detailed reconstruction of the successional Clements, F.E., 1916. Plant succession, an analysis of the devel- trends in communities in which Mauritia occurs opment of vegetation. Carnegie Inst., Washington. or could occur theoretically; and (6) the influence CLIMAP, 1976. The surface of Ice-Age Earth. Science 191, of climatic and human disturbance on the taxon's 1131 1137. distribution. Colinvaux, P.A., 1987. Amazon diversity in the light of the Finally, it is also worth mentioning that both paleoecological record. Quat. Sci. Rev. 6, 93 114. Colinvaux, P.A., Frost, M., Frost, I., Liu, K.B., Steinitz- the historical trends and the proposed further Kannan, M., 1988. Three pollen diagrams of forest distur- studies on Mauritia might be valid for other bance in the Western Amazon Basin. Rev. Palaeobot. taxa with similar environmental requirements. Palynol. 55, 73 81. Ecological phenomena take place at the com- Colmenares, O.A., Terfin, L., 1993. A biostratigraphic study of munity level, and concomitant local extinctions, Paleogene sequences in southwestern Venezuela. Palynology speciation processes, and ecological reorganiza- 17, 67-89. Connor, E.F., 1986. The role of Pleistocene forest refugia in tions in other lowland elements are expected. the evolution and biogeography of tropical biota. Trends Ecol. Evol. 1 (6), 165-168. Acknowledgements 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- The author is very grateful to Alan Graham and sea arkosic sands and Ice-Age aridity in Tropical South Thomas van der Hammen for critical revision, and America. Geol. Soc. Am. Bull. 81, 189 206. 120 V. Rull / Review ~[ Palaeobotany and Palynology 1O0 (1998) 109-122

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