Angiosperm A

Interplate dispersal paths for megathermal angiosperms

Robert J. Morley

School of Geography and Environmental Science, Monash University, Victoria, AustraliaDepartment of Geology, Royal Holloway University, Egham, Surrey, UK

Received: 29 January 2003 · Revised version accepted: 29 March 2003

Abstract

The dispersal of megathermal angiosperms between tectonic plates is reviewed on thebasis of fossil evidence for the Cretaceous and Tertiary periods, since the radiation of theangiosperms, and the period of break-up of Gondwana. The combination of tectonicplate disassembly and redistribution, coupled with phases of global warming followed bypronounced cooling, has resulted in the formation of intermittent dispersal opportunitiesfor frost-intolerant plants, and has been a major factor in determining the direction of an-giosperm diversification. The Early Cretaceous radiation of angiosperms seems to showlittle relationship to the formation of Tethys. However, for the Late Cretaceous and Ter-tiary nine relevant dispersal routes can be differentiated that can be divided into two dis-tinct categories: routes which formed following the break-up of Gondwana during theLate Cretaceous and Earlier Tertiary, when warm climates encouraged dispersal ofmegathermal elements globally, and routes which formed since the Middle Eocene, fol-lowing phases of plate collision, as global climates were cooling down, inhibiting such dis-persal. Most inter-plate dispersal of megathermal angiosperms took place in the Late Cre-taceous and Early Tertiary at a time when global climates were markedly different fromthose of today, and the global area of megathermal vegetation several times greater thanat present. Under such a scenario, it is likely than opportunities for speciation were muchhigher than for present-day megathermal plants.

Key words: angiosperms, dispersal routes, megathermal, plate tectonics

1433-8319/03/6/01-02-005 $ 15.00/0

Corresponding author: Robert J. Morley, School of Geography and Environmental Science, Monash University, Victoria 3800, Australia; and Dept ofGeology, Royal Holloway University, Egham, Surrey TW20 0EX, UK. Mailing address: Palynova/PT Eksindo Pratama, Vila Indah Pajajaran, Jl KertarajasaNo 12A, Bogor, Indonesia 16153; e-mail: [email protected]

Introduction

Angiosperms underwent their major phase of radia-tion and dispersal in unison with the break-up ofGondwana. At the same time, the global climatechanged dramatically from an essentially greenhouseworld during the Later Mesozoic and Earlier Tertiary,to the icehouse world of the Quaternary. The combi-nation of tectonic plate disassembly and redistribu-tion, coupled with phases of global warming followed

by pronounced cooling, resulted in the formation ofintermittent dispersal opportunities for frost-intoler-ant (‘megathermal’) angiosperms at different times,both within the low and mid latitudes and betweenlow and mid latitudes, and has been a major factor indetermining the direction of angiosperm diversifica-tion. Plate tectonic controls and patterns of global cli-mate change also account for many of the present-daydisjunctions seen in various groups of tropical flower-ing plants (Morley 2000).

Vol. 6/1,2, pp. 5–20© Urban & Fischer Verlag, 2003http://www.urbanfischer.de/journals/ppees

Perspectives in Plant Ecology,Evolution and Systematics

Fig. 1. Diagram showing the palaeolatitudinal relationship of first occurrences of angiosperm pollen. Open circles and lighter shading, monosulcate grains,reflecting a ‘grade’ of early dicotyledons and monocotyledons; closed circles and darker shading, triaperturate grains, reflecting eudicots. Geographic areas: (1)New Zealand, (2) Australia, (3) Patagonia, (4) Congo, (5) Brazil, (6) Peru, (7) Ivory Coast, (8) Israel, (9) Gabon, (10) Portugal, (11) Oklahoma, (12) PotomacGroup, (13) Maryland (Virginia and S England), (14) SW Siberia, (15) not known, (16) Denver Basin, (17) E England, (18) Saskatchewan, (19) E USA,(20) Wyoming, (21) W Central Siberia, (22) SE Alberta and SW Manitoba, (23) SE Siberia, (24) Central Canada, (25) Central Alberta, (26) Greenland, (27)Ellesmere Island and NE Siberia, (28) Yukon, (29) Kuk River (Alaska), and (30) Umiat Region (Alaska) (after Hickey & Doyle 1977).

The process of inter-plate plant dispersal remainspoorly understood; for instance, vicariance biogeogra-phers who view the time of separation of African andS American plates at about 100 Ma as the time of ces-sation of plant dispersal between these regions may bemaking an unwarranted generalisation since pollendata from the two sides of the South Atlantic showthat many dispersals continued well beyond that time.Reason suggests that dispersals are likely to continuebetween two tectonic plates well after the time of sepa-ration via wind, water and avian vectors. Also, the de-tails of land connections during times of initial plateseparation are equally poorly appreciated, and islandchains associated with mantle plume hotspots betweendiverging plates are likely to have provided opportuni-ties for ‘filter dispersal’ well after the time of ‘plateseparation’.

In this paper shortcomings in our understanding ofboth dispersal processes and the nature of detailedpalaeogeographies during times of plate separation orconvergence are circumvented through the examina-tion of the pattern of appearance of fossil (principallyangiosperm) pollen on adjacent plates in addition todata on plant macrofossils and faunal migrations. Theappearance of the same pollen types, or succession oftypes, on adjacent plates provides strong evidence for

interplate dispersal, and allows judgement to be madeas to the presence of a dispersal corridor or enablingfilter. Such judgements can be made whether or not theparent taxa of the pollen types are known. Emphasishas been placed on the fossil record of families consid-ered megathermal, or megathermal pro majore parteby van Steenis (1962), but to clarify some dispersalroutes, and to emphasise climatic barriers to dispersalof megathermal taxa during the Late Neogene andQuaternary, records of some mesothermal and mi-crothermal taxa are also discussed. In several in-stances, dispersal paths are identified using the recordof pollen types for which the parent plants are un-known; in such cases affinity with megathermal fami-lies cannot be demonstrated, it may be inferred on thebasis of palaeobiogeography. The classification usedhere follows Takhtajan (1969).

Angiosperm origins and initial radiation

Only a few years ago, the relationship between an-giosperms and other seed plants seemed more or lessclear-cut based on the Anthophyte theory, with an-giosperms being a sister group to Gnetales and extinctgroups, such as the Bennettitales, and presumably ini-

6 R. J. Morley

Perspectives in Plant Ecology, Evolution and Systematics (2003) 6, 5–20

Fig. 2. Early Cretaceous, Albian palaeogeography and climatic zones, according to Vakhrameev (1991), together with the approximate locations and ages (inMy) of the appearance of triaperturate pollen shown in Fig. 1.

the succession of pollen wall structures seen in extantprimitive angiosperm families, such as Anonaceae, wefind that the wall structures of the most primitive an-giosperm pollen types would have been indistinguish-able from those of many gymnosperms (Walker &Walker 1984), so the appearance of angiosperm pollenin the Early Cretaceous indicates the time of evolutionof pollen exines with a relatively complex tectate wallstructure. Until this time there is no clear way of deter-mining whether pollen was produced by angiospermsor gymnosperms unless it is found in situ in an an-giosperm flower, e.g. in the Magnoliid Lesomasitesfossulatus (Ward et al. 1989).

The global radiation of the first angiosperms thatproduced tectate pollen is illustrated by the fossilrecord of two contrasting pollen groups (Fig. 1). Therecord of tectate monosulcate pollen reflects radiationof monocots and Chloranthoid dicots, whereas therecord of triaperturate pollen reflects the appearanceand radiation of non-magnoliid dicots or eudicots(Crane et al. 1995). The oldest records for tectatemonosulcate pollen appear at about the same time inwidely separated localities, such as Gabon, Brazil andIsrael, in Western Gondwana, and England, Portugaland eastern USA in Western Laurasia (Crane 1987).Their pattern of appearance shows no relationship tothe separation of Laurasia and Gondwana by theTethyan Ocean, which was already well established inthe Mid-Jurassic, at about 160 Ma, or to climatic

tially appearing sometime in the Triassic, some 230Ma years ago (Crane et al. 1995). Recent molecularwork contests this view, suggesting that although un-ambiguously supported hypotheses of phylogenetic re-lationships among seed plants have not yet been ob-tained, angiosperms form a sister clade to all gym-nosperms (Bowe et al. 2000; Chaw et al. 2000; Madal-lon & Sanderson 2002), and hence point to an evenolder origin, in the Mid Palaeozoic. The oldest defini-tive angiosperm fossil is from the earliest Cretaceousof China (Sun et al. 2000), but this does not give a clueto the place of origin, since the deposits in which itwas found provide only one of the best settings forplant and animal fossil preservation from this period(Zhou et al. 2003); more localities would be neededbefore such judgements could be made.

It is not until well into the Early Cretaceous, withinthe Hauterivian (135–132 Ma) or Barremian (132–124 Ma) stages, that angiosperm fossils become suffi-ciently widespread to make judgements regarding pat-terns of radiation. The appearance of monosulcate tec-tate pollen referable to the pollen genus Clavatipollen-ites, identified with the magnoliid family Chloran-thaceae (Muller 1981; Walker & Walker 1984) is criti-cally important. From the palynological perspective,the absence of angiospermid fossils prior to the Bar-remian/Hauterivian is easily explained – if we look atthe wall structure of the oldest pollen grains confident-ly identified with angiosperms, and compare this with

Interplate dispersal paths for megathermal angiosperms 7


zones, suggesting that neither Tethys nor humidity-re-lated climatic belts formed a barrier to dispersal forthese early angiosperms. At the time of the first an-giosperm radiation terranes eventually to form STurkey, the Balkans and Italy were arranged as a linea-ment between Africa and Europe (Scotese 2001), andthis may have facilitated this initial dispersal phase.Angiosperms appear, however, to have arrived some10 Ma later on the Australian Plate (Burger 1991;Dettmann 1994), where the greatest concentration ofprimitive angiosperms occurs today, and in India. Wecan therefore hardly visualise Australia as their area oforigin, or their origin on a Gondwanan ‘shard’ or ter-rane that separated from Australasia in the Mesozoic(as proposed by Tahktajan 1987) but their area ofmost persistent survival.

The second ‘wave’ of angiosperm radiation, of earlyeudicots, shown by the record of triaperturate andtriaperturate-derived pollen, begins in the Aptian(113–108 Ma) of Gabon and Brazil (Doyle et al. 1977),during which time low-latitude climates were marked-ly subhumid, and probably much hotter than presentday equatorial climates. This group subsequently dis-persed poleward during the Albian (108–96 Ma) intozones of mesic ‘subtropical’ (still megathermal) climateand into areas with more mesothermal climates pole-ward of 60° N and S in the Cenomanian (96–92 Ma)

(Fig. 2). As in the case of the earlier monosulcate-pollen-producing angiosperms, the initial eudicotwave seems to have been little affected by the oceanicbarrier of Tethys possibly for similar reasons (seeabove).

By the Cenomanian/Turonian (96–90 Ma), an-giosperms became dominant components of vegeta-tion in most areas, with major centres of radiationacross the northern mid latitudes (Boreotropicalprovince), southern mid latitudes (Gondwananmegathermal province) and the equatorial region (Pal-mae province; Fig. 3). It is from this time onward thatthe angiosperm pollen record can be used readily toidentify periods of inter-plate dispersal (Morley 2000).

Dispersal routes fall into two distinct categories.The first category consists of routes which formed dur-ing and following phases of Gondwanan break-up, inthe Late Cretaceous and Early Tertiary, culminatingwith the Late Paleocene/Early Eocene thermal maxi-mum (60–49 Ma), when warm climates encourageddispersal of megathermal elements globally. The sec-ond category is of dispersal routes that formed be-tween the Middle Eocene and present day when dis-persals relate primarily to phases of plate collision,and during which time global climates have gonethrough a long phase of stepwise cooling which has in-hibited the dispersal of megathermal plants.

8 R. J. Morley


Fig. 3. Late Cretaceous, Turonian plate tectonic reconstruction and palaeogeography according to Smith et al. (1994), showing the three latitudinal belts withinwhich moist megathermal angiosperm taxa first evolved. Noteworthy dispersals of megathermal taxa and of Normapolles are indicated for the Turonian andSantonian/Coniacian, suggested by the palynological record (from Morley 2000, with modifications). Normapolles province boundary from Herngreen et al.(1996).

Late Cretaceous to Early Eocene,fragmentation of Gondwanaduring period of greenhouse climates

During the Late Cretaceous to Early Eocene (54–49 Ma),the fragmentation of Gondwana was well under way,with both the South Atlantic and Indian Oceans wideningrapidly (Figs. 3, 4 and 5). Six main interplate dispersalpaths for megathermal angiosperms need considerationduring this period: (1) a transatlantic path between Eu-rope and N America, (2) a route from Europe to Africa,(3) a land bridge between N and S America, (4) a transat-lantic path between Africa and S America, (5) routes be-tween Africa and India, and (6) a land bridge between SAmerica and E Gondwana (Antarctica/Australasia). Dur-ing this time global climates were warm, with tempera-ture maxima during the Turonian (92–90 Ma) and LatePaleocene/Early Eocene. A land connection also existedacross Beringia throughout this time, but was probablytoo far north (75°) for dispersal of megathermal planttaxa, although it formed a route for several microthermalBoreotropical taxa, reviewed by Manchester (1999).

Dispersal route from N America to Europe

Many megathermal angiosperm families originated inthe North American and Eurasian Boreotropical

province during the Late Cretaceous, when virtuallyfrost-free climates extended well into the mid-latitudes.Eurasia and N America would have been in tectoniccontact via Greenland through the Late Cretaceousand Early Tertiary, but except for very high latitudes,would have been separated by a narrow seaway untilthe Late Paleocene (60–54 Ma) and Early Eocene whena land connection was established across S Greenlandat latitudes of 45–50° N (Tiffney 1985a; Parrish 1987).Prior to the formation of this land connection, the tim-ing of possible dispersal routes is suggested from the si-multaneous appearance of pollen types in both regions.Pollen of the Normapolles group (affinity obscure butderived in part from early Juglandales), which charac-terizes Late Cretaceous pollen floras from the easternUSA to the Turgai Straits (Fig. 3) shows rapid diversifi-cation at this time (Tschudy 1981), with most similari-ties between the two areas during the Cenomanian andSantonian/Coniacian (88-84 Ma), suggesting the pres-ence of a land connection or dispersal filter. The parentplants of the Normapolles pollen group were not nec-essarily megathermal, but their prominence in areassuch as the Mississippi Embayment, considered to beartropical and paratropical vegetation at this time by Up-church & Wolfe (1987), suggests that strictly mega-thermal Late Cretaceous taxa which may have beencharacterized by less distinctive pollen could have fol-lowed the same route.



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Fig. 4. Early Tertiary, Paleocene, plate tectonic reconstruction and palaeogeography according to Smith et al. (1994), with occurrences of evaporites from Parrishet al. (1982) with additions, and closed-canopy megathermal rain forests according to Morley (2000). Noteworthy Maastrichtian and Earlier Paleocene dispersalsof megathermal taxa (prior to the Late Paleocene/Early Eocene thermal maximum) are indicated, as suggested by the palynological and macrofossil record.

Fig. 5. Early Tertiary, Early Eocene plate tectonic reconstruction and palaeogeography according to Smith et al. (1994), with occurrences of evaporites fromParrish et al. (1982) with additions, and closed-canopy megathermal rain forests according to Morley (2000). Noteworthy dispersals of megathermal plantsrelating the thermal maximum are indicated, as well as Middle Eocene dispersals into SE Asia, relating to the collision of the Indian and Asian Plates, assuggested by the palynological and macrofossil record.

unhindered dispersals over remarkably wide areas. Forinstance, at this time members of Bombacaceae andthe mesothermal Alangium were able to disperse be-tween Europe and N America on the one hand, and toAustralia on the other (Fig. 5).

Dispersal route from Europe to Africa

Opportunities for Late Cretaceous trans-Tethyan dis-persal from Europe of Africa is indicated by the com-mon occurrence of the Laurasian Normapolles groupin North Africa (Herngreen et al. 1996), delimiting thesouthern boundary of the Normapolles province(Fig. 3). With the northward drift of Tethyan terranesin the earliest Tertiary this dispersal route was proba-bly severed, and it was not until the Late Eocene thatdispersals of megathermal taxa between Europe andAfrica are recorded (Cavagnetto & Anadon 1995),with Mimosaceae, Amanoa, Alchornea, Caesalpinia,Crudia and Thespesia dispersing into the IberianPeninsula, only to disappear following Mid Tertiaryclimate deterioration.

Dispersal route from N to S America

The tectonics of the Caribbean region are complex andcontroversial, but the plate-kinematic model of Pindallet al. (1988) appears to be standing the test of time.With the western drift of both the N and S American

The development of a short-lived land connectionacross S Greenland during the latest Paleocene to EarlyEocene at about 45–50° N at the precise time of theLate Paleocene/Early Eocene thermal maximum provid-ed a clear route for the dispersal of megathermal ele-ments between the two areas for this short time period.Megathermal and mesothermal taxa that would havemade this crossing, summarized from macrofossil databy Manchester (1999) and pollen by Morley (2000) in-clude Mastixia and Toricella (Cornaceae), Gordonia(Theaceae), Symplocos (Symplocaceae), Alangium(Alangiaceae), Tapiscia (Staphyleaceae), Bombacaceaeand Sapotaceae (Fig. 5). There are also many examplesof mammalian dispersals between Europe and N Amer-ica at this time (Simpson 1946; McKenna 1973).

The Late Paleocene/Early Eocene thermal maxi-mum allowed megathermal angiosperms to extendtheir ranges further poleward than at any time duringthe period in which they were dominant, as empha-sised by records of Nypa macrofossils the EarlyEocene from the London Clay (41° N palaeolatitude)(Reid & Chandler 1933; Collinson 1983), and alsoTasmania (58° S palaeolatitude; Pole & MacPhail1996). The most remarkable aspect of global palaeo-geography at this time was the presence of landbridges between Europe and N America and between SAmerica and E Gondwana before the termination ofthe C American connection, allowing the possibility of

10 R. J. Morley


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whereas in the Paleocene, Bombacaceae and Symplo-cos (based on macrofossils) dispersed into S Americafrom the north, and Ilex (Aquifoliaceae), Anacolosa(Olacaceae) and members of Sapindaceae dispersedinto N America from the south (Fig. 4).

Dispersal route from S America to Africa

Direct land connections between the African and SAmerican Plates were severed at the end of the Albian(96 Ma), fitting approximately with faunal evidence(Parrish 1987). Up until that time there was a cleardispersal corridor for megathermal plants between thetwo plates, but the frequency of plant dispersals acrossthe newly formed ocean after this date – as shownfrom fossil pollen – suggests that some form of landconnection, or series of islands acting as steppingstones, remained throughout the Late Cretaceous andinto the earliest Tertiary, possibly in the region of theWalvis Ridge/Rio-Grande Rise and Sierra LeoneRidges. The Walvis Ridge, stretching from Angola toBrazil, consists of a seamount lineament (volcanic lin-eament formed on spreading oceanic crust above a sta-tionary mantle plume) with individual mounts datingfrom 82 Ma adjacent to Angola to 37 Ma within theMid Atlantic (MacDougal & Douglas 1988). For theRio Grande Rise on the other hand (which is less welldated), Theide (1977) conducted subsidence estima-tions, and suggested that the rise may have been abovesea level until the Oligocene. No such study has been

plates since the Mid Cretaceous, the interveningoceanic Caribbean Plate, lying directly to the west, de-veloped island arcs along both its leading and trailingmargins. During the Late Cretaceous, the leading arcformed an island arc, or land connection, between Yu-catan and Colombia (Fig. 6), which with further drifteastward subsequently separated from Yucatan afterthe Middle Eocene (49–39 Ma), resulting in the forma-tion of a marine trough permitting circulation ofoceanic waters from the Atlantic to the Pacific. Thearc subsequently drifted yet further east, but main-tained a connection with the S American Plate, andduring the Eocene-Oligocene is thought to haveformed a land mass, recently termed Gaarlandia (Itur-ralde-Vincent & MacPhee 1999), prior to fragmentinginto the present-day Caribbean islands. Evidence forthis dispersal route is forthcoming from vertebrate fos-sils (Bonaparte 1984; Rage 1988; Hallam 1994), withdispersals of hadrosaurian and ceratopian dinosaursfrom N America to S America in the Campanian, andof various snakes, lizards and titanosaurid dinosaursin the northward direction. Exchanges began in theCampanian and increased in the Maastrichtian andPaleocene.

The pollen record also provides evidence for disper-sals over this period. Late Cretaceous dispersals in-cluded the parent plant of Aquilapollenites pollen(thought to be derived from an extinct Loranthalean(Santalalean) group) from north to south, and Gun-nera (Gunneraceae) in the reverse direction (Fig. 3),



Fig. 6. Campanian reconstruction of Middle America (Pindall et al. 1988). The Greater Antilles and Aves Ridge, created by subduction of the Proto-CaribbeanSea by the Caribbean Plate, formed a land bridge between the Americas during the Late Cretaceous and Paleocene. The Caribbean Plate drifted eastward rela-tive to the Americas in the Mid-Tertiary, breaking this connection during the Eocene.

record appears to have developed mainly in theMaastrichtian. There are also some Eocene (54–36Ma) dispersal events, with pollen of the Amanoatype (Euphorbiaceae), Crudia (Leguminosae), Pel-liciera rhizophorae (Theaceae) and members ofMalpighiaceae and spores of a Schizaeaceous fern (Ci-catricosisporites dorogensis) appearing at the sametime on both continents (Fig. 5), and even someOligo-Miocene (36–5 Ma) crossings, such as the man-grove genus Rhizophora (Rhizophoraceae), a Sonner-atioid or Lythraceous mangrove indicated by thepollen species Verrutricolporites rotundiporus (Mor-ley 2000), the swamp-tolerant tree Symphonia globu-lifera (Guttiferae) and the climbing fern Lygodiumscandens (Schizaeaceae) which were probably in-stances of sweepstake (dispersal by rare or chanceevents across a major barrier) dispersal (Fig. 7).

The Walvis Ridge may also have acted as a disper-sal path for Southern Hemisphere elements, such asCasuarina (sl) (Casuarinaceae), which appears in bothsouthern Africa (Scholtz 1985) and southernmost SAmerica (Romero 1993) in the Paleocene (Fig. 4).

Dispersal route between S America and E Gondwana

Clear dispersal events occurred between S Americaand Antarctica through to New Zealand and Australia

performed on the Walvis Ridge, but clearly, thisseamount ridge is a strong contender for Late Creta-ceous and Palaeogene transatlantic dispersals. Duringthe Late Cretaceous, well after the time of separation,numerous major dispersals can be demonstrated fromsimilarities of the pollen record in West Africa andnorthern S America.

The most important pollen taxa appearing virtual-ly simultaneously on either side of the S Atlantic,summarised on Figs. 3 and 4, are: Triorites africaen-sis (ancestral Proteaceae) in the Cenomanian; Mono-colpopollenites sphaeroidites (ancestral Palmae) andDroseridites senonicus (possibly Droseraceae) in theTuronian; Auriculiidites spp., Cupanieidites spp.(Sapindaceae), and the Contantinisporis group (pos-sibly Myrtales) in the Santonian/Coniacian; Buttiniaandreevii (unknown), Aquilapollenites spp. (Loran-thales/Santalales), and Periretisyncolpites spp. (pos-sibly Illiciaceae) in the Campanian (84–74 Ma) andnumerous Proteaceae and Palmae, Ctenolophon(Ctenolophonaceae), Anacolosa and Restionaceae inthe Maastrichtian (74–66 Ma). The first evidence fordifferences, which might reflect Late Cretaceousprovincialism is suggested from Gnetalean pollen,which is more diverse in S America than in Africafrom the Turonian onward (De Lima 1980), but ob-vious provincialism, based on the angiosperm pollen

12 R. J. Morley


Fig. 7. Early Tertiary, Oligocene plate tectonic reconstruction and palaeogeography according to Smith et al. (1994), occurrences of evaporites from Parrish etal. (1982) with additions, and closed-canopy megathermal rain forests according to Morley (2000). Noteworthy dispersals are indicated for (a) Miocene sweep-stakes dispersals across the S Atlantic; and (b) Oligo-Miocene dispersals between the Australian and Asian Plates. The black southward-pointing arrows reflectthe proportions of Boreotropical taxa finding refuge in each of the three tropical rain forest blocks. Numbers refer to the number of Boreotropical genera whichare represented in each rain forest block (from Tiffney 1985b).

via the South Sandwich Islands and South Georgia,which would have formed a continuous connectionduring the Late Cretaceous and Paleocene. This pathwas followed by primitive angiosperms, such as win-teraceous Bubbia/Bellobium and Drimys (Dettmann1994), in the Campanian. Other taxa that followedthis route during the Late Cretaceous and Early Pale-ocene were Gunnera, ancestral Proteaceae, membersof Cunoniaceae, Didymelaceae and Myrtaceae (Figs. 3and 4). Taxa that dispersed to the north were Ilex,which has its oldest records in the Turonian of Aus-tralia (Martin 1977), Casuarina (sl) and Anacolosa. Itis likely that the S America–Antarctic connection in-volved a direct land bridge to facilitate dispersal oftaxa such as the microthermal Nothofagus (Fagaceae),although due to its near-polar position, this bridgewould strictly have acted as a filter for megathermalplants, inhibiting their dispersal. Climates were suffi-ciently warm, however, at the time of the Late Pale-ocene/Early Eocene thermal maximum, for a floristicinterchange with S American megathermal andmesothermal elements with members of Bombacaceae,Restionaceae, Sapindaceae and Polygonaceae dispers-ing to Australia (Fig. 5).

Dispersal route between Africa and India

At the time of the initial radiation of angiosperms, theIndian Plate was located too far south to include a sig-nificant angiosperm element, or to bear vegetation oftropical aspect. The Indian Plate separated fromGondwana in the Aptian, and drifted rapidly north-ward during the Mid Cretaceous, during which time itbore a Gondwanan flora similar to that of Australia.By Cenomanian to Turonian times, it lay in close prox-imity to Madagascar, and at this time, many plant taxawere able to disperse from Africa, via Madagascar andits associated islands, to India. One of the first of thesewas a winteraceous plant that produced Afropollispollen (IEDS 1996); it became extinct in the MidCenomanian. This pollen type was succeeded in theTuronian, or Early Senonian (90–74 Ma), as Indiadrifted across the southern hemisphere high-pressurezone, by the Constantinisporis group of pollen, whichis triporate with subequatorial pores and widelymisidentified with pollen of the palm Sclerosperma butmore likely of Myrtalean affinity (Harley 1996). TheConstantinisporis group, reported from the Late Cre-taceous of W Africa (Belsky & Boltenhagen 1963) andof S America (Muller et al. 1987), is recorded fromMadagascar by Chen (1978), and widely from theLate Cretaceous of India (Venkatachala 1974; Nandi1991). Other taxa that are likely to have followed thesame route (Figs. 3 and 4) are members of Sapin-daceae, Palmae (including nypoid palms and the Lon-

gapertites group), Myrtaceae, Ctenolophon (Srivasta-va 1987/88) and Normapolles (Nandi 1984, 1991).Dispersals ceased during the Maastrichtian, after theIndian Plate separated from Madagascar and began todrift rapidly northward toward Asia. The dispersalpath from Africa to India was likely to be a filter, asmany African taxa failed to make the crossing (Morley2000).

Some lines of evidence suggest that the Cenomani-an/Turonian dispersal path to India also reached Aus-tralia. The elater-bearing ephedroid pollen typeElateropollenites africaensis provides the best evidencefor this dispersal route, being widespread throughoutthe Cenomanian and Turonian of Africa and S Ameri-ca, but appearing suddenly in the Turonian of IrianJaya (Morley 2000). It is possible that some an-giosperms followed the same route, perhaps followingthe Kohistan Arc, with which the Indian Plate collidedduring its northward drift from Gondwana (Treloar etal. 1989; Treloar & Coward 1991). A further dispersalopportunity to the Australian Plate may have been cre-ated as India moved northward, since a string of is-lands formed along its southern trailing edge (Scotese2001). These islands form the NinetyEast Ridge andwere generated at the Kerguelen mantle plumehotspot. Although there is no direct evidence, theymay have facilitated dispersal between India/Mada-gascar and the Australian Plate in the Early Tertiary.Paleocene and Oligocene sediments from the Nine-tyEast Ridge have yielded pollen of Australian affinitywith one Madagascan endemic (Didymeles,Didymelaceae) and are thought to reflect long distancedispersal (Kemp & Harris 1975).

Middle Eocene to present,phase of plate collisioncoupled with global cooling

From the Middle Eocene to present day, three tectonicevents have had an impact on trans-oceanic an-giosperm dispersal routes as follows: (1) the collisionof the Indian Plate with Asia, (2) the complex collisionof the Australian Plate with the Philippine and AsianPlates, and (3) the formation of the Panamanian Isth-mus. During the period of these collisions, global cli-mates underwent stepwise cooling, strongly inhibitinginterplate dispersal of megathermal plants outside thetropical zone.

Collision of Indian Plate with Asia

At the time of the collision of the Indian Plate withAsia, during the Middle Eocene (50–39 Ma), both Indiaand SE Asia lay at similar latitudes and within the same



moist climatic belt (Fig. 5), as evidenced by the com-mon occurrence of coals within the sedimentary recordof both areas. At this time, SE Asian palynofloras showa sudden appearance of pollen of taxa that were charac-teristic of the Paleocene and Early Eocene of India, orwere of Gondwanan affinity, such as Durio type (Bom-bacaceae), Gonystylus (Thymelaeaceae), Iguanuroidpalms, aff Beauprea (Proteaceae), Restionaceae andMischocarpus type (Sapindaceae). At the same time,most of those SE Asian pollen types that cannot be re-ferred to modern taxa disappear from the record. Thisis thought to reflect the establishment of a moist corri-dor between India and SE Asia, with the dispersal of asomewhat aggressive Indian flora into SE Asia and theextinction of many local elements (Morley 1998,2000). Middle and Late Eocene (39–35 Ma) palynoflo-ras from Java are very diverse (Lelono 2000), suggest-ing that shortly after this phase of immigration, underthe influence of a warm and moist climate, the SEAsian flora had become notably quite species-rich.

Dipterocarpaceae are also likely to have dispersedto SE Asia from Africa via the Indian Plate, since theycurrently have representatives in both Africa, S Ameri-ca, Seychelles and Sri Lanka (Ashton & Gunatilleke1987) and fossil woods referable to the SE Asian sub-family Dipterocarpoidae are reported from the Ter-tiary of E Africa (Bancroft 1935). In SE Asia, their ear-liest fossil record is from the Middle Eocene of Myan-mar (Curiale et al. 1994) in the form on the geochemi-cal biomarker bicadenane (thought to be derived from

dipterocarp resins; van Aarssen et al. 1990) and is con-sistent with dispersal via the Indian Plate.

One reason why it has taken so long to realise thesignificance of the Indian Plate as a vector for rain for-est plants from Africa to SE Asia is that there are cur-rently very few wet climate refugia within the Indiansubcontinent. The ones there have mostly disappearedas a result of the development of monsoonal climatesfollowing the uplift of the Himalayas and the drift ofthe Indian Plate into the northern sub-tropical highpressure zone. A recent molecular evolution study byConti et al. (2002) on Crypteroniaceae and its alliesconfirm the importance of this dispersal route and ofthe ‘Out of India’ hypothesis.

Collision of the Australian Platewith the Philippine and Asian Plates

To explain the intermingling of Australasian and SEAsian floras following the collision of the Australianand Asian/Philippine Plates provides a major chal-lenge. Examination of present-day Sundanian florasshow clear examples of taxa of Australasian affinity,especially within the kerangas (heath forest) floras ofnutrient-poor podzolic soils, and in the mountains,whereas New Guinea and its associated islands inEastern Indonesia and the Pacific, with its predomi-nantly Gondwanan substrate, has a mainly SE Asianflora (Good 1962; Wiffin 2002). The rain forests ofEastern Australia also have some elements of SE Asian

14 R. J. Morley


Fig. 8. Noteworthy instances Pliocene and Pleistocene dispersals of microthermal taxa into the low latitudes. Distribution of closed canopy tropical rainforests during the last glacial maximum (Morley 2000).

affinity. This scenario for vascular plants must beviewed against that for the fauna of the region, onwhich Wallace’s Line was drawn, and which closelyfollows geological substrates, with faunas of predomi-nantly Australasian affinity in New Guinea and to theeast, and faunas of Asian affinity in Borneo, with amixture of affinities in intervening Wallacea (George1981).

The concentration of SE Asian elements in the flo-ras of Eastern Indonesia is thought to be relate to thetectonic history of the south-western arm of Sulawesi,which was attached to south-eastern margin of Borneoin the Middle Eocene prior to the formation of theMakassar Straits (Hall 1996, 2002), and has yieldedAsian affinity palynomorph assemblages of MiddleEocene age similar to those recorded through theEocene of Java and Kalimantan (Morley 1998). Fol-lowing uplift of New Guinea and its associated islandsin the Miocene, it is thought that this flora was able todisperse to the east without the need to cross Wallace’sLine and the Makassar Straits, which has been a bio-geographic barrier during the Younger Tertiary. Theantiquity of this flora explains why Wallace’s Line actsas a major dispersal barrier to the region’s fauna, buthas not been such an effective barrier to its flora.

Dispersals from Australasia to Sunda fall into threegroups: firstly, well-dispersed taxa which were proba-bly able to island hop following the initial phase ofcollision of the Australian Plate with the PhilippinePlate (Halmahera) close to the Oligo-Miocene bound-ary; secondly, chance dispersals across the MakassarStraits; and thirdly, dispersals of montane taxa follow-ing the Middle to Late Miocene uplift of New Guineaand other islands.

The first group of dispersals, relating to the initialphase of the collision of the Australian Plate with thePhilippine Plate, is based on pollen records of the Aus-tralasian taxa Phormium or Dianella (Hemerocalli-daceae), Casuarina (sl) and Dacrydium (Podocarpa-ceae) which suddenly appear in Java in the Mid-Tertiary (Fig. 7). This dispersal event was originallythought to be at about 21 Ma in the Early Miocene(Morley 1998, 2000), but recent unpublished worksuggests that it is likely to be a little earlier, within theLate Oligocene.

Following on this dispersal event, subsequent dis-persals occur at irregular time intervals, with Myr-taceae pollen showing a sudden increase in abundanceat about 17 Ma, interpreted by Muller (1972) as dueto dispersal from the east, followed by the appearanceof pollen of the mangrove tree Camptostemon (Bom-bacaceae) at about 14 Ma, and spores of the climbingfern Stenochlaena milnei (Blechnaceae) at 9 Ma. A fur-ther unpublished record suggesting dispersal fromAustralasia is of proteaceous pollen, recorded from the

Malay Basin by Jaizan Md Jais (1997). These disper-sals are thought to represent sweepstake dispersalsacross the Makassar Straits.

The third group of dispersals concerns mountainplants. The podocarp Dacrycarpus dispersed to NewGuinea from Australia in the Middle Miocene, andsubsequently to Borneo in the Mid Pliocene, at about3.5 Ma, but recent unpublished work suggests that itdid not disperse to Sumatra until 1.6 Ma, from whereit quickly spread to Indochina. The second was Phyl-locladus, which dispersed westward only to Borneo atabout 1.6 Ma. These taxa dispersed from New Guineaat a time when there was considerable uplift in thesouthern Philippines (van der Kaas 1991). The pres-ence of islands in this area probably allowed the bird-dispersed Dacrycarpus to ‘island hop’ to the Sunda re-gion. It is noteworthy that Nothofagus (Fagaceae) ac-companied Dacrycarpus to New Guinea, where it firstappears during the Middle Miocene (Morley 2000),but since Nothofagus is more poorly dispersed, it wasnever able to spread further west in the manner ofDacrycarpus. The montane dispersal route via NewGuinea may therefore be viewed as a filter for mon-tane elements with good dispersal mechanisms.

The Australian pollen record provides relatively fewdata that might reflect dispersals from the SE Asianarea following Neogene plate collision (Truswell et al.1987). Possible examples of dispersal from the north,based on Mid-Tertiary pollen appearances in Aus-tralia, are the climbing fern Stenochlaena palustris(Blechnaceae), Acacia, Caesalpinia and Crudia (Legu-minosae) and Merremia (Convolvulaceae). One sur-prising possible example is of Symplocos, which wasan element of the northern hemisphere Boreotropicalflora in the Palaeogene, but according to Martin(1994) reached Australia only in the Middle Miocene.

Formation of the Panama Isthmus

With further westward dispersal of the N and S Ameri-can Plates, and eastward movement of the CaribbeanPlate, the island arc that had formed along the trailingedge of the Caribbean Plate collided with the southernend of the N American Plate during the Mid Tertiary,and with Colombia in the Early Miocene (Pindall et al.1988). With subsequent uplift it formed a continuousland bridge between N and S America during thePliocene, which was followed by the faunal migrationsof the ‘Great American Interchange’ (Marshall et al.1979, 1982).

The most noteworthy plant taxa for which there is afossil record indicating times of dispersal are the mi-crotherm genera Juglans (Juglandaceae), Alnus (Betu-laceae) and Quercus (Fagaceae), which dispersed into



Table 1. Comparison of the ages of interplate dispersal routes as suggestedfrom plate tectonics with those of van Steenis (1962) in his Land Bridge theory(–, no information).

Steenis Land Bridge Age (van Steenis 1962) Age based on plate tectonics

1 Transatlantic Jurassic–Mid Cretaceous Early–Late Cretaceous

2 Transpacific Jurassic–Cretaceous Not needed

3 Madagascar– Mid–Late Cretaceous Late Cretaceous–EoceneCeylon

4 Subantarctic Mid–Late Cretaceous Mid Cretaceous–Paleocene

5 Bering Late Cretaceous–Tertiary Not applicable to tropicalplants

6 N Atlantic – Late Cretaceous–Eocene

7 C American – Late Cretaceous–Eocene + Pliocene

8 Europe–Africa – Late Cretaceous Eocene

9 Australia–SE Asia – Neogene

Colombia at about 2.2 Ma, 1.0 Ma and 300,000 Karespectively. Alnus and Quercus have subsequently be-come major components of Andean forests in Colom-bia (van der Hammen & Gonzalez 1964; Andriessenet al. 1993; Hoogheimstra 1984; Hoogheimstra &Ran 1994). An additional possible emigration from NAmerica during the Late Neogene is suggested by thesudden appearance of pollen of the montane foresttree Hedyosmum (Chloranthaceae) at about 4 Ma inColombia and the Late Miocene of Guyana (Wymstra1971). This genus has been shown by Colinvaux et al.(1996) to have been a component even of lowland veg-etation during glacial periods. It is also likely thatrelics of the Palaeogene Boreotropical forests (seebelow) followed this route, such as Trigonobalanus,Meliosma and Saurauia, all of which have Mid-Ter-tiary fossil records in Europe and N America, withpresent distributions including the Colombian Andes.

Demise of the Boreotropical flora

One of the consequences of Mid-Tertiary global cool-ing was the replacement of the northern mid-latitudeBoreotropical forests with frost-tolerant temperatevegetation (Fig. 7, 8). Frost-sensitive Boreotropicaltaxa had to disperse to the low latitudes, or face ex-tinction. A large number of these taxa found refuge inrain forests of S China and Sunda, dispersing south-ward along the permanent land connection betweenthe equatorial zone and northern mid-latitudes. Planttaxa were able to disperse between these two latitudi-nal zones in East Asia unhindered by oceanic barri-ers.

With respect to the European Boreotropical flora, acombination of latitudinal barriers, such as theMediterranean Sea, the Sahara desert and the uplift ofthe Alps prevented southward dispersal into theAfrican low latitudes. For this reason there are hardlyany Boreotropical elements finding refuge in Africanrain forests today. Tiffney (1985b) estimated that onlyeight Boreotropical genera are represented in the pre-sent African flora.

Within the Americas, Boreotropical elements wereprobably able to find refuge to some degree along thesouthern margin of the N American Plate, but couldnot disperse to equatorial latitudes until the formationof the Isthmus of Panama during the Late Mioceneand Pliocene. Those elements of the BoreotropicalProvince that were eventually able to find refuge in Cand S America, typified by Ampelopsis (Vitaceae),Gordonia, Meliosma, Saurauia, Symplocos andTrigonobalanus are now preserved as the amphi-Pacif-ic element of van Steenis (1962). Tiffney (1985b)recognised 22 Boreotropical genera in the present dayNeotropics.

In the Far East, there are many more representativesof Boreotropical forests, with, in addition to amphi-Pa-cific elements, Alangium, Cinammomum and Endian-dra (Lauraceae), Dracontomelon and Lannea (Anacar-diaceae), Mastixia (Cornaceae), and the palms Livis-tona, Nypa and Oncosperma. Tiffney (1985b) identi-fied 34 Boreotropical genera in Far Eastern forests. Therelative representation of Boreotropical elements in theFar East, Africa and Neotropics (Fig. 7) thus reflectsthe different opportunities for southward dispersal tothe tropical regions during the Late Tertiary (Morley2001) and explains the frequent references in the litera-ture to the relationship between Laurasian Early Ter-tiary floras with Malesia since the similarity was firstnoticed by Reid & Chandler (1933).

The possibility of dispersals of mesic Boreotropicalelements during the Late Cretaceous to Middle Eoceneshould also be given consideration since appropriateland bridges would have been in place to allowBoreotropical elements to disperse to both S Americaand SE Asia. Such taxa should also be poorly repre-sented in Africa, which was isolated from the northernmid latitude land masses from the Paleocene to MiddleMiocene. Symplocos might provide an example of thisdistribution type (Figs. 4, 7).

Discussion

The dispersal routes for megathermal plants as indi-cated from the fossil record correspond remarkablyclosely with the land bridges proposed by van Steenis(1962) with a single exception (Table 1). He proposed

16 R. J. Morley


rain forest expansion and contraction, is important forall taxonomic and molecular studies involving tropicalplants, even for those without a fossil record. Knowl-edge of the availability and timing of appropriate dis-persal pathways to a taxon limits the number of dis-persal opportunities open to it and allows the tax-onomist to make better judgements regarding phyloge-ny, even without the availability of a fossil record.

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