Journal of Biogeography, 30, 551–567
Neogene marine transgressions, palaeogeography
and biogeographic transitions on the Thai–Malay
Peninsula
David S. Woodruff* Ecology, Behavior, Evolution Section, Division of Biological Sciences,
University of California, San Diego, La Jolla, CA 92093-0116, USA
Abstract
Aim The aim of this review is to contribute to our understanding of the origination of
the Sundaic and Indochinese biotas in Southeast Asia. Numerous unsolved problems
surround the origination of the differences between these biotas and the determinants of
the breadth and current position of the transition between them.
Location Literature reviews show that phytogeographical and zoogeographical transitions between the Sundaic and Indochinese subregions lie on the Thai–Malay peninsula
just north of the Isthmus of Kra. A second, more widely recognized botanical transition
lies 500 km further south at the Kangar–Pattani line near the Thai–Malay border.
Results The phytogeographical transition involves 575 genera of plants, and a change
from wet seasonal evergreen dipterocarp rain forest to moist mixed deciduous forest.
The zoogeographical transition is best characterized for forest birds, and more than half
the species present in this region have species boundaries north of the Isthmus of Kra, at
11–13 N latitude. Although the phytogeographical transition is climate-related today,
and the avifaunal transition is viewed as being associated with the vegetation change,
there is no obvious present day geological, physiographical or environmental feature to
account for the origination of the provincial biotas. Similarly, known Neogene palaeoenvironmental changes on the tectonically stable peninsula, including those associated with periods of lower sea levels and the emergence of Sundaland, fail to account for
either the origination of the provincial differences or the current position of the transition.
Main conclusions Contrary to earlier palaeogeographical reconstructions, it is suggested that Neogene marine transgressions flooded the peninsula in two areas and created
circumstances leading to the biogeographical patterns of the present day. The Vail global
eustatic curve, supported by the oxygen isotope record, indicates that sea levels were
c. 100 m above the present-day level during the early/middle Miocene (24–13 Ma) and
again during the early Pliocene (5.5–4.5 Ma). Present topography suggests such high sea
stands would have created 30–100-km wide seaways north and south of the Nakhon si
Thammarat Range in the central peninsula (southern Thailand). Geological, palaeontological and phylogenetic evidence for such hypothetical seaways is scant (there have
been no focussed searches) but does not preclude their occurrence. The role of such
Neogene highstands in explaining present day biogeographical patterns in Southeast
Asia and elsewhere requires assessment.
Keywords
Biogeographical boundaries, sea levels, Tertiary palaeogeography, zoogeographical
transition, Indochinese biota, Sundaic biota.
*Correspondence: David S. Woodruff, Division of Biological Sciences, University of California, San Diego, La Jolla CA 92093-0116, USA.
E-mail: [email protected]
2003 Blackwell Publishing Ltd
552 D. S. Woodruff
INTRODUCTION
The biota of Sundaland has fascinated biogeographers since
Wallace’s (1869) narrative of his travels. The eastern
boundary of Sundaland has been the focus of much research
since he drew his famous line between Borneo and Sulawesi,
between the Sundaic subregion of the Oriental zoogeographical region and the Australian zoogeographical region
(Whitmore, 1981, 1987; Hall & Holloway, 1998; Metcalfe
et al., 2001) (Fig. 1). In contrast, the western boundary of
the Sundaic subregion, which meets the Indochinese subregion on the Thai–Malay peninsula, is poorly documented
and little discussed. This is surprising as twice as many
flowering plant genera have range limits there as at the
better known Wallace’s line (Steenis, 1950; George, 1981).
There are major phytogeographical and zoogeographical
transitions just north of the narrowest part of the peninsula
at the Isthmus of Kra (Hughes et al., 2003; Round et al.,
2003; Woodruff, 2003). The origination of this provincial
biogeographical boundary and the present control of its
geographical position are the focus of this paper.
A plethora of terms exists to describe the different regions
and biotas in Southeast Asia. I am interested in the phytogeographical interaction between the Continental SE Asiatic
Floristic Province and the Malayan Floristic Province of the
Indo-Malayan Subkingdom (Good, 1964) [the Indochinese
and Malesian floral regions of Takhtajan (1986)], the zoogeographical transition between the Indochinese Subregion
and the Malayan Subregion (Wallace’s Indo-Malayan Subregion) of the Oriental Region, and the transition between
the Indochina Bioregion and the Sunda Shelf and Philippines
Bioregion of Wikramanayake et al. (2002). Herein, I will use
Figure 1 Outline map of Southeast Asia
(adapted from Rainboth, 1996) showing
localities discussed in the text and the maximum extent of Sundaland 21–17 ka with sea
level at )200 m. Wallace’s Line which lies
immediately east of Borneo and Java, marks
the eastern limit of the Sundaic biogeographic
province. The western limit of the Sundaic
province, the transition between the Sundaic
and the Indochinese provinces, lies on the
Thai–Malay peninsula near the Isthmus of
Kra. Bar scale shows 300 km at 8 N.
2003 Blackwell Publishing Ltd, Journal of Biogeography, 30, 551–567
Seaways and Southeast Asian biogeography 553
the terms Indochinese and Sundaic for the northern and
southern provincial biotas, respectively, and divide the Thai–
Malay peninsula (sensu Wells, 1999) into three parts: northern (comprising the 450 km shared by Thailand and
Myanmar (Burma)(14–10 N), central (comprising the
400 km from the Isthmus of Kra to the Thai–Malay border
(10–630¢ N), and southern (the 800 km of peninsular
Malaysia, from 630¢ N–110¢ N). The palaeogeography of
the central peninsula during the Neogene (Miocene, Pliocene
and Pleistocene – the last 23.8 Myr, following the revised
Cenozoic chronology of Berggren et al., 1995) is of special
interest. The abbreviations Myr/kyr and Ma/ka are used to
mean million/thousand years and million/thousand years
ago, respectively.
The Indochinese and Sundaic provincial floras are distinctive and do not merge gradually on the Thai–Malay
peninsula (Ridley, 1911; Steenis, 1950; Whitmore, 1984;
Ashton, 1992; Richards, 1996). According to Steenis (1950)
575 genera of flowering plants reached their range limits
near the Thai–Malay border at 630¢ N. Of these, 375
Sundaic genera characteristic of evergreen dipterocarp rain
forest reach their northern limits, and the remaining 200
Indochinese genera, characteristic of the northern moist
mixed deciduous rain forests, reach their southern limits in
the same area. Steenis (1950) characterized the phytogeographical transition, which he termed as demarcation knot,
from a consideration of more than 1200 distribution maps of
all genera of phanerogams native in Malaysia. Unfortunately, as discussed in companion papers (Hughes et al.,
2003; Woodruff, 2003) the formal documentation has never
been published and the ranges of individual species have
never been mapped. It now appears that this transition is
more complex and occurs over a broader area, extending
north from near the Malaysian border at c. 630¢ N latitude
into the northern peninsula with most change occurring just
north of the Isthmus of Kra at 11–12 N latitude (Fig. 2).
The Indochinese–Sundaic zoogeographical transition is
even less well-documented (Hughes et al., 2003; Woodruff,
2003). Wallace (1876), placed the mammal transition zone
in the Tenasserim Range at 13–14 N. Wells (1976) located
the avifaunal transition near the Isthmus of Kra, finding that
150 species and subspecies of lowland forest birds had their
range limits there at c. 1030¢ N, three times more than at
any other site along the 1600 km peninsula. Zoologists are
aware of similar species–level transitions involving mammals, reptiles, amphibians, bumble bees and butterflies, but
no formal analyses have been published. Using the bird
distribution data, as the best currently available, my colleagues and I have analysed the range limits of 544 forest
associated birds along a 2000-km transect from north
Thailand (> 20 N) to the hills of central Malaysia (4 N).
We found a highly significant transition at 11–12 N, just
north of the Isthmus of Kra (Hughes et al., 2003) (Fig. 2).
We found 152 species (28% of those studied and more than
half of those found at these latitudes) had species limits in
this transition zone. When both species and subspecies are
counted together, a total of 269 taxa have range limits in the
same area. It is now clear that the provincial transition zone
2003 Blackwell Publishing Ltd, Journal of Biogeography, 30, 551–567
between Indochinese and Sundaic biotas lies in the northern
Thai–Malay peninsula just north of its narrowest point at
the Isthmus of Kra.
Many historical and ecological factors determine the
location and sharpness of transitions between biogeographical units (Hengeveld, 1990; Brown & Lomolino, 1998;
Peterson et al., 1999). Although it is to be expected that
individual plant and animal species differ in the position of
their geographical range limits, the significant concordance
of present-day range boundaries of both flowering plant
genera and forest bird species requires explanation. I review
here the various factors proposed to account for this pattern
and present a new hypothesis for further consideration. The
historical and ecological factors affecting species boundaries
of the present must be sought in the context of Southeast
Asia’s complex past. Unfortunately, regional palaeogeographical reconstructions have received little attention
except in the cases of (1) the large-scale interaction of continental plates and terranes, and (2) the Last Glacial Maximum (20 kyr BP) emergence of the Sunda Shelf. The
reconstruction of Neogene palaeoclimates and geography is
hampered by a dearth of data (much of it contained in obscure and proprietary reports relating to exploration of
petroleum and mineral deposits) and/or critical discussions
of the available data. Fortunately, this situation has improved recently (Hall & Holloway, 1998; Hall, 2001, 2002)
and the evidence now permits a first-order analysis that will
hopefully stimulate further research. Hall (1998) attributed
the differentiation of the regional biota to speciation in
Sundaland favoured by separation from Asia by climate and
unspecified topographical barriers, and from Australia by
marine barriers. In this review I shall attempt to identify
those unspecified topographical barriers that contributed to
the evolution of the Sundaic biota. Space limitations preclude the citation of all the meritorious papers published on
the diverse topics to be considered. Readers should consult
the references used by the authorities cited herein as a guide
to the vast relevant literature.
CLIMATIC CONTROL OF THE POSITION OF
THE PHYTOGEOGRAPHICAL TRANSITION
The position of today’s phytogeographical transition on the
Thai–Malay peninsula has been interpreted climatically
(Whitmore, 1984; Ashton, 1995; Richards, 1996). According to this explanation the changes in both flora and vegetation type parallel changes along a north–south climatic
gradient. Unfortunately, the evidence for this alleged relationship is not simple, and the problem has been compounded by the confusion of two different botanical
transitions that lie 500-km apart (Hughes et al., 2003;
Woodruff, 2003). There is a southern transition on the
Kangar–Pattani line (630¢ N) where the vegetation changes
from Malay-type evergreen rain forest to Thai–Burmese type
wet seasonal evergreen rain forest. The northern transition,
just north of the Isthmus of Kra, involves a more dramatic
change from wet seasonal rain forest to moist mixed
deciduous forest. Despite the distance between them both
554 D. S. Woodruff
Figure 2 Outline map of the Thai–Malay
peninsula showing localities mentioned in the
text, the position of the phytogeographical
transition between rain forest types at the
Kangar–Pattani line, and the southern edge of
the major biogeographical transition which
occurs north of the Ranong–Chumphon line
at the Isthmus of Kra. The )100 m isobath is
shown in the Andaman Sea, the Gulf of
Thailand is < 100 m deep. Hills above 100 m
are shaded and indicate the extent of land
above water following a 100-m rise in sea
level. If present-day topography is a useful
guide to Miocene and Pliocene paleogeographical reconstruction then seaways may
have crossed the central peninsula north and
south of the emergent Nakhon si Thammarat
Range. Bar scale shows 100 km.
transitions have been confused under the name Ôthe Kra
ecotoneÕ. The relationship between the two transitions and
van SteenisÕ (1950) floristic demarcation knot involving 575
generic range limits requires re-examination. I have assumed
herein that the major provincial transition is the northern
one, but note that some other discussants have focused their
attention on the southern transition between rain forest
types. The problem is well-illustrated by the following
quotation from Ashton (1995, p. 457) which begins with an
apparent confusion of southern and northern transitions:
ÔWhitmore (1984), has reviewed biogeographic evidence for
the major biogeographic boundary, which he terms the
Kangar–Pattani Line, which occurs in northwest peninsular
Malaysia across to southeast peninsular Thailand. This may
rather exactly correlate with a change from a climate (or soil
water conditions) which is effectively aseasonal to the south
of the Line, to one with a short annual period of lower
atmospheric moisture or soil water stress to the north. The
exact climatic relationship must await more detailed vegetation mapping…Õ. Unfortunately, the suggestion of a simple
association between rain forest type and the first appearance
of a month or more of drought is not well-supported by the
climatological data.
The southern peninsula from Singapore (1 N) to c. 6 N
is perhumid and was until recently covered by evergreen rain
forest. The southern transition is allegedly associated with
the occurrence, north of the line, of one or more months of
dependable drought. Along the west coast this is correct:
Alor Setar at 6 N is perhumid, Satun at 630¢ N experiences a 1–2-month dry season, Krabi at 8 N experiences a
2003 Blackwell Publishing Ltd, Journal of Biogeography, 30, 551–567
Seaways and Southeast Asian biogeography 555
3–4-month dry season. As one proceeds north along the west
coast of the central peninsula, the dry season gradually
becomes longer and more severe. At the level of the Isthmus
of Kra it extends from February until June on the west coast.
There is, however, no such pattern along the east side of the
central peninsula (Gaussen et al., 1967; key figure reprinted
in Wells, 1999, p. xix), where there is no dependable
drought until the latitude of Surat Thani (9 N), where it has
a duration of only 1–2 months. Attributing the transition
from Malay-type evergreen rain forest to Thai–Burmese type
wet seasonal evergreen rain forest does not appear to be
linked to the first appearance of a dry season on the east side
of the peninsula. As rainfall is high (2000–3000 mm) over
most of the central peninsula, other factors effecting water
availability may affect the position of the southern transition.
The northern phytogeographical transition, from tall wet
seasonal rain forest to lower moist mixed deciduous forest, is
less well-documented although it appears to be the more
significant. On the east coast, the transition is sharp and lies
near the town of Chumphon. On the wetter west coast, the
transition is much broader and extends over 200 km, north
to at least 14 N (with some Sundaic species occurring as far
as 17 N). Again, Whitmore (1984) and Richards (1996)
attribute its current position to climatic factors and although
this seems very reasonable I have to conclude that their
nature still requires documentation. On the east coast the
transition lies in an area where there is a dramatic increase in
seasonality. At Chumphon (1030¢ N) there are no dependably dry months, 100 km further north at Prachuap Khiri
Khan (1150¢ N) the dry season is 3–4 months in duration.
But such marked seasonality characterizes the entire west
coast from 7 N to at least 14 N. So, although the northern
transition is currently associated with the southernmost
point on the peninsula where a drought of 3–4 months
duration affects the entire peninsula, from coast to coast, the
climatic control of this transition also requires validation.
The climatic gradient hypothesis as presented above is
admittedly oversimplified. The north–south orientation and
elevation of the mountains (ridges at 1000 m in many places) creates local orographic anomalies. In the central and
northern peninsula there is a rain shadow on the east of the
ridges and higher rainfall on the west. The western hills
immediately south of the Isthmus of Kra receive over
4000 mm of rain annually (Thai Meteorological Department, 1977). It should not surprise us therefore that rain
forest extends further north on the west than on the east
coast. There are patches of evergreen rain forest on the west
coast of the central and northern peninsula, from Phuket
into the Tenasserim valley and north to c. 17 N. These
areas receive rain from both the southwest (Indian Ocean)
monsoon (May to October) and the northeast monsoon
(November to February). The fact that the transition of
vegetation types in the northern peninsula is sharp in the east
but very broad in the west suggests that seasonality coupled
with overall annual precipitation may well regulate its current position. The relationship is, however, more complex
and less well-documented than what the literature suggests.
2003 Blackwell Publishing Ltd, Journal of Biogeography, 30, 551–567
Wells (1976, 1999) interprets the position of the avifaunal
transition at the Isthmus of Kra as an adaptation to the
different forest types and it seems eminently reasonable that
the birds are tracking today’s vegetation change. Hughes
et al. (2003) and Round et al. (2003) document this transition and show that it is broader, especially in the west, and
lies a bit further north than previously understood. Their
analysis confirms that the avifaunal and phytogeographical
transitions are geographically concordant and occur between
11 and 14 N. If the avian transition is viewed as secondary,
then the challenge is really to explain the origination and
position of the phytogeographical transition. If the latter
were to be understood as an ecologically controlled boundary then any change in regional climate would be expected to
shift its position on the peninsula. This raises questions
regarding the position of the provincial boundary in the past,
and the circumstances that led to the evolutionary divergence of the different biotas on either side of it. This has
apparently never been examined and my prime purpose is
therefore to review the evidence for a barrier that may have
permitted the Indochinese and Sundaic biotas to diverge.
PALEOGEOGRAPHY OF THE THAI–MALAY
PENINSULA
The Malay Peninsula took shape when terranes rifted from
the eastern margin of Gondwana collided with Eurasia in the
Jurassic (Hutchison, 1989; Hallam, 1994; Hall, 1998, 2001;
Metcalfe, 2001). The northern peninsula, central peninsula
and western side of the southern peninsula originated as part
of the Indo-Australian Gondwanian plate called the Sinoburmamalaya or Sibumasu terrane. The eastern side of the
southern peninsula, in contrast, began as part of the Laurasian Cathaysian plate called Indosinia. The Indosinian terrane underlying eastern peninsular Malaysia is termed
Eastmalaya, or Eastmal. The hills of the modern peninsula
trace their origins to the conditions along the edges of
Eastmal and Sibumasu prior to collision (Paleozoic limestone and calcareous shale) and as a result of the collision
(intrusive granite plutons of Triassic–Cretaceous age) (Metcalfe, 1998). The central peninsula today is characterized by
a long north–south chain of granite mountains (the Nakhon
si Thammarat range) surrounded by Paleozoic limestone
hills on the west and a flat Quaternary coastal plain on the
east (Pongsabutra, 1991). At the northern end of the central
peninsula this pattern is offset by the major transcurrent
Khlong Marui fault, which crosses the peninsula from near
Phuket to Surat Thani. This fault, which was most active in
the Paleogene, produced the spectacular 100 km east–west
displacement of the mountains across the peninsula. North
of the fault, the mountainous spine of the peninsula is offset
c. 100 km to the west of the Nakhon Si Thammarat range
and although most of the Phuket range hills are Paleozoic
limestone and shale (with some Precambrian sandstone),
there are remnants of granitic plutons on Phuket and in the
Surin, Similan and Mergui islands offshore. The Phuket
range extends to the north along the Myanmar–Thai border
as the Tenasserim and Tanao Si (Bilauktaung) ranges that
556 D. S. Woodruff
run north–south through the northern peninsula. A second
major fault, the Ranong fault, crosses the peninsula at the
Isthmus of Kra, 150 km to the north of Khlong Merui. At
the isthmus, between the towns of Ranong and Chumphon,
the peninsula is only c. 45 km wide.
The regional paleogeographic reconstructions of Hall
(1998, 2001, 2002) are currently the best available. In the
middle Eocene, c. 45 Ma, the collision of the Indian plate
with Asia began to alter the overall geography of Southeast
Asia. Originally the position of the Thai–Malay peninsula
was closer to Indochina, and the Malay–Sumatra margin had
a NNW–SSE bearing [contrary to the E–W bearing of
Rangin et al. (1990) and other earlier reconstructions]. The
major Wang Chao–Three Pagodas Faults, which extend
from c. 16 N in Myanmar out across the South China Sea,
were active 40–30 Ma and mark the northern end or continental root of the original peninsula. The Miocene (23.8–
5.3 Ma) was also a period of significant regional tectonic
movement: the Philippine Sea plate rotating clockwise, the
Borneo plate and the northern Sumatra/southern Malay
peninsular plate rotating counter-clockwise. The northern
Thai–Malay peninsular plate rotated clockwise but remained
attached both to Indochina and to the southern Malay plate.
These movements led to the widening of sedimentary basins
in northern Thailand and the Gulf of Thailand, and by
c. 10 Ma the region had assumed its current geography.
In broadest terms, the Thai–Malay Peninsula has therefore
existed for over 100 Myr and during most of the Tertiary it
has probably been geographically similar to today, with a
spine dominated by north–south ranges of hills. In the
southern peninsula the hills are 100–200 km across and rise
to over 2000 m. In the central and northern peninsula the
ranges are much narrower and rise abruptly out of the low
eastern plains to ridge tops at c. 1000 m (highest point,
1835 m). The steepland boundary is often dramatically
sharp and typically lies c. 150 m a.s.l. These ranges have
been breached by the Khlong Marui and Ranong faults and
have been spectacularly eroded (especially in the regions of
limestone outcrop) by the high rainfall. Such erosion has
reduced the maximum elevation of the peninsula to <100 m
in two areas, both in the central peninsula: south of the
Khlong Marui fault in the north, and north-west of a line
between the towns of Alor Setar and Songkhla in the south.
In other places, in both northern and southern parts of the
peninsula, the minimum elevation of the ridge tops has
probably exceeded 300 m for the last 20 Myr. Minor local
tectonic activity will, of course, complicate such palaeotopographic reconstruction and, although isostatic changes
affect the coastal areas, especially the Gulf coast (Tjia,
1996), rates of regional uplift are probably on the same
order as rates of subsidence and erosion during the Neogene.
Throughout the Neogene continental Southeast Asia was
drained by the extended Chao Phraya river which, until its
headwaters were captured by the Mekong river in the
Pleistocene, was the largest river in the region (Rainboth,
1996). The river flowed across the Sunda Platform into the
South China Sea north of Sarawak (Tjia, 1980; Voris, 2000).
In the Gulf of Thailand it deposited up to 12,000 m of
non-marine and marginally marine clastic sediments. According to Hall (1998, 2001, 2002), the Sundaland platform
has been recognizable and tectonically relatively stable since
the late Eocene and, as discussed below, above sea level
during most of the Neogene.
I conclude this admittedly superficial account of the
paleogeography of the Thai–Malay Peninsula by noting that
there is nothing in its geological history or contemporary
physiography to clearly account for the position of the biogeographical transition under consideration at 11–12 N on
the present-day peninsula. No earlier workers have suggested any such simple physical association.
NEOGENE ENVIRONMENTAL CHANGES
IN SOUTHEAST ASIA
Failing to find climatological or geological evidence for
simple geographic barriers I will now review the evidence for
more complex palaeoenvironmental features that may have
functioned as biogeographic barriers. East of New Guinea,
the basaltic Ontong Java Plateau provides a stable platform
whose overlying sediments record the Pacific Ocean’s history
through the Neogene (Berger et al., 1993). Among the events
documented in these extraordinary benthic sequences are a
major cooling of deep ocean waters in the middle Miocene
associated with the growth of the East Antarctic and the
West Antarctic ice sheets, the closure of the Indonesian
Seaway (10–8 Ma), the early Pliocene warm period associated with a reduction of Antarctic ice volume to perhaps one
third of present volume (5–4 Ma), the closure of the Middle
American Seaway (c. 3.2 Ma), and the onset of Northern
Hemisphere glaciations (2.75 Ma). Global cooling since the
Eocene has been attributed to the growth of the Antarctic ice
sheets, decreasing atmospheric CO2 concentration, continental shifts, the uplift of the Himalayan–Tibetan plateau,
stepwise closure of Indonesian and Panamanian tropical
Pacific gateways, and vegetation–climate feedback (Dutton
& Barron, 1997; Filippelli, 1997). Antarctic ice sheets are
now known to be major determinants of Neogene environmental change and much older than previously suspected
(Abreu & Anderson, 1998). First evidence for the East
Antarctic ice sheet occurs c. 50 Ma in the lower Eocene; the
West Antarctic ice sheet formed by the middle Miocene, 18–
15 Ma. Both ice sheets have fluctuated greatly in volume
since then; both show signs of reduced ice volume during the
lower/middle Miocene and the lower Pliocene (Abreu &
Anderson, 1998; Zachos et al., 2001).
Morley (1998, 2000) reviews the palynological data
bearing on the reconstruction of plant communities in
Southeast Asia during the Tertiary. The hills of the Thai–
Malay peninsula and Sumatra were an important dispersal
route for Indochinese montane and submontane elements
during the late Cretaceous and Paleogene when Thailand
had an essentially Laurasian flora that included Pinus, Alnus
and Quercus and other species similar to those characteristic
of a modern northern temperate forest. Megafossil plants
indicate the presence of Ômonsoon forestsÕ in Asia as far back
as the Eocene (Guo, 1993). Since the Eocene the regional
2003 Blackwell Publishing Ltd, Journal of Biogeography, 30, 551–567
Seaways and Southeast Asian biogeography 557
phytogeographic record is interpretable in the context of
repeated climatic changes and the invasion of Gondwanan
elements from the Indian plate beginning 45 Ma (Eocene)
and from the Australian plate beginning 22 Ma (middle
Miocene). Following a drier Oligocene, the early Miocene
was wet and warm, with rain forests spreading and coals
forming across Sundaland. By the middle Miocene, tropical
forests had their greatest extent, reaching north to Korea and
Japan. Reef building coral also occurred at much higher
latitudes during the warmer early/middle Miocene (Wilson
& Rosen, 1998). Although the dipterocarps radiated 20 Ma,
they did not become dominant and widespread elements of
regional rain forests until Southeast Asia became progressively wetter c. 10 Ma, following the intensification of the
monsoons associated with the uplift of the Himalayas and
the Tibetan Plateau (Ashton 1988; Filippelli, 1997). Subsequently, global climate changes caused the repeated expansion and contraction of the rain forest. During periods of
drier, cooler and more seasonal climates (An, 2000) associated with the Pliocene and Pleistocene hypothermals, pine
woodland and savanna grassland expanded and completely
replaced rain forest over large areas (Morley, 2000).
Geographical changes associated with Pleistocene hypothermal phases were dramatic, as glacioeustatic falls in sea
level radically increased the width of the Thai–Malay peninsula. A marine regression of more than 100 m (as occurred
during several hypothermal phases) caused the Sunda shelf
to emerge and the Andaman Sea to retreat (Voris, 2000). On
the west of the northern and central peninsula, 100–500 km
of continental shelf became dry land. Further south, the
Straits of Malacca emerged and the entire southern peninsula was broadly connected to Sumatra. Changes were even
more dramatic along the eastern coast where the sea
retreated for 600–1000 km from the shallow Gulf of
Thailand far out into the South China Sea. The Thai–Malay
peninsula had a broad dry land connection to Borneo,
>500 km away. The emergent area (Sundaland), was first
characterized by Molengraaff & Weber (1921) and Molengraaff (1922) and is thought to have comprised a large gently
sloping plain extending out to about the present day 180-m
isobath. Although detailed palaeogeographical modelling
has not yet been undertaken, Sundaland probably emerged
to varying degrees for 5 Myr during the late Miocene, for
1 Myr in the late Pliocene and repeatedly for thousands of
years during each of the major Pleistocene hypothermals.
Some indication of the magnitude of these dramatic environmental fluctuations can be obtained by considering the
sea level changes during just the last 500 ka (Rohling et al.,
1998): oxygen isotope stage 2 (last glacial maxima): )140 m
at 18 ka, interglacial stage 3: )60 m at 45 ka, glacial stage
4: )80 m at 60 ka, interglacial stage 5: +7 m at 120 ka,
glacial stage 6: )150 m at 140 ka, interglacial stage 7: )5 m
from 200 to 240 ka, glacial stage 8: )120 m at 270 ka,
interglacial stage 9: +0 to 15 m at 325 ka, glacial stage 10:
)134 m at 340 ka, interglacial stage 11: +20 m from 390 to
415 ka, glacial stage 12: )140 m at 440 ka. Similar results
were derived independently by Lea et al. (2002) and
Lambeck & Chappell (2001) provide a more detailed review
2003 Blackwell Publishing Ltd, Journal of Biogeography, 30, 551–567
of the last glacial cycle. Voris (2000) correctly emphasizes
that the periods of maximum regression were relatively brief
and that sea levels were higher for most of this time. He
estimated that sea levels remained above )40 m for 50% of
the last 250,000 years. Using similar methods and a longer
palaeotemperature curve [Shackleton (2000), Lea et al.
(2002) and the modification of the 800,000-year Vincent &
2 Berger (1981) curve in Williams et al. (1998)], I estimate
that the average sea level was )60 m during the last
800,000 years. This is about the depth at which the Sunda
shelf emerges. Following the most recent glacial period the
sea did not re-enter the Gulf of Thailand until after 15 ka
(Somboon, 1988); it reached a maximum height of 4–5 m
above present mean sea level around 6 ka and has been close
to the present-day level for the last 3.5 kyr (Sinsakul, 2000).
Details of the local course of post-glacial flooding are provided by Hanebuth et al. (2000) who report a mean sea level
of )96 m at 14.6 ka, )80 m at 14.3 ka, )64 m at 13.1 ka
and )50 m at c. 11 ka. Rates of transgression were gradual
in this record except for the dramatic +16 m rise between
14.6 and 14.3 ka when sea levels rose at 5.3 cm year)1 for
300 years. Globally, such rapid rises in sea level following
the last glacial maximum are associated with the termination
of the Older Dryas and Younger Dryas cold phases, and with
the collapse of the Laurentide Ice Sheet between 8000 and
7700 ka. The latter event produced a near instantaneous
global rise of 20–40 cm and a 400-year rise of nearly 20 m
(Blanchon & Shaw, 1995). Obviously, the magnitude and
rate of these changes in physical geography complicate the
reconstruction of the history of the present-day transition.
The vegetation of the exposed Sunda shelf is poorly
known but during the Last Glacial Maximum (LGM) precipitation is thought to have been 30–50% less than that
prevailing at present (Kershaw et al., 2001). Rainfall was
reduced because the SW monsoon passed over the exposed
Sunda shelf rather than the warm South China Sea. The
evidence for such drier hypothermal conditions includes
palynolgical records from a mid-Pleistocene site near Kuala
Lumpur, from a deep-sea core from the Lombok Ridge
encompassing three glacial periods, and from Bandung Lake,
west Java (Morley, 2000). Evergreen rain forest was widely
replaced by large expanses of lowland pine and deciduous
woodland, by savanna grassland (Morley & Flenley, 1987;
Morley, 2000). Large freshwater swamps may have characterized depressions in the centre of the exposed shelf
3 (Emery & Nino, 1963). A savanna corridor apparently
facilitated the dispersal of plants and animals (including
elephants, mastodons, stegodonts and Homo erectus) from
Thailand to Java, Borneo and even Sulawesi across the drier
4 interior of Sundaland (Heaney, 1991). In montane west
Java, Kaars & Dam (1995) found that conditions were very
warm and wet 135 ka (interglacial) but dry during the LGM
(20–18 ka). At this site (665 m a.s.l.) the shift from swamp
forest to vegetation dominated by grasses and sedges implies
a fall in mean annual temperature of 5–7 C at 20 ka (Kaars
& Dam, 1997; Kershaw et al., 2001). At that time Southeast
Asian sea-surface temperatures were 4 C lower than that
at present. A similar glacial/interglacial change in sea
558 D. S. Woodruff
temperatures was reported by Pelejero et al. (1999).
Although upper montane rain forest communities and the
firn line were depressed 600–1000 m (Flenley, 1996; Kaars
& Dam, 1997; Morley, 2000) it is unlikely that plant communities migrated up and down the hillsides intact. It is
more likely that each species responded to climatic change
individually and that following climatic amelioration, the
mountains were re-colonized by new assemblages of available lowland taxa (Morley, 2000).
During Pleistocene hypothermal periods, the evergreen
rain forest and associated animals are thought to have contracted into ÔPleistocene refugiaÕ the location of which is still
conjectural. Glacial refugia models were first developed for
Amazonia (Haffer, 1969) but are invoked in other tropical
regions (Flenley, 1979; Richards, 1996). As the refugia
concept has been criticized and some of its premises refuted
elsewhere (Brown & Lomolino, 1998; Nores, 1999; Colinvaux et al., 2000; Hewitt, 2000; Runge, 2000), a re-assessment of its application to Southeast Asia is overdue.
Nevertheless, Brandon-Jones (1998) viewed two cycles of
contraction and expansion of rain forest as the prime
mediator of primate species diversity in Sundaland. In his
reconstruction, glacial droughts eliminated all but a few
small pockets of rain forest. He hypothesized that refugia
existed in north Sumatra, the Mentawi islands, north
Borneo, west Java, northeast Indochina and the Western
5 Ghats of south India (see Brandon-Jones, 2001: Fig. 1).
According to this view, rain forest disappeared almost
entirely from the Thai–Malay peninsula and currently forested areas were vegetated with open pine woodland and
savanna grassland. More recently, Gathorne-Hardy et al.
(2002) have used current termite community distribution
patterns to infer the location of the rain forest refugia and
reached similar conclusions. The present-day termite community composition reflects Quaternary patterns of rain
forest and savanna distribution and is concordant with the
limited evidence based on palynology (Stuijts, 1993; Kershaw et al., 2001), geology (Verstappen, 1997; Thomas,
2000) and fossil and current plant and animal distributions
7 (van den Bergh, 1999; Cranbrook, 2000). Gathorne-Hardy
et al. conclude that during the last glacial maximum, there
were three major rain forest refugia: in northern and eastern
Borneo, in northern and western Sumatra, and in the Mentawi Islands. Most of Thailand, peninsula Malaysia, eastern
and northern Sumatra, western and southern Borneo, and
Java were probably covered with grass–Pinus–savanna
woodland, as was the emergent Sunda shelf except at places
where it was crossed by rain forest edged rivers and fringed
by mangrove forests. Morley (2000) reached conclusions
similar to these but believes that there was more rain forest
on the emergent shelf. Kershaw et al. (2001) also believe that
the emergent shelf was covered with rain forest during the
LGM. They analysed pollen records for ten sites (lowland
and ocean cores) with comparable data for the LGM and
mid-Holocene. Except for the emergent shelf in the northern
part of the South China Sea (where Artemisia steppe grassland prevailed), they found little evidence for the dry corridor hypothesized by earlier workers. Instead, they found
evidence for rain forest between peninsular Malaysia and
Borneo. Although these forests may have been restricted to
riverbanks in a savanna–grassland matrix, it is clear that rain
forest did not disappear completely into small refugia during
the LGM. Furthermore, although the montane rain forest
species clearly shifted their ranges during the cooler and
drier periods, Kershaw et al. found no broad regional
changes in the distributions of vegetation types. Regional
patterns of distribution of evergreen and semi-evergreen rain
forest and deciduous forest must have developed long before
the LGM. All these reconstructions imply that the current
position of the transition(s) between Indochinese and Sundaic biotas is very recent in development as current plant
community distributions are <15,000 years old.
Unfortunately, the distribution of Indochinese and Sundaic plant and animal species during Pleistocene hypothermals is still poorly or completely undocumented. I will not
speculate here about the response of individual species to
these repeated environmental changes. We must keep in
mind, however, that documented species responses elsewhere are typically individualistic; communities are not
stable, persistent entities over long stretches of time
(FAUNMAP Working Group, 1996; Roy et al., 1998;
Morley, 2000). It is likely that the hypothermal vegetation
assemblages, particularly on emergent Sundaland, differed
from present-day plant communities and may have no
modern analogues. This makes it difficult to reconstruct
past species distributions based on current associations and
account for present patterns. The fact that the transition is
currently near the Isthmus of Kra may be misleading as its
position must have changed repeatedly during the last 3 Myr
and markedly during the last 800 kyr.
In conclusion, there is nothing previously reported about
the environmental history of Southeast Asia during Miocene,
Pliocene or Pleistocene that will account for the development
of the major biogeographical provincial transition. Despite
the large and repeated changes in species distributions, the
region appears to have had a spatial continuity that would
preclude the development of the observed provincial patterns
during the Neogene. Either the provincial biotas diverged
earlier or the cause of their divergence has not yet been
found.
MARINE TRANSGRESSIONS DURING
THE NEOGENE
There is one palaeoenvironmental phenomenon that appears
to have been overlooked in discussions of Southeast Asian
palaeogeography – marine transgressions. Although Corbet
(1941) felt certain that Malaya was formerly separated from
the mainland by an arm of the sea, most regional biogeographers have followed the conventional wisdom (Haile,
1971; Batchelor, 1979) that the present-day sea levels are
within 7 m of their highest since the Miocene. Apart from
that, I reported evidence for a +20 m highstand at 400 ka
8 and a +7 m highstand at 120 ka (Hearty et al., 1999, Rohling et al., 1998). I also present evidence for +100 m high sea
level stands in both the Miocene and the Pliocene that may
2003 Blackwell Publishing Ltd, Journal of Biogeography, 30, 551–567
Seaways and Southeast Asian biogeography 559
Figure 3 Eustatic changes in global sea level (after Haq et al., 1987, 1988; Mitchum et al., 1994; Abreu & Anderson, 1998). The time
scale used is from Berggren et al. (1995).
have had hitherto unsuspected effects on the palaeogeography of the Thai–Malay peninsula and argue that seaways
breached the central peninsula on two occasions of >1 Myr
duration. Such straits may have served as obstacles to the
dispersal of terrestrial organisms and may be causally related
to the present-day concordant species range boundaries and
the provincial transition.
Vail’s sea level curve (Vail et al., 1977), as revised by Vail
& Hardenbol (1979), Haq et al. (1987, 1988), and Mitchum
et al. (1994, for the Pliocene–Pleistocene) provides the basis
for this discussion (Fig. 3). Vail and colleagues reconstructed
eustatic history by applying sequence stratigraphy to a global
array of proprietary Exxon Production Research data comprising seismic profiles, well logs and outcrop records.
Hallam (1992) discusses the curve and its significance in
detail and independent studies (described below) support its
major features. Figure 3 shows that during most of the
Paleogene, sea levels were relatively high. From the Paleocene until the middle Oligocene, from 65 to 30 Ma, sea
levels fluctuated around +200 m (relative to the present day)
and fell briefly to +100 m on only five occasions. In the
middle Oligocene (28.5 Ma) there was a sudden spectacular
180 m regression to the present-day sea level, marking the
Rupelian/Chattian Stage boundary. Sea level then rose episodically during the late Oligocene and was at +100 m by
the early Miocene, 23.8 Ma. Although the following
account is based on the dates published in the various cited
reports and appears to be very precise, it must be noted that
the precision of absolute dating has improved dramatically
in the last decade and Zachos et al. (2001) should be consulted for some recent revisions of the chronology.
During the early and much of the middle Miocene, from
24 to 13 Ma (Aquitanian, Burdigalian, Langhian and first
half of the Serravallian Stages), sea levels were typically 100–
150 m above the present-day sea level. The Miocene highstand persisted for c. 11 Myr up to c. 13 Ma, when sea level
fell below +100 m for 6–7 Ma. During the 11 Myr of generally high sea levels – 24–13 Ma – there were three significant regressions, two of c. 100 ka and one of 1.0 Myr
duration, when sea level fell to c. +50 to 70 m. Sea levels fell
2003 Blackwell Publishing Ltd, Journal of Biogeography, 30, 551–567
three times during the Serravallian Stage (at 13.8, 12.8 and
11 Ma) and by the late Miocene (earliest Tortonian) were
at )90 m (below the present day). They rose during the
remainder of the Tortonian and early Messinian to +40 m
and then fell rapidly to below )50 m. In the late Messinian/
early Pliocene sea levels rose sharply to +100 m and fluctuated near +90 m for nearly 1.4 Myr, from 5.5 to 4.2 Ma.
Following this early Pliocene highstand, sea levels fell to
)20 m c. 3.5 Ma and to )100 m c. 2.75 Ma and then began
a series of relatively rapid glacioeustatic fluctuations with
23-, 41- and 100-kyr periodicities associated with orbitally
induced variation in insolation and Northern Hemisphere ice
volumes. The precise number of these fluctuations depends
on definition but there were on the order of twenty 100-kyr
eccentricity cycles in the last 2 Myr and their climatic
implications increased dramatically 700 ka. Significantly,
the present-day global temperatures are as warm as only 9%
of the last 3 Myr (Webb & Bartlein, 1992) and, as discussed
above, sea levels have not risen above +20 m during the last
500 kyr.
The eustatic curve presented in Fig. 3 is based on stratigraphic sequence data obtained by seismic methods. Independent confirmation for the general features of the curve is
now available from newly constructed composite benthic
9 foram oxygen isotope (d18O) curves based on numerous
drilling cores from deep sea and continental slope sites
(Miller et al., 1996, 1998; Abreu & Anderson, 1998; Lea
et al., 2002). The d18O curves are dated and correlated with
the geomagnetic polarity timescale and show remarkably
concordant timing of sequence boundaries with the stratigraphic data (Abreu & Haddad, 1998). Both types of curves
are regarded as reasonable proxy indicators of global
eustasy, which for the last 50 Myr has been determined
principally by glaciation. The isotope curves (not shown
here) indicate that the East Antarctic ice sheet expanded
during the Eocene and Oligocene, and decreased in volume
in the early Miocene. This corresponds with the initiation of
the 11 Myr highstand seen in the stratigraphic sequence
data. Initiation of the West Antarctic ice sheet, beginning
18–15 Ma, may account for the regression events during this
560 D. S. Woodruff
Miocene highstand. The isotopic record also supports the
pronounced late Miocene fall in sea levels (13.8–10.5 Ma),
presumably associated with increases in Antarctic ice volume
(Garcés et al., 1997). The Miocene/Pliocene boundary
highstand, associated with a period of extreme warming and
possible deglaciation between 6.0 and 4.4 Ma, is also supported by oxygen isotope ratios and other data (Abreu &
Anderson, 1998). The Miocene and Pliocene sea level
changes had amplitudes of c. 90 m – variation consistent
with the estimated ice volume changes in Antarctica. Sea
level fluctuations became greater with the start of Northern
Hemisphere glaciation c. 2.8 Ma when Antarctic volume
changes begin to respond to the rise and fall of sea level
caused by the expansion and contraction of Northern
Hemisphere ice sheets. Low amplitude fluctuations tracking
the 41-kyr Milankovitch orbital obliquity cycles account for
the pattern up to c. 800 ka when 100-kyr cycles become
dominant. These Pleistocene cycles are associated with 130–
170-m sea level fluctuations.
Although the Miocene and Pliocene highstands have been
widely ignored by regional biogeographers, Hutchison
(1989) suggested they were even higher than the +100 m as
claimed earlier; he estimated the maximal Miocene and
Pliocene highstands in Southeaast Asia at +220 and +140 m,
respectively. Others derived estimates of lower amplitude sea
level changes and lower relative highstands using data from
onshore backstripping (Miller et al., 1998) and changes in
ocean ridge volume (Kominz et al., 1998). It appears that
depositional sequences are a result of relative sea-level change
(eustasy and subsidence) while the isotopic signal is related
10 more to ice volume (Abreu & Haddad, 1999). Geologists
thus differ in their interpretation of these different sea level
estimates and, although the following hypotheses are based
on the data currently available, they must be regarded as
preliminary. Hall (1998) provided an explicit discussion on
the regional problems of determining the extent of land and
sea from the available record. His reconstructions (1998, Figs
11–16) showed an exposed Sundaland throughout most of
the Neogene but his Plates (1998, Figs 5–10) showed the
same areas as ÔsubmarineÕ. Neither series showed Miocene or
Pliocene highstands or transpeninsular seaways – but in
fairness, his focus was on larger scale regional plate tectonic
reconstruction and eustasy was appropriately not considered.
An earlier reconstruction (Hall, 1996; Wilson & Rosen,
1998) shows more clearly that parts of the peninsula were
submerged between 13 and 20 Ma, although the mapping
scale is too coarse for the type of palaeogeography of interest
here. Hall (2001, Figs 8–9) also shows shallow seas over parts
of southern Thailand in 25 Mya, 20 Mya and possible
15 Mya reconstructions, while the area of the Gulf of Thailand is shown as land, but he explicitly discusses the difficulties of determining precise water depths. Moss and
Wilson’s (1998) more detailed reconstruction of a much
smaller area, the region of Wallace’s Line, showed that it will
eventually be possible to recover information on the location
of former coastlines in western Sundaland too. Their reconstruction shows that Borneo was broadly connected to
Southeast Asia c. 8 Ma, a time of lower sea levels, and that
this land bridge may have been submerged at 4.0 Ma, a time
of higher sea levels.
The Vail sea level curve, supported by the d18O curve,
leads to a scenario where straits may have cut the Thai–
Malay peninsula during two periods in the last 24 Ma; first
in the early-middle Miocene for c. 11 Myr beginning at
24 Ma, and second in the early Pliocene for another 1.0–
1.4 Myr beginning at 5.5 Ma. Although the Miocene and
Pliocene seaways probably breached the peninsula in the
same areas, one cannot prove this by using the present-day
topographic maps. Nevertheless, an examination 1:250,000
map sheets of the Thai government shows that a +100 m
marine highstand would flood the central peninsula in two
areas at present (Fig. 2). In the north, a seaway would open
from Krabi in the west to near Surat Thani on the Gulf. It
would be c. 120-km long and 30–100-km wide. In the south,
a second seaway would run c. 100 km from Alor Setar and
Satun on the Andaman Sea to Songkhla and Pattani on the
Gulf. Based on the present-day topography, the southern
seaway would be 40–50-km wide. In both cases the straits
would be oriented roughly north–south and contain a
number of prominent islands. Between the northern and
southern seaways, much of the east side of the central peninsula would be submerged under shallow seas and forest
habitat would be greatly reduced to fragments on the
emergent Nakhon si Thammarat Range and western hills.
TESTING THE NEOGENE SEAWAY
HYPOTHESIS
Local geological evidence
Although some early geologists, including Scrivenor (1911,
p. 13) concluded from observations between Kedah and
Songkhla, that Ôin recent times the peninsula was an islandÕ,
this possibility has received little attention in the recent scientific literature. Nevertheless, the latest reviews of regional
geology (Tjia, 1988; Hutchison, 1989) provide some evidence for these marine transgressions and hypothetical seaways. The thick sedimentary deposits in the Gulf of Thailand
Basin and the Malay Basin provide strong evidence for a
Miocene highstand. Similarly, the Gulf of Thailand, Malay
and Natuna Basins become marine c. 5 Ma and provide
evidence for the early Pliocene transgression (Hall, 2002).
Onshore sections of marine Neogene sediments in the central
peninsula, as in other sections, have apparently been eroded
away, as been evidenced for the brief +5 m highstand
of 7000 ka. Eight small Tertiary sedimentary basins are
recognized in the central peninsula: three in the vicinity of
the northern strait (Khen Sa, Thung Yai, Krabi Kangtang),
two on the central west coast (Huai Yoi, Kangtang) and
three in the vicinity of the southern strait (Sadao-Bukit
Arang, Songkhla, Lankawi) (Hutchison, 1989, Fig. 3.3).
Unfortunately, with the exception of the Krabi Basin, these
basins are of poorly defined ages and have yielded few or no
fossils to date. The mammalian fossils in the Krabi deposits
are now known to be of upper Eocene age and represent a
seasonal tropical forest community (Ducrocq et al., 1995).
2003 Blackwell Publishing Ltd, Journal of Biogeography, 30, 551–567
Seaways and Southeast Asian biogeography 561
In the southern peninsula, Tjia (1988) described numerous
field indicators of +30 m sea stands but ascribes Pleistocene
ages to these features and interpreted higher (+75 m) features as either pre-Quaternary strandlines or features that
have been raised isostatically. From central Myanmar there
are recent reports of a Miocene highstand (marine flooding
from early to middle Miocene) punctuated by at least two
short regressions (Khin & Myitta, 1999). The five basins in
northern Thailand are 16–14 Mya and provide no evidence
for sea level changes (Ducrocq et al., 1995).
As earlier workers were unaware of the possible occurrence of +100 m marine highstands during the Neogene the
available geological and geomorphological data should be
re-examined as a first direct test of the Neogene seaway
hypothesis. Geological maps should be checked for marine
sediments, coral reefs, displaced erosional and aggredational
shorelines, evaporites, etc. that are not simply attributable to
local uplift. Geomorphological evidence for notched valleys
and cave drainage features indicative of high sea stands
should also be sought. Erosional features at 75–140 m a.s.l.
on the 1000 m limestone cliffs bordering the Khlong Saeng
Valley (west of Surat Thani in the northern central peninsula) are intriguing in this regard. Such features are now
accessible for the first time as a result of the flooding of the
valley associated with the construction of the Chiew Larn
reservoir (reservoir level >73 m a.s.l., pers. obs.). Such features may have formed on a northern embayment of the
northern seaway. As such cliffs are widespread in the central
peninsula they deserve scrutiny for evidence of Neogene
marine transgressions.
The overall topography of areas bounding the proposed
straits might also be examined for evidence of erosional
shaping by passing currents. The prevailing direction of such
currents is unknown as the possible interplay of the Andaman Sea and Gulf of Thailand has never been modelled. At
present currents flow south along both coasts of the central
peninsula but they are constrained by the shallow Straits of
Malacca in the west and the orientation of the Gulf of
Thailand in the east. Although the N–S orientation of the
seaways leads to the prediction of a flow from the Gulf into
the Andaman Sea, models of palaeo-currents would also
have to consider the effects of SW and NE monsoons (which
intensified c. 8 Ma), and differences in salinity and tidal
ranges in the two seas.
Palaeontological evidence
The limited fossil data available are neutral on the question
of the occurrence of the hypothetical Neogene seaways.
Ultimately, however, it is to be expected that fossil data will
clarify both the temporal history of the emergent Sundaland
platform and the nature and duration of the proposed
highstands. Few Miocene and Pliocene fossiliferous deposits
in the central peninsula are known and these have yet to be
carefully studied (Hutchison, 1989). Late Eocene (a time of
high sea level) deposits in the Krabi Basin on the western
side of the central peninsula contain a rich mammalian
fauna (Ducrocq et al., 1995) indicative of humid tropical
2003 Blackwell Publishing Ltd, Journal of Biogeography, 30, 551–567
conditions and a seasonal forest. More recent plant fossils
show the trans-Sundaland movement of three Australianderived groups but do not resolve whether their dispersal
was by island hopping or across an emergent platform:
Myrtaceae c. 17 Ma, the mangrove Camptostemon
c. 14 Ma, and the fern Stenochlaena c. 9 Ma (Baker et al.,
1998; Morley, 2000).
Chaimanee (1997) provides a first glimpse of the dynamism of the zoogeographical transition since the Miocene.
Changes in species composition in rodent communities at
twenty fossil sites throughout Thailand support the hypothesis of regional change from seasonal evergreen rain forest
with interspersed grassland to widespread moist deciduous
forest in the Pliocene. More interestingly, she found evidence
that species range limits have shifted over hundreds of kilometres. Fossils from early Pleistocene sites suggest that the
provincial transition lay 500 km south of the present site. At
Khao Rupchang on the Thai–Malaysia border, she found
one species restricted to China today and four others not
currently found south of the Isthmus of Kra. At probably
younger early Pleistocene sites at Ban Nasan near Surat
Thani, she found that one species was now restricted to
Malaysia and three others were now found only north of
Isthmus of Kra. At two middle Pleistocene sites (possibly
400 ka) north of Krabi (Khao Toi, Phang Nga) she found
fossils of three Sundaic (Indonesian) species 500 km north of
their present ranges and never before found in Thailand,
together with two Indochinese species now occurring only
north of the Isthmus of Kra. The coexistence of these Sundaic and Indochinese species were interpreted by Chaimanee
as suggestive of dense evergreen forest habitat and no
grassland, and also as indicative of the fact that the transition between provincial faunas was wider at that time.
Unfortunately, three sites in the northern peninsula including
Khao Tinpet, near Chumphon, are not informative with
respect to the transition; and older Pliocene sites are also
uninformative, and/or have only extinct species. The Pleistocene rodent communities thus appear more diverse than
that at present and may have no modern analogues. Chaimanee’s data will become more valuable as the ages of the
various species assemblages are clarified.
Tougard (2001) reviews records of large mammal species
(primates, carnivores, proboscidians, perissodactyls and artiodactyls) from thirteen Middle Pleistocene sites and five Late
Pleistocene sites in Southeast Asia. The fossil records indicate
that the provincial faunas differentiated long before the
Middle Pleistocene and Tougard interprets some records as
indicating that the present day provincial transition, which she
places at the Isthmus of Kra, may have shifted south to northern peninsular Malaysia during Pleistocene hypothermals.
With additional fossil discoveries and a coupled molecular
genetic survey of extant forms (see below), we may be able to
reconstruct the Neogene history of the transition.
Phylogenetic and phylogeographical evidence
Although I have not conducted an exhaustive search of the
recent literature of phylogenetic evolution in Southeast Asia,
562 D. S. Woodruff
I found only two scientists using a palaeogeographical model
involving a seaway across the Thai–Malay peninsula. Corbet
(1941) cited butterfly distributions as evidence for an old sea
channel, proposed earlier on the basis of plant distributions
by Ridley (1911, p. 59), and corresponding to the southern
seaway postulated here. Corbet concluded however that any
hypothetical seaway probably lay further north, and that the
explanation of the current faunal and floral transition was
largely climatic. Ridder-Numan (1998) argued that the
Isthmus of Kra was easily inundated during periods of high
sea level that occurred sometime between the mid-Eocene
and the Pliocene. She presented a cladistic analysis of
morphometric features of a genus of lianas, Spatholobus,
that ranges from India to the Philippines and Sulawesi and
found the first major split in the area cladogram separating Indochinese from Sundaic taxa on the peninsula south
of the Isthmus of Kra. Her paleogeographical reconstruction
was based on Hutchison’s (1989) extreme estimates of
Neogene highstands and the regional tectonic reconstruction
of Rangin et al. (1990). Although I agree that marine straits
separated southern parts of the Malay peninsula from
Indochina, I differ from Ridder-Numan on three issues. First,
I can find no evidence to support the existence of a seaway
at the Isthmus of Kra. Secondly, the palaeogeographical
reconstructions of Rangin et al. and more recently Hall
(1998, 2002) are not concerned with eustasy (although this
is considered by Hall, 2001). Their focus is on plate tectonics
and evidence for global sea level changes must be sought
elsewhere. Finally, in contrast with Ridder-Numan’s (1998,
p. 271) view that it Ôis impossible to indicate more exactly in
what period this splitting off took placeÕ, I think the evidence
permits the spatial and temporal aspects of the hypothetical
seaways to be modelled with much greater precision.
Regardless of these differences, she has presented a number
of eminently testable hypotheses; one example, concerning
the origin of S. ferrugineus clade A42 from clade A48,
appears well-suited to the type of phylogeographical testing
advocated below.
The degree to which these hypothetical seaways functioned
as barriers or obstacles to dispersal of terrestrial organisms
will have varied tremendously. Significant reductions of
available habitat must also have had a variable impact.
Hughes et al. (2003) discuss the probable response of bird
taxa of different ages to the hypothetical Pliocene seaway and
argue that the examination of bird phylogeny, phylogeography and ecology may throw light on the origin and history of
the transition. Molecular genetic comparisons of populations, subspecies and species permit the estimation of the age
of such taxa (e.g. Klicka & Zink, 1999; Hewitt, 2000; Eggert
et al., 2002), the determination of the direction of dispersal
or colonization (e.g. Woodruff et al., 1999), and the assessment of the isolating effects of historical barriers (e.g.
Jackson et al., 1996; Avise, 2000). Unfortunately, such
studies are in their infancy in Southeast Asia, and no genetic
data are yet available to test the seaway hypothesis. By
comparing sister taxa – related populations of similar
taxonomic rank (subspecies or species) – on either side of the
current provincial transition, one should be able to establish
the existence of concordant patterns of genetic differentiation
of either Pliocene or of Miocene age. Phylogenetic and
phylogeographical analyses of genetic variation of selected
organisms should answer the question of whether a hypothetical barrier created the circumstances that permitted the
evolution of the present-day pattern. One might also look for
evidence of the hypothetical seaways by comparing conspecific populations and congeneric species of various marine
organisms with low vagility from either side of the peninsula.
Comparative studies of marine organisms across the Isthmus
of Panama (Jackson et al., 1996; Lessios, 1998) are most
informative in this regard and show that barriers do not have
to be fully formed to separate populations genetically and
permit divergence and speciation (Vermeij, 1993).
As proposed above, both Miocene and Pliocene seaways
would have existed long enough for populations of plants
and animals isolated on either side of them to diverge evolutionarily. The fact that the hypothetical seaways occupied
the same geographical positions may have reinforced their
impact on the biotas although the individual species involved
would probably have turned over in the intervening 8 Myr.
As the time required for allopatric divergence associated
with speciation (not to be confused with the duration or age
of a species once formed) in North American songbirds and
arthropods is 0.5–1.0 Myr (McCune & Lovejoy, 1998;
Klicka & Zink, 1999; Avise, 2000), and far less in cases of
sympatric speciation or when ecological adaptation is under
strong selective pressure (Orr & Smith, 1998), there is no
doubt that barriers with durations of >1 Myr have the
potential of causing increases in regional species-level
diversity. The Miocene seaways, with cumulative durations
of 11 Myr, may be associated with what we see today as
generic level diversification in some groups, and the impact
of the 1 Myr Pliocene seaways are very probably recognizable in the present-day species (Avise, 2000). There is little
evidence to support the traditional view that the dramatic
cyclical late Pleistocene environmental changes are responsible for widespread speciation in either the temperate region
or the tropics (Patton & da Silva, 1998; Klicka & Zink,
1999; Hewitt, 2000).
A historical anecdote
Although some recent scientists may have missed signs of
Neogene seaways across the peninsula, historians have not.
Gerini (1909) describes his visit to Ligor (now Nakhon si
Thammarat) in 1885 and his impression–based on geomorphology and subfossil molluscs–was that a shallow seaway
once extended from the Gulf coast between Ligor and
Singora (now Songkhla) south to the west coast between
Kedah and Korbie. He cites anecdotal historical evidence
that sea-going ships found passage across this southern
seaway until the middle of the first century AD when it
became impracticable. In particular, he invokes the former
seaway to explain the paradox of Marinos of Tyre and
Ptolemy who both described the Golden Chersonese (the
Malay peninsula) as a peninsula while earlier European
geographers all refer to it as Khryse Insula or Golden Isle, as
2003 Blackwell Publishing Ltd, Journal of Biogeography, 30, 551–567
Seaways and Southeast Asian biogeography 563
Ôc. 350 mÕ and the Ô5.5 mÕ Pliocene highstand is described as
being +140 m above present. The original curve, as revised
by Abreu and Anderson (1998) and used here, as Fig. 3, is
much easier to interpret.
Finally, I reiterate that although there is far more written
about the palaeogeography of Southeast Asia than I have
been able to cite and discuss here, the type of resolution
attainable is still minimal. With more research and better
interpretation of existing data we will be able to tackle the
really interesting questions presented by the biogeographical
transition on the Thai–Malay peninsula. The origination of
the Sundaic biota from the Indochinese biota, the history
of the transition between the two biogeographical provinces,
the hypotheses that climatic and/or sea level changes fragmented biotas and led to cyclical vicariance (the species
pump model) accounting for the amazing regional biodiversity, are among the great opportunities for further
research. I submit that the investigation of the Indochinese–
Sundaic transition will be as rewarding scientifically as the
far more advanced studies of Wallace’s Line and the Great
American Interchange.
does even the earlier Ramayana (c. 300 BC). Evidence for this
historical seaway can now be sought, as can that for the
transgression that may have occurred during the hypsithermal, 6000–7000 ka.
GLOBAL IMPLICATIONS OF THE NEOGENE
TRANSGRESSIONS
Whether or not Miocene or Pliocene seaways cut the Thai–
Malay peninsula, the marine highstands suggested by the Vail
curve have widely unappreciated global implications. The
Miocene highstand suggested by the global curve has been
verified on the coastal plain and continental shelf of New
Jersey (Miller et al., 1996). This highstand is also associated
with the recently postulated Amazon Seaway that may have
connected the Caribbean with the South Atlantic and the
Paranense Sea of Argentina (Hoorn, 1993; Räsänen et al.,
1995; Webb, 1995; Nores, 1999). Alternatively, The proposed early Pliocene transgression appears to be earlier than
the +45-m highstand associated with the Orangeburg Scarp in
the southeastern US which formed 3.2–2.5 Ma and may represent the second Pliocene highstand seen in the curve (Paz11 zaglia & Gardner, 1993). Another Pliocene seaway, the
Suwanee Straits, cut across the top of the Florida peninsula
and affects phylogeographical patterns in diverse vertebrates
in the southeastern US (Avise, 1994). Evidence for Neogene
(early/middle Miocene or early Pliocene, or both) highstands
also occurs on the continental margins of the Mediterranean,
in western Africa, southern Australia, and on numerous
islands and other peninsulas (Hallam, 1992; Upton & Murphy, 1997; Moss & Wilson, 1998; Westphal, 1998). Recently,
Koegh et al. (2001), in explaining a biogeographical pattern
involving snakes and other organisms, cite Hall (1998) as
evidence that the entire northeastern half of Sumatra was
under water from 20 to 10 Mya – a period roughly concordant
with the Miocene high stand defined more precisely above.
Why, given the level of interest in biogeographical
boundaries and in the effects of Pleistocene glacioeustatic
changes, were the earlier Neogene transgressions ignored?
Evidence for Miocene and Pliocene highstands based on the
Exxon’s seismic and sequence stratigraphy has been available for over 20 years (Vail et al., 1977). One possible reason is that the widely published Vail curve was labelled in a
manner that suggested there were no transgressions greater
than +10 m throughout the Tertiary. For example, Vail &
Mitchum (1979) and Haq et al. (1987) present figures
showing global cycles of relative sea level fluctuating between zero and 1.0, and between zero and 10, respectively,
but the units are not defined on the figures themselves. Some
workers subsequently changed the scale labels to show sea
levels changing between 0 and 10 m (e.g. Hutchison, 1989,
fig. 3.4). All this suggests to the casual reader that highstands
of +100 m are unsupported by the evidence. A close reading
of the original reports shows, however, that the scales
identify relative rates of sea level change in time and not
absolute changes in height. Hutchison (1989) makes this
clear in his adaptation of the 1979 curve by annotating the
curve such that a Late Cretaceous Ô10 mÕ highstand is labelled
2003 Blackwell Publishing Ltd, Journal of Biogeography, 30, 551–567
ACKNOWLEDGMENTS
I thank the following for their useful discussions on the ideas
presented here or comments on the manuscript: Peter Ashton, Wolfgang Berger, Warren Brockelman, Joseph Curray,
Jennifer Hughes, Philip Round, Kaustuv Roy, Thanawat
Jarupongsakul [formerly Somboon], Sompoad Srikosamatara, Peter Vail, Geerat Vermeij, David Wells, Tim Whitmore. This paper is dedicated to the memory of the late
Edmund D. Gill, paleontologist, Museum of Victoria, who,
as Australian correspondent on the INQUA Quaternary
Shorelines Committee first interested me in paleogeography.
This research was supported, in part, by the U.S. National
Science Foundation and the University of California.
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BIOSKETCH
David Woodruff is a professor of biological sciences at
UCSD. He has investigated the phylogeography of such
diverse groups as Australian frogs, Bahamian land snails,
Neotropical and Asian freshwater snails and their
schistosome parasites, chimpanzees and elephants. He
has conducted collaborative research on the evolution
and conservation of animal species in Thailand for
20 years.