Malagasy/Indo-australo-malesian Phytogeographic Connections
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EOCENE / OLIGOCENE "LEMURIAN STEPPING-STONES"
primarily Laurasian elements
distributions in Africa, Madagascar, the Seychelles, India/Sri Lanka, W. and E. Malesia, China, Japan
closely-related species in the W. Indian Ocean/Africa
Calling attention to a distributional pattern that included
(Africa)/Madagascar, the Seychelles, (India)/Sri Lanka and W.
Malesia, van Steenis (1962),
in his now infamous paper on the
land-bridge theory, asserted that "there must have been an
isthmian connection between Madagascar and Ceylon over the
Seychelles-Comores bank" operating during the middle to upper
Cretaceous, which he named "Lemuria". Modern understanding of
the geological history of the Indian Ocean refutes both the
timing and existence of such a direct land route
(Mckenzie & Sclater 1973),
but the distributional pattern nevertheless still
stands, and for some taxa also includes China, Japan, and E.
Malesia. Such a pattern often involves closely-related species
in, and around the periphery of, the Indian Ocean and Africa. As
India assumed its current position from the early Eocene onward,
global sea levels were dropping, with a marked regression at the
Rupellian/Chattian boundary during the Oligocene, i.e., ca. 30
MYA (Haq et al. 1988).
|At that time, significant portions of the
Chagos/Laccadive Plateau and the contiguous (at that time)
Mascarene Plateau (including the Seychelles Bank, which
encompasses over 50,000 km2 now at an average depth of 75 m)
could have been emergent, and served as stepping-stones for
dispersal of essentially Laurasian (mesic) elements between
Laurasia/W. Malesia and Africa/Madagascar via India/Sri Lanka as
depicted in Figure 2.
Figure 2. Schematic representation of migration pathways active
during Eocene/Oligocene after India has attained close proximity
to Asia, through to post-collision (and continuing to present), a
period during which sea-levels were dropping. Mesic Laurasian
and W. Malesian elements enter Madagascar/Africa via India and
"Lemurian Stepping-stones" in the W. Indian Ocean (emergent
portions of the Chagos/Laccadive Plateau and Mascarene Plateau
including the Seychelles). Xeric elements are exchanged between
N.E. Africa/Arabian Peninsula and Laurasia/India and may disperse
to dry western Madagascar. Thickness of arrow represents
probability of dispersal.
As such a dispersal track coincides with
the "Lemuria" land-bridge of van Steenis, I propose that it be
named the "Lemurian Stepping-stones". Xeric affinities between
Madagascar and India (e.g., Commiphora (Burseraceae), Delonix
(Fabaceae), Moringa (Moringaceae) are best explained by overland
migration through N.E. Africa/Arabian Peninsula, and do not
provide additional support for the "Lemuria" land-bridge" as Wild (1965) suggested. Although the stepping-stone pathway may only have begun operating during the Eocene, there is no reason to believe that it has not remained active to the present day
(Madagascar to Seychelles - 1,000 km; Seychelles to India - 2,600
km), or that migration has not also occurred in an eastward
direction (e.g., Brexia (Brexiaceae), Grisollea (Icacinaceae)).
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