The role of the ice ages in the dispersal of the genus Phelsuma.
to break up during Late Triassic to Early Jurassic time. Major mantle
plumes such as the Karoo-Ferrar Plume that first split Gondwana at
about 182 Ma, the Paraná-Etendeka plume at 132 Ma that split South
America and Africa, the Marion plume at 88 Ma that split Madagascar
and India and finally the Reunion hotspot that split the Mascarene
Plateau from India at 64 Ma.
The separation of the Mascarene
Plateau was accompanied
by a tremendous outpouring of basaltic lava from this hotspot, giving
birth to a mass of igneous rock called the Deccan Traps, which today
covers much of western India. Because the
Mascarene Plateau was just beginning to separate from India at the
time, much of this basalt covers the plateau as well, though the
eruptions did not smother all of the older granite.
Some of the granite remains still above
sea level, forming the granitic islands of the Seychelles, located in
the northern tip of the Mascarene Plateau. While the plateau drifted
northwards over the Réunion Hotspot a series of now submerged islands
were formed. This process started 64 Ma, after the separation of the
Mascarene Plateau and India, about 35 Ma the Saya
de Malha bank was formed, later the Nazareth bank and the Cargados Carajos Shoals (Saint Brandon).
Cargados Carajos today still counts 22 small islands. About 7 Ma the
northern islands of Mauritius and Mauritius itself where
formed. Reunion only reached the surface about 2 million years ago,
and is the youngest island originating from the Réunion Hotspot.
The Mascarene Plateau now extends approximately 2,000 km between
Seychelles and Mauritius and is one of the few submerged features
clearly visible from space. It covers an area of over 115,000 km² of
shallow water with depths ranging from 8 m to 150 m on the plateau,
plunging to abyssal depths of 4000 m at its edges. It is granite at
its core, with a mantle of basalt and limestone.
limestone banks found on the plateau were once coral reefs, indicating
that the Mascarene Plateau once formed an archipelago of islands much
bigger then Mauritius or Réunion today. Erosion, subsidence and sea
level fluctuations made the islands disappear.
Over time there has been a continuous change in sea levels due to
periods of glaciation near the poles (ice ages) that determined the
climate and sea levels on earth. The last of the ice ages reached its
maximum roughly 18,000 years ago, and then gave way to warming. Sea
level rose rapidly between 18,000 and 6,000 years ago (about 130 m) ,
drowning almost the entire Mascarene Plateau. Today only a few areas
remain un-submerged forming the islands of Saint Brandon, Albatross
Island and some offshore islets near Mauritius (incl. Round Island).
The rest is submerged, large areas by only about 10-40 meters below
current sea level.
tectonic processes like subsidence, subduction and sea-floor spreading
are not to be neglected even if they occur on much longer time scales
than glacial cycles. If the subsidence rate of the plateau is only 0.5
mm per year then the plateau never has been submerged during earlier
sea-level fluctuations. (see
Even during the ice ages, average temperatures in the area were
only 1° to 3° C lower than today. It is thus very likely that some,
now submerged, islands of the plateau were inhabited by the genus
Phelsuma. The morphology of the current Mascarene species
indicates an early dispersal from Madagascar. The young age of
Mauritius does not suspect this, meaning that the ancestor of the
Mascarene forms probably used the islands of the Nazareth bank and
the Cargados Carajos Shoals as stepping stones to colonize in a later