Introduction
All Mascarene species are closely
related, indicating they must have had a common ancestor. On the other
hand are the islands to young to have generated such a diverse group,
with Phelsuma gigas on one end and Phelsuma inexpectata on
the other. Most likely the source pool of this group was situated away
from the fast speciation process and taxon cycle on Madagascar. 20.000 Years ago, Saya de Malha and
Nazareth (incl. Cargados Carajos) formed large islands about 700 km
north-east of Madagascar. The first of these islands (Saya de Malha)
was formed about 35 Ma and was probably never submerged until the
last ice age 18.000 years ago.
Materials and Methods
In order to compare the status, distribution, and other aspects
related to the biogeography of the genus Phelsuma, a thorough
literature review was done. A map of the region has been
generated with the sea levels present 20.000 years ago based on
the bathymetry and today's sub-sea level topography.
Several specimen where examined, both alive and preserved.
Comparisons between the different mascarene species were
conducted.
Results
Geography -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.
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
island of Mauritius was formed.
Reunion only reached the surface about 2 million years ago, and is
the youngest island originating from the Réunion Hotspot.
 The
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.
Plate
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.
Biology -All Mascarene species are
closely related to each other as similar morphological features are
present. The location of the nasal cavity in between the nostril and the
first super labial clearly distinguishes all Mascarene species, including
Phelsuma guentheri and Phelsuma edwardnewtoni, from the other members of the genus.
The vertical pupil and the colouration of
Phelsuma guentheri indicates that it evolved from an ancestor that
colonized the islands in a very early stage. Probably during the
time the genus just became diurnal on Madagascar. Especially the presence of
another semi-nocturnal species, Phelsuma gigas (now extinct),
on the very young island of Rodrigues (1.5 Million years) indicates
that this species has reached the island recently from a much older
source pool.
Discussion
Most likely the now submerged islands played a very
important role in the evolution of the genus Phelsuma as
stepping stones to colonize the current islands of the Mascarene
Archipelago, Mauritius, Réunion and Rodrigues. But also Agalega was
probably colonized from these now submerged islands. 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 stage Mauritius.
The Mascarenes were probably colonized from the more northern Nazareth bank
(now submerged) and Cargados Carajos Islands. There, the early colonists, arriving from Madagascar,
evolved at a slower paste due to the limited topographic features.
Speciation occurred with little radiation (anagenesis). When Mauritius,
Rodrigues and later Réunion where colonized with forms that slowly
evolved here, adaptive radiation became more frequent and the speciation
process was supported by the many topographic features and the
ecological niches on the islands.
|
