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ability in such a way that it can be perceived in the skeleton.
For example, the flightless ibises from Hawaii are known only
from islands dating to less than 1.8 Ma (Olson and James,
1982). Thus it is necessary to find a cause for this absence of
flightless forms. We have good reason to believe that this cause
may be found in the volcanic past of the island.
Reunion is situated on a hotspot that gave rise to the Deccan
traps during the Cretaceous, then the Chagos-Laccadive Ridge,
the Mascarene Plateau, and Mauritius (Bonneville, 1990). It is
made up of two volcanoes: the oldest is Piton des Neiges, in the
northwest, the volcanic activity of which now consists only of
thermal springs, whereas the more recent Piton de la Foumaise,
in the southeast, is still active.
Piton des Neiges is a strato-volcano, made up of hundreds of
lava flows spread out on gentle slopes. Built up on a 4000 m
deep seafloor, it emerged about three million years ago
(m.y.a.), but the oldest dated rocks are 2.1 Ma in age. Its construction
includes two main phases, one with tholeiitic and
transitional basalts, dating back 2.1 to 0.43 Ma, and one with
differentiated alkaline lavas, dating from 0.35 Ma to less than
30,000 years ago (Kieffer, 1990; Deniel et al., 1992; Kieffer et
al., 1993). The alkaline products appeared in the course of very
explosive emptions that resulted in several calderas.
Piton de la Foumaise appeared above the ocean about one
m.y.a. but collapsed several times on its eastern side. After
each collapse a new cone appeared farther westward. The first
collapse is marked by the caldera of Riviere des Remparts,
which is very close to the massif of Piton des Neiges.
The second phase of Piton des Neiges, with differentiated alkaline
lavas, shows three major explosive episodes. The first
one, before 230,000 years ago, emitted pyroclastites that fell
down on the whole western flank and probably also on the
northern flank. The Quenched Bombs Formation, which belongs
to this episode, is found from the Dos d'Ane, in the
northwest, to Saint-Pierre, in the south of the Piton des Neiges
massif. Thus at this time, the massif must have been almost entirely
covered by pyroclastites. The second one, known as the
dalle soudee (welded slab) episode, dates back to 230,000
years. It was produced by huge lava fountains that shot up several
km into the air and came down while still fluid, welding
after falling. After this explosive event, the center of the volcano
collapsed, resulting in a caldera, the diameter of which was
eight to 10 km. The third explosive episode, dated back to
188,000 years, produced ignimbrites that flowed down mainly
on the eastern flank. Other explosive phenomena occurred subsequently,
but the material emitted remained mainly inside the
caldera. Thus, during these explosive volcanic emptions, almost
all the massif of Piton des Neiges was covered by incandescent
products. In the meantime, the Piton de la Foumaise,
which had no explosive episodes, collapsed several times into
the ocean.
It may be supposed that Reunion Island was colonized by the
same birds that colonized Mauritius and Rodrigues, that is, a
pigeon ancestor of the Dodo and Solitaire, a rail ancestor of the
genera Aphanapteryx and Erythromachus, and a parrot ances­
SMITHSONIAN CONTRIBUTIONS TO PALEOBIOLOGY
tor of the genera Lophopsittacus and Necropsittacus, and that
they evolved on Reunion as on the other islands, progressively
losing their ability to fly. These flightless or almost flightless
forms disappeared during the explosive episodes of the second
phase of activity of Piton des Neiges, between 300,000 and
180,000 years ago. Either they disappeared instantaneously or
their environment was so depleted that they were not able to
survive. The island was colonized again by forms from Africa
or Madagascar, such as the ibis, Alopochen, falcon, and night
heron, or by forms from Mauritius, such as Anas theodori and
Fulica newtonii, and none of these forms had enough time to
become flightless.
Two genera, however, Mascarinus and Fregilupus, are endemic
to Reunion. It is probable that if they had arrived on
Reunion after 180,000 years ago, they would not have had
enough time to become generically distinct. One may propose
the hypothesis that the ancestral forms of these genera arrived
on Reunion at a more ancient period and survived the holocaust.
The presence of a flightless rail of the genus Dryolimnas is
compatible with this hypothesis because we have the example
of D. cuvieri aldabranus, which has become flightless, whereas
the Madagascan subspecies, D. cuvieri cuvieri, is still able to
fly. Aldabra Island has undergone several cycles of emergence
and submergence, and its most recent emergence occurred
about 80,000 years BP (Braithwaite et al., 1973). The Aldabra
White-throated Rail must have arrived on the island after the
last emersion, so 80,000 years or less were enough for its skeleton
to be considerably modified.
The study of bats and of the Reunion land tortoise, Cylindraspis
borbonica, does not refute the foregoing hypothesis.
The bats include two extinct species, Pteropus niger (Kerr) and
Pteropus subniger (Kerr), another that is probably extinct, Scotophilus
borbonicus (E. Geoffroy) {=S. leucogaster), and two
species that are still present, Mormopterus acetabulosus (Hermann)
and Taphozous mauritianus E. Geoffroy. All but Scotophilus
once lived, or are still living, on Mauritius (Cheke and
Dahl, 1981; Moutou, 1982). Scotophilus borbonicus also occurs
in Africa and Madagascar and probably colonized
Reunion in recent times (Cheke, 1987).
The genus Cylindraspis, endemic to the Mascarenes and now
extinct, is represented on Reunion by a single species, whereas
two species are known on Mauritius, and another two are
known on Rodrigues. The two Mauritian species, Cylindraspis
neraudii (Gray) (=C. inepta; =?C. indica) and Cylindraspis
triserrata (Gunther) (=?C. graii), can be distinguished from
each other by their skeleton and particularly by their skull, the
latter showing a specialization of the triturating surfaces (supplementary
ridges). These two sympatric and synchronous species
indicate either a long in situ evolution, with speciation, or
two successive colonizations from a common ancestral stock.
On Rodrigues the two species Cylindraspis rodericensis
(Gunther) (=?C. vosmaeri) and C. pel tastes (Dumeril and Bibron)
(the smallest form in the genus) exhibit a striking synapomorphy
(predominance of the palatine arterial circulation