which drained the eastern slope of an emergent Gibraltar Isthmus to the Mediterranean Basin at the end of the Messinian. Consequent canyons are still found in the
marine topography towards the Alboran Sea. The present Spartel and Camarinal
Sills both originated from the coeval mass sliding of the northern bank of the strait
(Blanc 2002).
This narrative leads to thinking that the bulk of the Mediterranean biota has been
received from the Eastern Atlantic through the Strait of Gibraltar, populating a basin
whose flora and fauna may have been practically eliminated by the Messinian
Salinity Crisis. The idea that all Mediterranean biotic elements should proceed
from the Atlantic was first put forward by Paul Pallary, a schoolteacher in Oran
who published extensive work on the Mollusca of North Africa. Without being
aware of the salinity crisis, Pallary (1907) wrote (in translation) that “the Mediterranean having received its population from the Ocean, there is no such thing as a
Mediterranean fauna”.
Nonetheless, some aspects of the Mediterranean biogeography are discordant
with this scenario. The rate of endemism in the Mediterranean Sea is high, contrary
to what would be expected in a subset of the Atlantic biota. The overall rate of
endemism is more than 20% (Bianchi et al. 2012) and a good number of endemic
Mediterranean species are paleoendemics, not recently segregated from an Atlantic
ancestor but with a relict distribution limited to the Mediterranean. The most
spectacular example is the marine seagrass, Posidonia oceanica, a flagship species
of Mediterranean biota, which has its westernmost populations in the Alboran Sea
and its nearest relatives in Australia. Other species have a still more puzzling
distribution, as is for example the case of the small gastropod Homalopoma
sanguineum (Fig. 11.2), which has no close relatives in the Atlantic and avoids
even the Alboran Sea. The history of these Mediterranean endemics and
paleoendemics is still far from being understood (Ekman 1953). Where were those
species during the salinity crisis? The adjacent Paratethys was reduced, in its western
part, to the brackish Lake Pannon, which could only harbour a fauna similar to that
of the “Lago-Mare” episode. Two possibilities are left: either some parts of the
Mediterranean proper were neither evaporated nor brackish and could give refuge to
Posidonia and other marine biota, or the adjacent part of the Atlantic could provide a
suitable habitat. The first alternative is not supported by any fossil record anywhere,
and the second would suppose that the deep embayment of the southwestern Iberian
Peninsula and Northwestern Morocco was then not so much sediment-clad as it is
nowadays.
In any case, the species richness of the Mediterranean was fully restored during
the Pliocene with many species that are now extinct or are restricted to more southern
waters off northwestern Africa. At that moment the history of the Mediterranean
biota was already linked to the populations of the Atlantic. Not so widely renowned
as the Italian Pliocene (Monegatti and Raffi 2001), the outcrops around the Western
part of the Alboran Sea near Estepona have provided an extremely diverse Pliocene
fauna mostly studied for molluscs (see Landau et al. 2009 and references therein).
From there on, the biogeographic history of the Mediterranean is essentially the
result of a series of climate changes, resulting in repeated biotic shifts. There were
11 Biogeographical and Macroecological Context of the Alboran Sea
435
marine topography towards the Alboran Sea. The present Spartel and Camarinal
Sills both originated from the coeval mass sliding of the northern bank of the strait
(Blanc 2002).
This narrative leads to thinking that the bulk of the Mediterranean biota has been
received from the Eastern Atlantic through the Strait of Gibraltar, populating a basin
whose flora and fauna may have been practically eliminated by the Messinian
Salinity Crisis. The idea that all Mediterranean biotic elements should proceed
from the Atlantic was first put forward by Paul Pallary, a schoolteacher in Oran
who published extensive work on the Mollusca of North Africa. Without being
aware of the salinity crisis, Pallary (1907) wrote (in translation) that “the Mediterranean having received its population from the Ocean, there is no such thing as a
Mediterranean fauna”.
Nonetheless, some aspects of the Mediterranean biogeography are discordant
with this scenario. The rate of endemism in the Mediterranean Sea is high, contrary
to what would be expected in a subset of the Atlantic biota. The overall rate of
endemism is more than 20% (Bianchi et al. 2012) and a good number of endemic
Mediterranean species are paleoendemics, not recently segregated from an Atlantic
ancestor but with a relict distribution limited to the Mediterranean. The most
spectacular example is the marine seagrass, Posidonia oceanica, a flagship species
of Mediterranean biota, which has its westernmost populations in the Alboran Sea
and its nearest relatives in Australia. Other species have a still more puzzling
distribution, as is for example the case of the small gastropod Homalopoma
sanguineum (Fig. 11.2), which has no close relatives in the Atlantic and avoids
even the Alboran Sea. The history of these Mediterranean endemics and
paleoendemics is still far from being understood (Ekman 1953). Where were those
species during the salinity crisis? The adjacent Paratethys was reduced, in its western
part, to the brackish Lake Pannon, which could only harbour a fauna similar to that
of the “Lago-Mare” episode. Two possibilities are left: either some parts of the
Mediterranean proper were neither evaporated nor brackish and could give refuge to
Posidonia and other marine biota, or the adjacent part of the Atlantic could provide a
suitable habitat. The first alternative is not supported by any fossil record anywhere,
and the second would suppose that the deep embayment of the southwestern Iberian
Peninsula and Northwestern Morocco was then not so much sediment-clad as it is
nowadays.
In any case, the species richness of the Mediterranean was fully restored during
the Pliocene with many species that are now extinct or are restricted to more southern
waters off northwestern Africa. At that moment the history of the Mediterranean
biota was already linked to the populations of the Atlantic. Not so widely renowned
as the Italian Pliocene (Monegatti and Raffi 2001), the outcrops around the Western
part of the Alboran Sea near Estepona have provided an extremely diverse Pliocene
fauna mostly studied for molluscs (see Landau et al. 2009 and references therein).
From there on, the biogeographic history of the Mediterranean is essentially the
result of a series of climate changes, resulting in repeated biotic shifts. There were
11 Biogeographical and Macroecological Context of the Alboran Sea
435