The Biogeography of the Maltese Islands, Illustrated by the Clausiliidae

Abstract

Published data on the geology and biogeography of the Maltese islands are reviewed. The islands consist of Oligo-Miocene limestones of marine origin, and probably rose above sea level during the Late Miocene. During the Messinian crisis, the Straits of Sicily were probably above sea level, and terres- trial organisms could migrate between Europe and Africa. It is likely that the flooding of the Mediterranean basin which took place at the start of the Pliocene separated Malta from both Africa and Sicily. If any land connection with Lampedusa or North Africa persisted, this must have been destroyed by the extensive subsidence in the middle of the strait of Sicily which occurred during the Pliocene. Tectonism on the Malta-Sicily submarine ridge was less extensive and may have occurred during the Pleistocene. Late Pleistocene eustatic sea level lows during glacials were sufficient to connect Malta to Sicily. The Maltese biota closely resembles the biota of Sicily, but there are also several endemics. A few species occur on Malta and North Africa, but are not found in Sicily. The distributions of the various Maltese species of Alopiini (Mollusca: Clausi- hidae) are figured. Most of the species are endemic to the archipelago and exhibit allopatric or parapatric distribution patterns. The genus Lampedusa (Imitatrix) possesses a relict distribution, with three species in as many isolated refugia. These species appear to be descended from a single ancestral species which was displaced by members of the genus Muticaria invading from Sicily during glacials when the intervening continental shelf was exposed. Invasion byMuticaria appears to have occurred twice, the earlier invaders evolving into an endemic species (Muticaria oscitans). Maltese biogeography is interpreted in terms of sea level changes and tectonic events which established or severed land connections with Sicily. The following hypothesis is proposed: Malta received terrestrial biota from Europe and presum- ably also from North Africa during the Messinian crisis. A long period of isolation during the Pliocene was followed by an Early-Middle Pleistocene connection with Sicily, during which some Sicilian forms invaded the islands. A second period of isolation followed, perhaps due to subsidence on the Malta-Sicily submarine ridge during the Middle Pleistocene. The islands' endemics evolved during the two periods of geographic isolation. The latest Pleistocene eustatic fluctuations were extensive enough to connect Malta to Sicily during glacials, and the islands' biota acquired an essentially Sicilian character. The North African elements in the Maltese biota seem interpretable in terms of active dispersal across the sea or