Considerations on the affinity and origin of the pteridological flora of the Mediterranean Region

Abstract

First of all, this research deals with the delimitation of the Mediterranean Region, which is circumscribed in a narrow sense. The species and subspecies of pteridophytes recorded within its limits amount to 124. They belong to 38 genera and to 23 families and represent barely 0.88% of all the extant Pteridophyta. Thus, the pteridoflora of the Mediterranean Region must be regarded as a'poor flora. Starting from the above mentioned data, the research has been extended to the level of ploidy and geographical distribution of each species and subspecies (Table I). The number of diploids is clearly higher than that of the polyploids and on the basis of this ratio the pteridoflora has to be considered as a very ancient one.The geographical distribution of the species and subspecies of the Mediterranean Region, in the adjoining territories and other parts of the world shows that the Mediterranean pteridoflora has its greatest affinity with the flora of the Submediterranean, Atlantic and Asiatic regions.

A brief outline of the most important palaeogeographical events in the Mediterranean area, and some palaeoclirnatic notes precede the part dealing with the origin of the Mediterranean pteridoflora. This is mainly based on the chorological and epiontological study of the most significant species of past and present times, of their ancestors and allies. Particular attention is also paid to the tracks of migration followed by some species to reach the Mediterranean area from their first centres of origin in Asia and Africa (Fig. 5). The most ancient of these species, such as Woodwardia radicans, Culcita macrocarpa and Selaginella balansae presumably represent the relicts of a late Cretaceous flora. Others are the relicts of the tropical flora of the Paleogene and mainly of the subtropical laurel flora of the Oligocene. Other species reached the Mediterranean area from the eastern Euroasiatic lands during the Messinian and still others from the northern countries during the pleistocenic glaciations. Finally a rather consistent group of pteridophytes is indigenous to the Mediterranean Region. According to some authors the laurel flora of Oligocene, mentioned above, had its origin in the Canary Islands, but in the author's opinion the more reliable hypothesis is that the oligocenic laurel flora and vegetation originated in the western Mediterranean area and later transmigrated to the Canary Islands, where they survived up to the present time. This hypothesis is supported by recent geological and palaeogeographical research according to which the eastern islands of Canary Archipelago did not emerged from the sea earlier than the Miocene when the laurisilvae had already been widely represented in the Mediterranean area for several milion years. Furthermore, according to some geologists, the eastern Canary Islands are interpreted as a microcontinent or a sialic continental fragment detached from the western margin of northern Africa. Its existence would explain in the best manner the transmigration from Africa to the Canary Islands of both the laurisilvae and the ratite birds whose fossil eggshells have been found in the calcarenites of Miocene-Pliocene in Lanzarote Island.

A picture of the history of the Mediterranean pteridoflora in the light of the palaeogeographical and palaeoclimatic events is briefly outlined as a conclusion of the paper.