THE NEOGENE EURO-NORTH AFRICAN SIRENIA: PHYLOGENETIC AND SYSTEMATIC STUDY, PALAEOECOLOGICAL AND PALAEOBIOGEOGRAPHIC CONSIDERATIONS

The evolution of the Neogene sirenians along the Euro-North African coasts (southern North Sea, northeastern Atlantic, Mediterranean and Paratethys) has been investigated in correlation with the palaeogeographic and palaeoclimatic evolution of the region. A better chronostratigraphic setting and a systematic review of the main specimens (listed in a database) have been provided and nine valid Neogene Euro – North African sirenian species have been recognized: Miosiren kocki, Prohalicore dubaleni, Rytiodus capgrandi, Rytiodus sp. from Libya, Metaxytherium krahuletzi, M. medium, M. petersi, M. serresii and M. subapenninum; while “Halitherium” bellunense, previously referred to the Early Miocene, has been determined to be Late Oligocene in age and confirmed to be a dugongine close to Rytiodus spp.. These species, except Prohalicore dubaleni known only from an incomplete mandible and M. petersi under study by Domning and Pervesler, have been included in a cladistic analysis. This analysis has been carried out with the PAUP version 40b10 program; developed for 31 species of sirenians and the primitive Proboscidea Phosphatherium escuilliei as outgroup, and based on 57 characters of cranium, mandible and dentition. The results confirm the anatomical observations and show a close correspondence between palaeobiogeographic and chronostratigraphic distribution of the taxa and their positions on the tree. A reconstruction of Euro-North African sirenian palaeoecology and palaeobiogeography during the Neogene has been proposed in relation to the long-term trend of climatic cooling and to the decreasing of marine influence, which characterized the evolution of the region. The sirenian palaeoecology has been reconstructed by combining data from seagrass evolution and ecology, sirenian distribution and morphology, stable isotope (C and O) analyses of sirenian tooth enamel. On the whole, the Neogene Euro-North African sirenians appear to be fully aquatic herbivorous mammals strictly linked to coastal environments. During the Early Miocene, which was a period of global warming, at least three different sirenian lines lived along the Euro-North African coasts: the euryhaline mixed-feeder Miosiren kocki (Trichechidae, Miosireninae), living in isolation on the southernmost margin of the North Sea; the generalist small-tusked Metaxytherium krahuletzi (Dugongidae, Halitheriinae), which appears to have been widespread along the Euro-North African coasts; and the specialized large-tusked genus Rytiodus (Dugongidae, Dugonginae), recorded with confidence just in the Aquitaine Basin (SW France) and in the Sirte Basin (Libya), where it most probably lived sympatrically with Metaxytherium krahuletzi. From the Middle Miocene onward, however, the only sirenians surviving in the Euro-North African region belonged to the generalist cosmopolitan halitheriine genus Metaxytherium. The only exception is the miosirenine trichechid Prohalicore dubaleni, suggesting that Trichechidae survived until the Serravallian, probably living together with the generalist Metaxytherium. During the Late Miocene the genus Metaxytherium became extinct throughout the world except along the Euro-North African coasts, and by the end of the Miocene it appears to have been survived just in the Mediterranean Basin with the relictual dwarfed species M. serresii. M. serresii specimens are recorded before and after the Messinian Salinity Crisis: at about 7.24 Ma from southern Italian deposits; at about 6.8 Ma from Libyan deposits; at 5.25 - 4.98 Ma from Spanish deposits; and at 4 – 5.3 Ma from French deposits. The M. serresii record is considered continuous from the uppermost Tortonian to the early Pliocene in the Mediterranean; according to recent data suggesting that normal marine conditions were present in the Mediterranean during the Messinian Salinity Crisis. The dwarfism in M. serresii is interpreted as a method to increase the fitness of a relictual species, permitted by geographic restriction and reduction in the genetic pool. Moreover, the increase in tusk size observed in M. serresii, in comparison with its supposed ancestors M. krahuletzi and M. medium, could denote a shift in behaviour, with tendencies to territoriality and competition for females and space. The subsequent, and latest, Metaxytherium species M. subapenninum was larger and also with larger and, probably, sexually dimorphic tusks. The sexually dimorphic tusks of M. subapenninum could be interpreted as a successive step in change in behaviour, an indication of competition for females and space that favoured the development of a sexual dimorphism similar to that observed in the living Dugong dugon. On the contrary, the rapid increase in body size could be interpreted as a response to the long-term climatic cooling, as the hydrodamaline sirenians did in the North Pacific; which survived into historic times, eating algae in cold waters. The anatomical feeding adaptations and the stable isotope analyses of M. subapenninum do not indicate a clear change in diet and habitat in comparison with the Miocene species. M. subapenninum did not respond to climatic cooling by changing its diet and habitat as the Hydrodamalinae did, and therefore it became extinct during the Middle Pliocene, most probably during the acceleration of climatic cooling record after 3.1 Ma. But the history of Euro-North African sirenians does not appear to be concluded; in fact, the remains coming from the Pliocene deposits of the Atlantic coast of Morocco referred to Zanclean and assigned to Metaxytherium serresii, have been demostred to be Piacenzian in age and to bear more resemblances to the Pliocene caribbean dugongine Corystosiren varguezi.