Methodologies for paleontological field research: early Pliocene sirenian paleosite in southern Tuscany

We report the preliminary description of recent discoveries of Pliocene marine fauna from Arcille (Grosseto, Italy) and application of Ground Penetrating Radar to locate fossil vertebrate. During the past years four partially articulated sirenian skeletons were found in shallow siliciclastic deposits exposed in the Arcille area. All these fossil remains can be referred to Metaxytherium subapenninum, an extinct species of haliteriine dugongid (Mammalia, Sirenia) spread along the northwestern coasts of Mediterranean Basin and became extinct in the upper part of the Pliocene because of the progressive climatic cooling occurred after 3.1 Ma (Sorbi et al. 2008). Planktonic foraminiferal assemblages are consistent with an attribution of the sirenian specimens to the lower part of the Zanclean, in particular to the MPl2 zone of Cita (1975) dated between 5.08 and 4.52 Ma (age after Lourens et al., 2004). Three specimens were collected in a sand quarry and one of this is almost complete. The fourth specimen, discovered in a sunflower field near the quarry, was collected as a consequence of a preliminary georadar prospection. This technique is a non destructive method based on propagation and reflection of electromagnetic (EM) waves and it is sensitive to variations of the EM parameters in the subsoil, especially the dielectric constant and electric conductivity (Davis and Annan, 1989). The GPR survey was performed using the Radar System device of IDS Company, equipped with a monostatic antenna of 200 MHz. Two adjacent areas have been mapped. Time slices at various depths indicated the presence of several reflective zones: one of this was located at the centre of the first area at 0.20 m from surface. The other zones were smaller and not persistent and rapidly changing in shape with depth. In the second area, two small zones were recorded. We decided to verify the nature of this GPR signals through an excavation. In corrispondance with the central reflective zone of the first area, some fossil bones emerged: in particular, the skull with tusks, the mandibles with teeth, some cervical and dorsal vertebrae and a considerable number of ribs. Other bones (humerus, radius and scapula) were found in correspondance with smaller reflective zones of the same area. In the second area, we discovered several caudal vertebrae in correspondence of one of the two reflective zones. The results obtained from this study are very interesting, although further experimentations are required for a better refinement of this surveying method for paleontological research. Cita M. B. (1975). Studi sul Pliocene e sugli strati di passaggio del Miocene al Pliocene. VIII. Planktonic foraminiferal biozonation of the Mediterranean Pliocene deep sea record. A revision. Rivista Italiana di Paleontologia e Stratigrafia 81, pp. 527-544. Davis J. L., Annan A. P. (1989). Ground Penetrating Radar for high-resolution mapping of soil and rock stratigraphy. Geophysical Prospecting, 37, 531-551. Lourens L. J., Hilgen F. J., Laskar J., Shackleton N.J., Wilson D. (2004). Appendix 2. Orbital tuning calibrations and conversions for the Neogene period, pp. 469―484 in: F. Gradstein, J. Ogg, and A. Smith (eds), A Geologic Time Scale (2004). Cambridge University Press, Cambridge. Sorbi S., Vaiani S. C., Bianucci G. (2008). Metaxytherium subapenninum: l’ultimo sirenio del Mediterraneo. Giornate di Paleontologia VIII edizione, Simposio della Società Paleontologica Italiana (Siena, 2008), pp. 97-99.