Palaeoenvironmental analysis of the Messinian macrofossil floras of Tossignano and Monte Tondo (Vena del Gesso basin, Romagna Apennines, northern Italy)

Fossil leaves of Messinian age (ca. 5.7 Ma) from two sites of the Vena del Gesso Fm. (Monte Tondo and Tossignano, Ravenna and Bologna provinces, Italy) were studied morphologically to assess the taxonomic composition of the assemblages and carry out a palaeoenvironmental analysis. The flora of Tossignano so far comprises 41 vascular plant taxa with 8 conifers and 33 angiosperms (30 dicots and 3 monocots). In the plant material from Monte Tondo 74 vascular plant taxa were recognized, with 11 conifers and 63 angiosperms (59 dicots, 3 monocots, 1 plant incertae sedis). Several fossil species well known in the Miocene of central Europe and Italy were detected. The most frequent being Taxodium dubium, Pinus cf. rigios, Daphnogene polymorpha, Laurophyllum sp. 1 and 2, Platanus leucophylla, Quercus pseudocastanea, Quercus roburoides, Fagus gussonii, cf. Ailanthus pythii, Leguminosae gen. et sp. indet. 1. The general palaeovegetation pattern of the Monte Tondo and Tossignano plant assemblages correspond to the vegetation transect of the Evaporitic Messinian with swamp, riparian vegetation, and zonal “subtropical humid forest”. The results of IPR vegetation analysis for the “subtropical humid forest” zone indicate a transitional (ecotone) vegetation type between “Broad-leaved Evergreen Forest” and “Mixed Mesophytic Forest”. The Coexistence Approach estimates that both floras existed under comparable climatic conditions, with MAT 14–16.2 °C for Tossignano and 16–16.5 °C for Monte Tondo. This is warmer than the estimates derived from the Leaf Margin Analysis (MAT 13.8 °C and 14.3 °C) and Climate Leaf Analysis Multivariate Program (MAT 13.2 °C and 12.0 °C), and present day conditions in the Mediterranean Zone (MAT around 13.5 °C). ■ flora, vegetation, palaeoclimate, Messinian, Italy. Received June 17, 2015 Issued December, 2015 housed, collected over the years at two different sites, Tossignano (municipality of Borgo Tossignano, Bologna province) and Monte Tondo (municipality of Riolo Terme, Ravenna province), only a few kilometres apart and both with exposed the Vena del Gesso Fm. The name “Vena del Gesso” is also a popular name for a continuously outcropping gypsum belt extending in a northwest-southeast direction for some 15 km in the western Romagna Apennines and which offers spectacular outcrops of selenitic Messinian gypsum (Roveri et al. 2006). The story of the two collections is substantially different: the core of the Tossignano palaeoflora is the fossil collection that P. Viaggi donated to MSF in 1983 (Viaggi 1989). This collection, consisting of some hundreds of fossils (especially fishes and leaves), was recovered by the donor between the years 1976–1982 from an outcrop in the SPES gypsum quarry (Text-fig. 1). This quarry, located on the right side of a small canyon created by the Sgarba creek, remained in operation from 1969 to 1986. Other specimens were added to the initial collection during subsequent years, always recovered from the same location, thanks to various volunteers of MSF, including in particular M. Diversi. The recovery of the Monte Tondo palaeoflora occurred much more recently (during 2012–2014) as a result of the multidisciplinary project initiated by the Speleological Federation of Emilia Romagna. This was concentrated on the gypsum area immediately to the south of the town of Borgo Rivola, which had been subjected to intense mining activity since 1958 (Text-fig. 1). Through this project some MSF volunteers (i.e., one of the authors, M.S., ably assisted by T. Benericetti) had the opportunity to sample a site in which decades of digging had made difficult to access. This field work produced several hundreds of fossil leaf samples, in addition to fossil remains of fishes and insects. The palaeoflora from Tossignano, although already known since the second half of the 19th century (Scarabelli 1864), was also later quoted in a few other general palaeobotanical papers (e.g., Knobloch and Gregor 1997, Kovar-Eder et al. 2006, Bertini and Martinetto 2008, 2011). The palaeoflora of Monte Tondo was first described in a preliminary study by Sami and Teodoridis (2013). Geological setting The Romagna Apennines, extending from the Sillaro valley on the west to the Marecchia valley on the east, are part of the northeast-verging Northern Apennine arc and are characterized by an exposed belt of siliciclastic deposits of early Miocene to Pleistocene age, overlying buried Mesozoic to Cenozoic carbonates. The uppermost structural unit cropping out along the Po Plain side of the Romagna Apennines consists of the Langhian to Tortonian Marnoso-arenacea Formation turbiditic complex (Ricci Lucchi 1975, 1981) while the Apennine foothills are a gentle northeast dipping monocline of Messinian to Pleistocene deposits resting above the Marnoso-arenacea Fm. The Tortonian–Messinian succession of the Apennine orogenic wedge records the fragmentation and closure of the Marnoso-arenacea foredeep basin during the extended thrustfront propagation and concurrent depocenter migration toward the foreland (Ricci Lucchi 1986). This process implied the formation of small thrust-top basins; some of them were characterized during the Messinian by the deposition of shallow-water, primary evaporites (mainly selenitic gypsum), coeval with the Lower Evaporites of Mediterranean marginal basins (Krijgsman et al. 1999a, b). However, these basins are rarely preserved because of subsequent tectonic deformation thus the Vena del Gesso basin is a valuable exception.