Since 1970, after the drilling of the ocean floor of the Mediterranean Sea by the research vessel Glomar Challenger, many scientific papers have been produced for or against the theory of complete desiccation of the Messinian Mediterranean - from 5.97 to 5.33 My - (Hsu et. al., 1973; Krijgsman et al., 1999). In particular, since 2006 a new meaning was given to the study of the Messinian ichthyofauna with the intent to understand in a better way the Messinian Salinity Crisis; in fact fishes represent a tool for the interpretation of environmental conditions on a large scale because of their migratory character (Carnevale et al., 2006; Carnevale, 2008). This project aims to provide a detailed stratigraphic revision of the Messinian fossil site of Monte Castellaro and to describe a recently discovered deposit of Messinian fish fauna: Monte Tondo (Ravenna, Chalk Vein of Romagna). The stratigraphic succession of Monte Castellaro field includes three distinct ranges:  the roof of Schlier Formation (Tortonian - Lower Messinian);  the Primary Lower Gypsum Formation (lower and medium Messinian);  San Donato Formation (medium Messinian) or directly the Colombacci Formation (Upper Messinian), (Savelli & Wezel, 1979; Roveri et al., 2005). The sampling, the description and the stratigraphic revision of fossil levels of Monte Castellaro belonging to the Primary Lower Gypsum Formation show a sudden change in the composition of the ichthyofauna, at the filing of the first level of evaporitic gypsum (Messinian). This represents a shift from a rich fish fauna of the marine layers underneath (Tortonian) of which are known about 90 species, to a fish fauna with few marine genera and gradually to a purely eurhyaline fauna including small Cyprinodonts, such as Aphanius crassicaudus (Landini & Sorbini, 1989; Bedosti, 1999). Two phenotypes of the species A. crassicaudus are represented in the Monte Castellaro deposit: one with a normal and thin skeleton, another with a thickened skeleton: the "pachyostosic - one" (Gaudant, 1979). The pachyostosis in A. crassicaudus is a bone mass increase covering the whole skeleton and is known in other Messinian eurhyaline fish, such as Gobiidae, but has never been found in living fish belonging to these two genera. The pachyostosis, whose etiology is still not well-defined even if reconnected to the excessive salinity of the waters of evaporitic period, is studied in this thesis, since it has been investigated in paleontological, sedimentological, biostratigraphical, radiographical, histological and geochemistrical terms. The detailed study of the ichthyofauna of the contemporary field of Monte Tondo in the Chalk Vein Park (Emilia-Romagna) has permitted the reconstruction of the Messinian paleo-environment which is characterized by a context of around-coast or coastal plain, similar to a coastal lagoon occasionally communicating with the open sea. The discovery of eurhyaline fishes such as Aphanius crassicaudus and Gobius sp., even in the deposit of Monte Tondo, has allowed us to deepen the phenomenon of pachyostosis afflicting these two species. In particular, radiographic, histologic and geochemical research (made either with the mass spectrometer Plasma - ICP-MS, both with the ESEM) of the two pachyostosic phenotypes of Aphanius crassicaudus and of Gobies, from the field of Monte Castellaro, from deposit Monte Tondo and from the farm "Bread and Wine" (Tuscany), has allowed to investigate the possible causes of the appearance of pachyostosis. The spectrographic analyzes carried out on specimens belonging to the two phenotypes of A. crassicaudus and on a pachyostosic exemplary of Gobius sp. have emphasized a significant decrease (often even absence) of the concentration of Mg ions in the pachyostosic bone tissue of both the eurhyaline species. Since the Mg ion is an inhibitor of the hydroxyapatite formation in the human bone tissue (Bigi et al., 1992; Percival, 1999; F. Valentini, 2010) and in the fishes one (Eidelman, 2009), we assume that the appearance of pachyostosis is due to the excessive dilution of the Mg ion in the Messinian waters where the eurhyaline fishes lived. From the geochemical point of view an uptake of Mg ion from the water can occur in evaporitic environment during the process of dolomitization; in the environment with high temperature and in water in which carbonate calcite is already formed; Ca ion is then spontaneously replaced by the Mg ion, forming dolomite MgCa(CO3) 2. (Prof. Bruno Capaccioni, geochemist - University of Bologna, personal communication; Eugster, 1980; Babel & Schreiber, 2014). The phenomenon of pachyostosis in Aphanius crassicaudus was also observed in the post-evaporitic deposits of Tuscany and of Piedmont (Carnevale, 2006; Carnevale 2008; Colombero, 2014). In such environments it may occur an uptake of Mg ions from the water because of a general dilution of salts, due to fresh water coming from the rivers, in lagoon areas subject to periodic oscillations of physical - chemical parameters of water ( Gaudant 1979; Gaudant et al., 1988; Roveri et al., 2008; Campbell et al., 2009; Colombero et al., 2014; Li, 2015). Moreover study of the isotopic composition of sulfate and carbonate in Sicilian evaporites showed that the contributions of fresh water during the precipitation of gypsum was so heavy that we can consider more important the role of rainwater than marine water during the Messinian salinity crisis (Longinelli, 1978). Further confirmation to this effect comes from the discovery of a frog in Piedmont evaporites that, although it shows the presence of fresh water, it was fossilized together with fishes in rocks containing gypsum (Morisi, 1983). The discovery of dragonfly larvae and twigs or leaves nearby Aphanius crassicaudus in the same fossil slab (Monte Castellaro excavations 2011 – 2014), testify to the river intake and therefore the presence of freshwater. The discovery in Crete of several pachyostosic Aphanius crassicaudus in the same slab of 200 cm2 of chalky marl is referred to a deposit of gypsum in non hypersaline waters; in fact there was a Sardina sp. in the same slab (Gaudant, 2014). Hsianwenia Wui, Gen. et sp. Nuova is the only other species, besides fossil Aphanius and Gobius, which presents a pachyostosis uniformly extended to the whole skeleton. This species lived in arid conditions in the salt lake in the Qaidam Basin (northern Tibet, 2,700 meters above the sea level). The environment in which the minnow lived is similar to the one of Aphanius: high concentration of Ca and gypsum, a vegetation of arid area, but with algae in fresh and shallow waters, together with a plant growing along the margins of lakes and rivers (Chang et al., 2008). The uptake of Mg ions, inhibitors of hydroxyapatite formation seem to be attributable to a metasomatic replacement of calcium ions with magnesium ions, by the action of hypersaline brine or fresh water on calcareous sediments (Longinelli, 1978; Li, 2015). Both in evaporitic and in post-evaporitic environment it must however be considered a sudden change of Mg ions concentration in waters, evidenced by the presence in some thin skeleton of Aphanius crassicaudus (Monte Castellaro) of small spherules of hydroxyapatite within blood vessels. These spherules are characteristics of a very fast increase because of their shape and size. These spherules are formed because of the excess of hydroxyapatite, due to the decrease of the ion Mg, inhibitor of its formation. Probably the phenomenon occurred in a transition environment because it is present in chalk marl rocks. A contribution to a wider paleogeographic reconstruction of the phenomenon of pachyostosis in Aphanius crassicaudus has been given studying the numerical incidence of pachyostosis in populations of this small Cyprinodontidae from Tyrrhenian, Adriatic and Piedmont deposits. These populations have been divided into different groups considering their age; the bone mass increase is not linked to the fish age, distributing indifferently between the young and the adults. During paleontological digs at Monte Castellaro deposit in July 2011 and July 2014, were found marine species of Teleostei; they belong to the families: Trichiuridae, Myctophidae, Gonostomatidae, Paralepididae and Gadidae. In the lower and middle Messinian levels until now it has been found only eurhyaline species remains united to the mainland ones, such as insects, plants, birds, rodents and lagomorphs (Sorbini, 1987; Landini & Sorbini, 1989). In conclusion, the discovery of mesopelagic and batipelagic fish represent a support to the latest theory of the not complete desiccation of the Mediterranean during the MSC (Carnevale, 2006 Carnevale, 2008).

La crisi di salinità del Messiniano: il contributo degli ittioliti del giacimento di Monte Castellaro (Pesaro)

BEDOSTI, NICOLETTA
2016

Abstract

Since 1970, after the drilling of the ocean floor of the Mediterranean Sea by the research vessel Glomar Challenger, many scientific papers have been produced for or against the theory of complete desiccation of the Messinian Mediterranean - from 5.97 to 5.33 My - (Hsu et. al., 1973; Krijgsman et al., 1999). In particular, since 2006 a new meaning was given to the study of the Messinian ichthyofauna with the intent to understand in a better way the Messinian Salinity Crisis; in fact fishes represent a tool for the interpretation of environmental conditions on a large scale because of their migratory character (Carnevale et al., 2006; Carnevale, 2008). This project aims to provide a detailed stratigraphic revision of the Messinian fossil site of Monte Castellaro and to describe a recently discovered deposit of Messinian fish fauna: Monte Tondo (Ravenna, Chalk Vein of Romagna). The stratigraphic succession of Monte Castellaro field includes three distinct ranges:  the roof of Schlier Formation (Tortonian - Lower Messinian);  the Primary Lower Gypsum Formation (lower and medium Messinian);  San Donato Formation (medium Messinian) or directly the Colombacci Formation (Upper Messinian), (Savelli & Wezel, 1979; Roveri et al., 2005). The sampling, the description and the stratigraphic revision of fossil levels of Monte Castellaro belonging to the Primary Lower Gypsum Formation show a sudden change in the composition of the ichthyofauna, at the filing of the first level of evaporitic gypsum (Messinian). This represents a shift from a rich fish fauna of the marine layers underneath (Tortonian) of which are known about 90 species, to a fish fauna with few marine genera and gradually to a purely eurhyaline fauna including small Cyprinodonts, such as Aphanius crassicaudus (Landini & Sorbini, 1989; Bedosti, 1999). Two phenotypes of the species A. crassicaudus are represented in the Monte Castellaro deposit: one with a normal and thin skeleton, another with a thickened skeleton: the "pachyostosic - one" (Gaudant, 1979). The pachyostosis in A. crassicaudus is a bone mass increase covering the whole skeleton and is known in other Messinian eurhyaline fish, such as Gobiidae, but has never been found in living fish belonging to these two genera. The pachyostosis, whose etiology is still not well-defined even if reconnected to the excessive salinity of the waters of evaporitic period, is studied in this thesis, since it has been investigated in paleontological, sedimentological, biostratigraphical, radiographical, histological and geochemistrical terms. The detailed study of the ichthyofauna of the contemporary field of Monte Tondo in the Chalk Vein Park (Emilia-Romagna) has permitted the reconstruction of the Messinian paleo-environment which is characterized by a context of around-coast or coastal plain, similar to a coastal lagoon occasionally communicating with the open sea. The discovery of eurhyaline fishes such as Aphanius crassicaudus and Gobius sp., even in the deposit of Monte Tondo, has allowed us to deepen the phenomenon of pachyostosis afflicting these two species. In particular, radiographic, histologic and geochemical research (made either with the mass spectrometer Plasma - ICP-MS, both with the ESEM) of the two pachyostosic phenotypes of Aphanius crassicaudus and of Gobies, from the field of Monte Castellaro, from deposit Monte Tondo and from the farm "Bread and Wine" (Tuscany), has allowed to investigate the possible causes of the appearance of pachyostosis. The spectrographic analyzes carried out on specimens belonging to the two phenotypes of A. crassicaudus and on a pachyostosic exemplary of Gobius sp. have emphasized a significant decrease (often even absence) of the concentration of Mg ions in the pachyostosic bone tissue of both the eurhyaline species. Since the Mg ion is an inhibitor of the hydroxyapatite formation in the human bone tissue (Bigi et al., 1992; Percival, 1999; F. Valentini, 2010) and in the fishes one (Eidelman, 2009), we assume that the appearance of pachyostosis is due to the excessive dilution of the Mg ion in the Messinian waters where the eurhyaline fishes lived. From the geochemical point of view an uptake of Mg ion from the water can occur in evaporitic environment during the process of dolomitization; in the environment with high temperature and in water in which carbonate calcite is already formed; Ca ion is then spontaneously replaced by the Mg ion, forming dolomite MgCa(CO3) 2. (Prof. Bruno Capaccioni, geochemist - University of Bologna, personal communication; Eugster, 1980; Babel & Schreiber, 2014). The phenomenon of pachyostosis in Aphanius crassicaudus was also observed in the post-evaporitic deposits of Tuscany and of Piedmont (Carnevale, 2006; Carnevale 2008; Colombero, 2014). In such environments it may occur an uptake of Mg ions from the water because of a general dilution of salts, due to fresh water coming from the rivers, in lagoon areas subject to periodic oscillations of physical - chemical parameters of water ( Gaudant 1979; Gaudant et al., 1988; Roveri et al., 2008; Campbell et al., 2009; Colombero et al., 2014; Li, 2015). Moreover study of the isotopic composition of sulfate and carbonate in Sicilian evaporites showed that the contributions of fresh water during the precipitation of gypsum was so heavy that we can consider more important the role of rainwater than marine water during the Messinian salinity crisis (Longinelli, 1978). Further confirmation to this effect comes from the discovery of a frog in Piedmont evaporites that, although it shows the presence of fresh water, it was fossilized together with fishes in rocks containing gypsum (Morisi, 1983). The discovery of dragonfly larvae and twigs or leaves nearby Aphanius crassicaudus in the same fossil slab (Monte Castellaro excavations 2011 – 2014), testify to the river intake and therefore the presence of freshwater. The discovery in Crete of several pachyostosic Aphanius crassicaudus in the same slab of 200 cm2 of chalky marl is referred to a deposit of gypsum in non hypersaline waters; in fact there was a Sardina sp. in the same slab (Gaudant, 2014). Hsianwenia Wui, Gen. et sp. Nuova is the only other species, besides fossil Aphanius and Gobius, which presents a pachyostosis uniformly extended to the whole skeleton. This species lived in arid conditions in the salt lake in the Qaidam Basin (northern Tibet, 2,700 meters above the sea level). The environment in which the minnow lived is similar to the one of Aphanius: high concentration of Ca and gypsum, a vegetation of arid area, but with algae in fresh and shallow waters, together with a plant growing along the margins of lakes and rivers (Chang et al., 2008). The uptake of Mg ions, inhibitors of hydroxyapatite formation seem to be attributable to a metasomatic replacement of calcium ions with magnesium ions, by the action of hypersaline brine or fresh water on calcareous sediments (Longinelli, 1978; Li, 2015). Both in evaporitic and in post-evaporitic environment it must however be considered a sudden change of Mg ions concentration in waters, evidenced by the presence in some thin skeleton of Aphanius crassicaudus (Monte Castellaro) of small spherules of hydroxyapatite within blood vessels. These spherules are characteristics of a very fast increase because of their shape and size. These spherules are formed because of the excess of hydroxyapatite, due to the decrease of the ion Mg, inhibitor of its formation. Probably the phenomenon occurred in a transition environment because it is present in chalk marl rocks. A contribution to a wider paleogeographic reconstruction of the phenomenon of pachyostosis in Aphanius crassicaudus has been given studying the numerical incidence of pachyostosis in populations of this small Cyprinodontidae from Tyrrhenian, Adriatic and Piedmont deposits. These populations have been divided into different groups considering their age; the bone mass increase is not linked to the fish age, distributing indifferently between the young and the adults. During paleontological digs at Monte Castellaro deposit in July 2011 and July 2014, were found marine species of Teleostei; they belong to the families: Trichiuridae, Myctophidae, Gonostomatidae, Paralepididae and Gadidae. In the lower and middle Messinian levels until now it has been found only eurhyaline species remains united to the mainland ones, such as insects, plants, birds, rodents and lagomorphs (Sorbini, 1987; Landini & Sorbini, 1989). In conclusion, the discovery of mesopelagic and batipelagic fish represent a support to the latest theory of the not complete desiccation of the Mediterranean during the MSC (Carnevale, 2006 Carnevale, 2008).
2016
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