I bivalvi della Formazione a Bellerophon (Permiano superiore) delle Dolomiti nord-occidentali nel contesto dell’estinzione di massa di fine Permiano

The Bellerophon Formation (upper Permian) in the Western Dolomites (Badia and Gardena valleys) is mainly constituted by solfatic evaporites (gypsum and anhydrite) and dolomites in the lower part, and by marls, dolomites and micritic to biocalcarenitic limestones, in the upper part. During the 19th and 20th centuries several palaeontologists described many new bivalve species from this formation, but the material on which these studies were based is often badly preserved, and lack of stratigraphic setting. The Bellerophon Formation records the last stage of the Palaeozoic marine life, just before the greatest mass extinction in the history of life that marks the transition between the Permian and Triassic periods. The present study aims to investigate the variation in the diversity of the bivalve communities through systematic and biostratigraphic analyses in order to define the extinction and survival rates, and to contribute in the investigation on the dynamics of the end Permian extinction, when 90 % of marine species and 70% of terrestrial vertebrate species disappeared. The peak of the extinction has been recently dated to 252,28 ± 0.08 million years ago; it seems to have been caused by a global warming linked to a strong and protracted volcanic activity of the Siberian traps and methane emission. The bivalves object of the present study have been collected from extensive outcrops of the Bellerophon Formation located in the Badia and Gardena valleys. The stratigraphic distribution of the bivalves has been reconstructed through a detailed bed-by-bed sampling of the more complete successions from the two above mentioned Ladinian valleys. Most of the sections, although known since the XIX century, have been described and sampled for the first time in the present thesis. The sections analysed in the Gardena valley are: Bulla (Ortisei, 30 m thick), Balest (Ortisei, 130 m thick), Pic (S. Cristina, 46 m thick); the sections analysed in the Badia valleys are Preroman (San Martino in Val Badia, 52 m thick), Seres (Longiarù valley, 37 m thick, which is the only one the be already described in detail). All the studied sections are located on a southwest-northeast oriented transect, along which an increasing of the marine conditions, ranging from coastal and restricted to open, occurred. Nevertheless, even during the transgressive phase peaks, the beds originated always in euphotic and eurytopic conditions, with variations of some environmental limiting factors, such as oxygen and nutrients. The Bellerophon Formation succession shows a transgressive trend, punctuated by some regressive episodes that allowed to recognize third order depositional sequences, which were used to correlate the bioevents between the analysed sections. The systematic analysis performed on the collected material and the historical collections of the bivalves from the Bellerophon Formation studied by STACHE, CANEVA, GORTANI and MERLA, allowed to classify 17 genera and 25 species, with 2 new species and one new genus, which will be formally named and described in a future work. The species described in the present study can be subdivided on the basis of their mode of life as follows: • Infaunal suspension feeders: Permophorus tenuistriatus NAKAZAWA & NEWELL, 1968; Permophorus cfr. tenuistriatus NAKAZAWA & NEWELL, 1968; Permophorus cfr. octocostatus (WELLER, 1916); Pleurophorella jacobi (STACHE, 1878); Pleurophorella sp.; Schizodus obscurus (SOWERBY, 1821); Edmondia cf. rudis MC COY, 1853; • Semi-infaunal endobyssate suspension feeders: Modiolus n. sp.; Volsellina aurioides (YIN & GON, 1978); Bakevellia ceratophaga (SCHLOTHEIM, 1816); Towapteria sp.; Marmaronia peracuta (STACHE, 1878); • Epifaunal suspension feeders, epibyssate and/or free swimmers in adult stage: Aguilerella n. sp.; Palaeolima sp. 1; Palaeolima sp. 2; Parallelodon cf. multicostatus GIRTY, 1908; ”Pecten” trinkeri (STACHE, 1878); Eumorphotis praecurrens MERLA, 1930; Eumorphotis striatocostata (STACHE, 1878); Eumorphotis lorigae POSENATO, 2005; Pernopecten tirolensis (STACHE, 1878); Pernopecten symmetricus NEWELL, 1940; ?Scythentolium sp.; • infaunal facultatively mobile deposit feeders : Janeia typa (WAAGEN, 1881); Janeia cfr. cheni LI & LI, 1997 Stenotipic organisms well adapted to undisturbed and predictable in the abiotic modifications open see environments, such as brachiopods, shows very high extinction rates, while euritopic organisms, such as the bivalves, well adapted to coastal disturbed and unpredictable environments, show higher survival rates, becoming the dominant benthic taxonomic group during the Early Triassic. In the Bellerophon Formation, 6 bivalve associations have been recognized. They record the different phases of a marine regression, ranging from the open marine environment, almost undisturbed and predictable in the abiotic modifications, where stenotopic organisms such as brachiopods were adapted, to lagoon environments with oxygen depletion, high temperature and high levels of salinity, inhabited by low diversified benthic communities dominated by opportunistic species. Association A has the highest levels of richness. This association is related to the maximum flooding surfaces, when the most open marine conditions occurred and the abiotic parameters were affected by weak fluctuations. The high levels of oxygen allowed the presence of a benthic fauna rich in bivalves such as Eumorphotis, Schizodus, Palaeolima, Edmondia, Janeia, Bakevellia, Permophorus, Pernopecten, Modiolus, Parallelodon. This association contains both infaunal and epifaunal bivalves. Few specimens of Aviculopecitinidae gen. nov., Marmaronia and Pleurophorella and abundant individuals of nautiloids, bellerophontids, and Comelicania characterize the assemblage. Association B. It is linked to an environment with high values of salinity and temperatures. The benthic communities still show high richness: Aviculopecitinidae gen. nov., Pleurophorella jacobi, Modiolus, Volsellina, Schizodus and few individuals of Eumorphotis striatocostata. However, it differs from the Association A for the lacking of species typical of more open and stable marine environment such as Pernopecten, Paleolima and Paralledon. Nautolioids and bellerophontids are rare, while the brachiopod Comelicania is missing. Association C shows a moderate richness in bivalves. It is characterized by the abundance of Pleurophorella jacobi and "Pecten" trinkeri, by few Eumorphotis striatocostata and small-sized individuals of Marmaronia peracuta, Modiolus and ?Bakevellia. This association can be related to low levels of oxygen, as suggested by the dominance of Aviculopectinidae, which are considered as marker of disaerobic environments. Association D is essentially composed by the two species Pleurophorella jacobi and “Pecten” tinkeri suggesting an environment of oxygenated muddy substrate, allowing the settlement of infaunal species such as P. jacobi. Association E is a monospecific association with Pleurophorella jacobi suggesting euryhaline conditions. Association F is a monospecific association with “Pecten” trinkeri, which suggests disoxic bottom conditions. The differences in biodiversity of bivalve assemblages are related both to palaeogeographic and stratigraphic setting, with the highest diversity occurring in the Val Gardena area and in the Bulla Member, which represent the top of the formation. In this unit the presence of a rich benthic association, dominated by brachiopods and a few individuals of Eumorphotis striatocostata, "Pecten" trinkeri, ?Scythentolium sp. and Bakevellia sp. should be interpreted as the early effects of the palaeoenvironmental variations prior the extinction event. During the sedimentation of the Bellerophon Formation there is no evidence of extinctions in the community of bivalves, but only regional disappearance connected to the establishment of adverse environmental conditions, such as the disappearance of some nautiloids and bivalves of the Association A (e.g., Palaeolima, Edmondia, Janeia, Pernopecten and Parallelodon) occurring above the Nautiloid Horizon B in the Balest, Pic and Preroman sections and in the lowermost part of the sequence Lo 5 in the Preroman section. This event seems to be connected to the disappearance of suitable niches for the settlement of the above quoted genera. This habitat lost was probably caused by the fast transgression which characterizes the base of the Bulla Member, where articulate brachiopods appear already since the basal decimetres. In the Bellerophon Formation 25 species of bivalves have been classified. Between them, 16% survived to the peak of end Permian extinction (Eumorphotis striatocostata, Marmaronia peracuta, Bakevellia ceratophaga and ?Scythentolium sp.). It is not clear if the mentioned taxa occurs in bed younger than the Tesero Member of Werfen Formation. On the basis of this analysis 84% of the bivalve species found in the Bellerophon Formation extinguished at end of the Permian. In the Bellerophon Formation 17 genera have been described. The extinction rate at generic level reached 41%, corresponding to the following 7 genera: Edmondia, Schizodus, Volsellina, Pleurophorella, Janeia, Towapteria /Marmaronia and Aviculopectinidae gen. nov., while 10 (59%) survived the extinction. Five genera (Permophorus, Towapteria or Marmaronia, ?Scythentolium, Bakevellia and Eumorphotis) can be found in the Werfen Formation. They all died out during the Early Triassic, while Permophorus and Bakevellia disappeared during the Jurassic. When looking at the Lazarus taxa two of them (Palaeolima and Pernopecten) re-emerges and then dies out before the end of the Upper Triassic, Aguilerella re-appears during the Jurassic, Parallelodon during the Triassic and survives until the Cretaceous and Modiolus is still alive. The end-Permian extinction seems to have had a remarkable impact on the bivalve associations of the Bellerophon Formation. This extinction mostly affected the infaunal suspension feeding and deposit feeding bivalves while the semi-infaunal and epifaunal ones show higher percentages of surviving. The high rate of extinction at species level is likely taphonomically biased, because many Lower Triassic species have been classified with different names from the Permian ones, because their poor preservation. From a generic point of view, the bivalves show an extinction rate decidedly lower than other benthic invertebrate groups, with a survival rate of 65%. This allowed to these molluscs to proliferate in the strongly stressed Lower Triassic environments, when they occupied the most part of ecological niches previously occupied by the articulate brachiopods. This replacement represents the third and last evolutionary stage of the marine fauna (Modern Marine Fauna, SEPKOSKI, 1984).