The Moltrasio Formation also known as the Lombardische Kieselkalk Formation is a geological formation in Italy and Switzerland. This Formation mostly developed in the Lower or Middle Sinemurian stage of the Lower Jurassic, where on the Lombardian basin tectonic activity modified the current marine and terrestrial habitats.[2] Here it developed a series of marine-related depositional settings, represented by an outcrop of 550–600 m of grey Calcarenites and Calcilutites with chert lenses and marly interbeds, that recovers the Sedrina, Moltrasio and Domaro Formations.[2] This was mostly due to the post-Triassic crisis, that was linked locally to tectonics.[3] The Moltrasio Formation is considered a continuation of the Sedrina Limestone and the HettangianAlbenza Formation, and was probably a shallow water succession, developed on the passive margin of the westernmost Southern Alps.[3][4] It is known due to the exquisite preservation observed on the Outcrop in Osteno, where several kinds of marine biota have been recovered.[5]
Fossils
Exceptional fossilization
Exposed Moltrasio Fm in Southern Switzerland
Apart from the Eocene of Monte Bolca, the Sinemurian of Osteno is the only fossil deposit in Italy in which soft bodies are preserved. The Osteno site was discovered in 1964. It was recovered from a series of 6 metres (20 ft) package of fine laminated, gray, spongiolitic, micritic limestone.[6]Coroniceras bisulcatum allowed to date the outcrop as the Bucklandi zone, lower Sinemurian.[6] The outcrop is a good documentation of a particularly complete fauna and flora of the Lower Jurassic which is not exactly common in the Southern Alps.[6] The Osteno outcrop, part of the formation, is worldwide known due to the exceptional preservation of mostly marine biota, including rare fossilized components, helping to understand the ecosystems of the local Sinemurian margin of the Monte Generoso Basin.[7] The high local variety of fossils found is most likely due to unique conditions of preservation, where phosphatized soft tissues have not been observed in any fish or polychaetes, but they are common in crustaceans (33%) and also occur in a smaller percentage of teuthids (14%).[8] Soft part preservation through phosphatization in this deposit includes the muscles and branchia of Crustaceans, fish tissues, and the digestive tracts of coleoids, polychaetes, and nematodes. These fossils are interpreted as having been preserved in a stagnant, restricted basin with anoxic conditions likely within the sediment pore waters.[5]
Structural scheme illustrating the syn-rift setting of the western Lombardian Basin during late Triassic and early Jurassic (most of the early jurassic terms belong to the calcare di Moltrasio and secondarily to Calcare di Domaro and Rosso Ammonitico Lombardo). The approximated location in the context of Osteno lagerstätte is indicated. References in the image.
Artistic reconstruction of a hypothetical scene related to the Osteno deposit (Sinemurian-Lower Jurassic). Tilacocephali Ostenocaris cypriformis are hunting cephalopods (Ostenoteuthis) and other arthropods (Coleia, Phlyctisoma). Two feed on the carcass of a conspecific. The sediment, which is very rich in siliceous sponge spicules, indicates that these forms were common and formed true “grasslands” on the seabed. The shark-like Paleospinax pinnai passes in the background.
A polychaete, member of the group Phyllodocemorpha with incertae sedis assignation. This polychaete was probably an open swimmer more than a substrate dweller.[11]
A decapod, type member of the family Aegeridae. This genus is the most abundant local crustacean, and was a shrimp-like creature that was probably necrophagous.[13]
A decapod, member of the family Erymidae. The species was originally included in the genus Eryma as E. meyeri, although the specimens reveal morphological characters diagnostic of Palaeastacus.[19]
A horseshoe crab, member of the family Limulidae. This genus represents the oldest Jurassic limulid and only the second xiphosuran known from Italy.[22] Also indicates that this family were a diverse and widespread clade during the Jurassic.[22]
A thylacocephalan, type member of the family Ostenocarididae inside Conchyliocarida. Originally Interpreted as a bizarre Cirriped. Ostenocaris is the most common fossil of the formation, and the main identified thylacocephalan from the formation. In the first interpretations, the genus was shown as a filter-feeding organism, which used the cephalic sac as a burrowing organ to ensure adhesion to the substrate. Based on the presence of Coprolites associated to the genus, with abundant masses of alimentary residues (hooks of cephalopods, vertebrates, remains of Crustacea) in the stomach of these organisms, Ostenocaris cypriformis was probably a necrophagous organism, and the cephalic sac can be tentatively interpreted as being a burrowing organ employed during the search for food, or as an organ of locomotion with intrinsic motility.[25]
A coleoid, type member of the family Ostenoteuthidae. A unique group of coeloids, only found at this location. Coleoids with ten arms each bearing pairs of hooks, Ostenoteuthis measure between 18 and 30 cm length.[27]
An acorn worm, member of the family Harrimaniidae inside Enteropneusta. It was a large sized member of its family. This genus probably lived linked to sand bottoms, where it made burrows to hide and find food.[29]
An euselachiian, type member of the family Ostenoselachidae inside Elasmobranchii. A unique genus of shark, only found in the Osteno Formation. With an elongated eel-shaped morphology, the specimens found measure at least 240 mm in length, with a neurocranium with a long, complex rostrum.[16] It has been suggested it was an animal able to produce electric shocks, probably to kill prey.[16]
A primitive actinopterygiian, member of the order Palaeonisciformes. A late surviving representative of the family. The specimens may not represent members of the genus.[35]
A primitive actinopterygiian, type member of the family Pholidophoridae inside Teleostei. Open water swimmer, probably forming schools. It is the main fish recovered on the formation.
Primitive actinopterygiians, suggested to be members of the order Palaeonisciformes. The features inferred to refer this specimens aren´t enought to confirm the family association.[37]
MSNM V659, almost complete specimen lacking the tail
Osteno Outcrop
A primitive actinopterygiian, member of the family Peltopleuridae inside Neopterygii. Probably represents the only known Jurassic survivor of the order Peltopleuriformes
A Coelacanth, member of the family Latimeriidae inside Coelacanthiformes. Isolated 20 mm scales of coelacanths analogous to the genus Holophagus are quite frequent on the Osteno deposits, suggesting a taxon up to 75 cm long.
A Horsetail, member of the family Equisetaceae inside Equisetales. A freshwater-related plant, that grew as fast as modern bamboo on tropical settings.
^ abArduini, P.; Pinna, G.; Teruzzi, G. (1983). "Eophasma jurassicum ngn sp., a new fossil nematode of the Sinemurian of Osteno in Lombardy". Atti della Società italiana di scienze naturali e del museo civico di storia naturale di Milano. 124 (1–2): 61–64.
^Poinar, G.O. (2011). The Evolutionary History of Nematodes: As revealed in stone, amber and mummies. Nematology Monographs and Perspectives. 9. Brill Publishers. ISBN9789047428664.
^ abcPinna, G. (1967). "Decouverte d'une nouvelle faune a crustaces du Sinemurien inferieur dans la region du Lac Ceresio (Lombardie, Italie)". Atti Soc. It. Sci. Nat. Museo Milano. 106 (4): 183–185.
^ abcPinna, G. (1969). "Due nuovi esemplari di Coleia vialiii Pinna, del Sinemuriano inferiore di Osteno in Lombardia (Crustacea Decapoda)". Ann. Mus. St. Nat. Genova. 77 (12): 626–632.
^ abcLamsdell, J. C.; Teruzzi, G.; Pasini, G.; Garassino, A. (2021). "A new limulid (Chelicerata, Xiphosurida) from the Lower Jurassic (Sinemurian) of Osteno, NW Italy". Neues Jahrbuch für Geologie und Paläontologie - Abhandlungen. 300 (1): 1–10. doi:10.1127/njgpa/2021/0974. S2CID234814276.
^Arduini, P.; Pinna, G.; Teruzzi, G. (1984). "Ostenocaris nom. nov. pro Ostenia". Atli Del/A Societcl Iraliwzn di Sc: Ienze Naturali e del Museo Civico di Storia Naturale di Milano, Miiano. 125 (1–2): 48.
^Arduini, P.; Pinna, G.; Teruzzi, G. (1982). "Il giacimento sinemuriano di Osteno in Lombardia". Palaeontology, Essential of Historical Geology. Mucchi, Modena. 134 (2): 495–522.
^Pinna, Ð. (1972). "Rinvenimento di un raro cefalopode coleoideo nel giacimento sinemuriano di Osteno in Lombardia". Atti de la SocietaÁ Italiana di Scienze Naturali e del Museo Civico di Storia Naturale di Milano. 113 (1): 141–149.
^Duffin, C. J. (1987). "Palaeospinax pinnai n. sp., a new Palaeospinacid shark from the Sinemuvian (Lower Jurassic) of Osteno (Lombardy, Italy)". Atti della Società italiana di scienze naturali e del museo civico di storia naturale di Milano. 128 (1): 185–202.
^ abcdefgBonci, M. C.; Vanucci, G. (1986). "I vegetali sinemuriani di Osteno (Lombardia)". Atti della Società italiana di scienze naturali e del museo civico di storia naturale di Milano. 127 (2): 107–127.
^ abcdefghSordelli, F. (1896). "Flora liasica ed infraliasica"(PDF). Flora fossilis Insubrica. Studi sulla vegetazione di Lombardia durante i tempi geologici: 38–55.
