Moltrasio Formation

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 Hettangian Albenza 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]

Moltrasio Formation
Stratigraphic range: Middle Sinemurian-Lower Pliensbachian ~196–188 Ma PreꞒ Ꞓ O S D C P T J K Pg N
Outcrop
Type	Geological formation
Unit of	Monte Generoso Basin
Sub-units	Osteno Outcrop
Underlies	Domaro Formation
Overlies	Sedrina Limestone
Thickness	200 m
Lithology
Primary	Limestone
Location
Coordinates	46°00′N 9°06′E / 46.0°N 9.1°E / 46.0; 9.1
Approximate paleocoordinates	33°12′N 15°36′E / 33.2°N 15.6°E / 33.2; 15.6
Region	Lombardy
Country	Italy   Switzerland
Type section
Named for	Moltrasio
Named by	Antonio Stoppani^[1]
Year defined	1857
Moltrasio Formation (Italy) Show map of Italy Moltrasio Formation (Lombardy) Show map of Lombardy

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]

Nematoida

Genus	Species	Material	Location	Notes	Images
Eophasma^[6]^[9]	Eophasma jurasicum	A complete specimen	Osteno Outcrop	A nematode, type member of the family Eophasmidae inside Desmoscolecida.^[10] A rare find, since nematode fossils are absent in most Mesozoic marine rocks.^[9]

Annelida

Genus	Species	Material	Location	Notes	Images
Melanoraphia^[5]^[6]^[11]	Melanoraphia maculata	A nearly complete specimen	Osteno Outcrop	A polychaete, member of the group Phyllodocemorpha with incertae sedis assignation. This polychaete was probably an open swimmer more than a subtrate dweller.^[11]

Crustacea

Genus	Species	Material	Location	Notes	Images
Aeger^[5]^[6]^[12]^[13]	Aeger foesteri Aeger muensteri Aeger robustus Aeger rostrospinatus Aeger elongatus Aeger macropus	Multiple specimens fairly preserved	Osteno Outcrop	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]
Coleia^[5]^[6]^[12]^[14]^[15]^[16]	Coleia vialiii Coleia medilerranea Coleia cf. antiqua Coleia pinnnai	Multiple specimens fairly preserved	Osteno Outcrop	A decapod, type member of the family Coleiidae. Probably a bottom dweller predatory crustacean.^[14]
Eryma^[17]^[18]	Eryma sinemuriana	41 specimens, fairly preserved	Osteno Outcrop	A decapod, type member of the family Erymidae. The species was originally identified as Pustulina sinemuriana.^[19]
Glyphea^[17]	Glyphea tricarinata	137 complete and fragmentary specimens	Osteno Outcrop	A decapod, type member of the family Erymidae. The species was originally identified as Pustulina sinemuriana.^[19]
Mecochirus^[17]	Mecochirus germari	81 complete and fragmentary specimens,	Osteno Outcrop	A decapod, member of the family Mecochiridae. Rather rare compared to other local crustacean genera.
Ostenosculda^[20]	Ostenosculda teruzzii	Single Specimen fairly preserved	Osteno Outcrop	A mantis shrimp
Palaeastacus^[17]^[19]	Palaeastacus meyeri	Multiple specimens fairly preserved	Osteno Outcrop	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]
Phlyctisoma^[17]	Phlyctisoma sinemuriana	10 specimens, in a fairly good state of preservation	Osteno Outcrop	A decapod, type member of the family Erymidae. The species was originally identified as Pustulina sinemuriana.^[19]
Pseudoglyphea^[5]^[12]^[16]	Pseudoglyphea amylochelis	7 complete specimens, in a fairly good state	Osteno Outcrop	A decapod, member of the family Litogastroidae. Probably a bottom dweller predatory crustacean.^[14]
Teruzzicheles^[21]	Teruzzicheles popeyei	Various specimens fairly preserved	Osteno Outcrop	A decapod belonging to the family Polychelidae

Xiphosurida

Genus	Species	Material	Location	Notes	Images
Ostenolimulus^[22]	Ostenolimulus latus	A nearly complete specimen	Osteno Outcrop	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]

Thylacocephala

Genus	Species	Material	Location	Notes	Images
Ostenocaris^[5]^[6]^[23]^[24]	Ostenocaris cypriformis	Multiple specimens, complete and incomplete	Osteno Outcrop	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]

Cephalopoda

Genus	Species	Material	Location	Notes	Images
Arietites^[5]^[6]	Arietites bucklandi	Shells	Osteno outcrop	An ammonitidan, type member of the family Arietitidae inside Psiloceratoidea. The main ammonite identified locally.
Angulaticeras^[5]^[6]^[16]	Angulaticeras sp.	Shells	Osteno outcrop	An ammonitidan, member of the family Schlotheimiidae inside Psiloceratoidea. It was not the most abundant ammonite, but was rather common.
Coroniceras^[5]^[6]^[16]	Coroniceras bisulcatum	Shells	Osteno outcrop	An ammonitidan, member of the family Arietitidae inside Psiloceratoidea. It was less common than other ammonites, although index for some locations.
Ectocentrites^[5]^[6]^[16]	Ectocentrites sp.	Shells	Osteno outcrop	An ammonitidan, member of the family Lytoceratidae inside Lytoceratina. It was not the most abundant ammonite, but was rather common.
Ostenoteuthis^[5]^[6]^[16]^[26]^[27]	Ostenoteuthis siroi	Several specimens with decent preservation	Osteno outcrop	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]
Oxynoticeras^[5]^[6]	Oxynoticeras oxynotum	Shells	Osteno outcrop	An ammonitidan, type member of the family Oxynoticeratidae inside Ammonitida. It was not the most abundant ammonite, but was rather common.
Uncinoteuthis^[6]^[27]	Uncinoteuthis cuvieri	An incomplete specimen consisting of the arm crown only	Osteno outcrop	A coleoid, member of the family Ostenoteuthidae. A unique group of coeloids, only found at this location.^[27]

Echinoderms

Genus	Species	Material	Location	Notes	Images
Hispidocrinus^[28]	Hispidocrinus scalaris	Multiple ossicles	Pradalunga	An Crinoidean, member of the family Isocrininae inside Isocrinida.
Isocrinus^[28]	Isocrinus sp.	Multiple ossicles	Ubiale - Clanezzo	An Crinoidean, member of the family Isocrininae inside Isocrinida.
Millericrinus^[28]	Millericrinus cf. adneticus	Multiple ossicles	M. di Grone	An Crinoidean, member of the family Millericrinidae inside Millericrinida.
Palaeocoma^[6]^[16]	Palaeocoma milleri	Multiple specimens	Ubiale - Clanezzo	An ophiuridan, member of the family Ophiodermatidae inside Ophiodermatina. Extant tropical species like Ophioderma are benthic predators and scavengers that show the same short spines seen in Palaeocoma.^[6]

Hemichordata

Genus	Species	Material	Location	Notes	Images
Megaderaion^[29]	Megaderaion sinemuriense	A nearly complete specimen with soft tissue	Osteno Outcrop	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]

Chondrichthyes

Genus	Species	Material	Location	Notes	Images
Hybodontiformes^[30]	Hybodontiformes indet	Teeth	Osteno Outcrop	A shark, member of the family Hybodontiformes.
Myriacanthidae^[30]	Gen et sp. nov	Virtually complete skull with associated parts of the branchial skeleton	Osteno Outcrop	A chimaerid, member of the family Myriacanthidae inside Chimaeriformes.
Ostenoselache^[6]^[16]	Ostenoselache stenosoma	One complete articulated specimen	Osteno Outcrop	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]
Palaeospinax^[16]^[31]	Palaeospinax pinnai	A single specimen in four contiguous pieces and including parts of the dentition and squamation	Osteno Outcrop	A Shark, type member of the family Palaeospinacidae inside Synechodontiformes. Open water swimmer.
Squaloraja^[16]^[30]^[32]	Squaloraja sp.	One complete articulated specimen	Osteno Outcrop	A chimaerid, type member of the family Squalorajidae inside Squalorajoidei. This genus belongs to a lineage of ray-like chimaeras.

Actinopterygii

Genus	Species	Material	Location	Notes	Images
Cosmolepis^[33]	Cosmolepis ornatus	MSNM V617, complete specimen	Osteno Outcrop	A primitive actinopterygiian, member of the order Palaeonisciformes. A late surviving representative of the family.
Dapedium^[33]	Dapedium sp.	MSNM V630, caudal fin with associated scales	Osteno Outcrop	A primitive actinopterygiian, type member of the family Dapediidae inside Holostei. Probably a Coral Environment wanderer.
Furo^[33]	Furo ("Eugnathus") sp.	MSNM V659, skull & anterior truncal body	Osteno Outcrop	A primitive actinopterygiian, type member of the family Furidae inside Ionoscopiformes. An enlongated Gar-alike taxon.
Pholidophorus^[33]	Pholidophorus cf.bechei	MSNM V621, complete specimen MSNM V556, isolated complete skull Multiple referred specimens	Osteno Outcrop	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.
Pholidolepis^[33]	Pholidolepis sp.	MSNM V409, complete specimen MSNM V594, isolated complete skull Multiple referred specimens	Osteno Outcrop	A primitive Actinopterygiian, member of the family Pholidophoridae inside Teleostei. Open water swimmer, probably forming schools.
Peripeltopleurus?^[33]^[34]	Peripeltopleurus sp.	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
Pteroniscus^[33]	Cf.Pteroniscus sp.	MSNM V618-19, skull & toracic section specimen	Osteno Outcrop	A primitive actinopterygiian, member of the order Palaeonisciformes. A complicated genus know from younger-aged freshwater deposits, whose modern status is rather dubious.

Sarcopterygii

Genus	Species	Material	Location	Notes	Images
Undina^[6]^[33]	Undina (Holophagus) cf. gulo	MSNM V587, isolated scales	Osteno Outcrop	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.

Plants

Genus	Species	Material	Location	Notes	Images
Brachyphyllum^[6]^[35]	Brachyphyllum sp. A Brachyphyllum sp. B Brachyphyllum sp. C	Fronds	Osteno Outcrop.^[33]	A coniferophytan, member of the family Cheirolepidiaceae inside Pinopsida.
Equisitites^[6]^[35]	Equisitites bunburyanus	Stems	Osteno Outcrop.^[33]	A Horsetail, member of the family Equisetaceae inside Equisetales. A freshwater-related plant, that grew as fast as modern bamboo on tropical settings.
Otozamites^[6]^[35]	Otozamites bunburyanus Otozamites sp.	Fronds	Osteno Outcrop.^[33]	A spermatopsidan, member of the family Bennettitales inside Bennettitopsida. Cycad-like flora, common on dry settings.
Pachypteris^[6]^[35]	Pachypteris cf. rhomboidalis	Fronds	Osteno Outcrop.^[33]	A tree fern, member of the family Umkomasiaceae inside Peltaspermales. Fronds of large tree ferns, related to humid ecosystems.
Pagiophyllum^[6]^[35]	Pagiophyllum sp. A Pagiophyllum sp. B	Fronds	Osteno Outcrop.^[33]	A coniferophytan, member of the family Araucariaceae inside Pinopsida.
Williamsonia^[6]^[35]	Williamsonia sp.	Fronds	Osteno Outcrop.^[33]	A spermatopsidan, member of the family Bennettitales inside Bennettitopsida. Cycad-like flora, common on dry settings.
Zamites^[6]^[35]	Zamites sp.	Fronds	Osteno Outcrop.^[33]	A spermatopsidan, member of the family Bennettitales inside Bennettitopsida. Cycad-like flora, common on dry settings.

See also

• List of fossiliferous stratigraphic units in Italy
• Calcare di Sogno, Toarcian fossiliferous formation of Lombardy
• Saltrio Formation, Sinemurian fossiliferous formation of Lombardy
• Rotzo Formation, Pliensbachian fossiliferous formation of Veneto

References

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